putting 33s on a 12ht

[rowan]

howdy all- thinking about putting 33s on my 60-

really hesitant as i get 10km / L on highway at the moment and i don't want to upset the miraculous balance that gives me those sort of fuel figures.

what diff will 33s make- bigger diameter so therefore less revs, but heavier? more friction (more aggressive tread)- at the moment i'm running michelan LTX 31s, which are pretty much a road tread- i drive mostly sand, so the mild tread isn't an issue, but i could do with a bit more clearance-can you get 33s with a mild tread or are they all aggressive tread patterns?

cheers

[Shadow]

I get 13l/100 from my 2H pushing 33x12.5's

you will probably be somewhere between 11 and 12L/100 if you go to 33's.

[rowan]

so you reckon fuel consumption will definitely get worse, not better?

[Shadow]

so you reckon fuel consumption will definitely get worse, not better?

definantly worse

[vSAHARAx]

heya i put 33's on my 12ht bout 2 weeks ago, fuel consumption went from bout 12.5l/100 to 13l/100 (all city driving, with heavy right foot)

but on the freeway my speedo is pretty well spot on. the old fella pick up his new D40 navara on friday and with his cruise on 100 i was doin bout 101km/h. Before i would have to sit on around 105-110 to keep up with traffic in a 100 zone, Hell 33's to me was the best money ive spent on it!!!!

Troy

[tuffsahara]

i run 35's on mine if i drive it like an old man i get just over 500 to a tank but yeh im usually on the loaud pedal listening to ther sweet sound of a 12h-t so usually around 460 kays to a tank whatever that is per 100 i dunno haha

[rowan]

hey Troy i was reading somewhere (on this forum i think) that the 60s were designed for a 32.7 inch tyre, even tho they came from the dealer with 31s. My speedo is also out by 5-10k/hr, so that makes perfect sense-putting 33s on will make the speedo almost perfect.


What sort of 33s did you fit? how are they so far?

what other size tyres can you put on a 15 inch rim? most 33s are in a 12.5, and i don't want to go wider, just taller. 11.5 would be ok. are there metric equivalents? 255/80? etc

[tuffsahara]

yes when mine had 33's the speedo was pretty much spot on

[vSAHARAx]

mate i put 15X10" mickey thompson rims and Mickey Thompson BAJA MTZ's And so far they are bloody awsome in the wet,dry,mud,gravel etc

Mine are a 33x12.5. Do you want to keep the factory 7" wide rims hence the narrower tyre?

Heres a pic of what it looks like now.


As for metric equivalents for a 15" rim i couldnt tell ya. You could try a 32x11.5xR15, I used to have the BFG A/T in that size before i changed to the 33's and the new rims. How much lift have you got under yours? the 32's with the 3" spring lift used to look good, tyres used to fill a bit of the guard nicely. Then i got the 2" body lift and it started to look a bit skateboard-ish.

Troy.

[tuffsahara]

troy that thing is lookin great!! as for the equivilant of a 16 inch rim for a 33 itd be 285/75/16 same height just a little narrower

[rowan]

i'm not sure how wide my rims are-they're those alloy ones with the two black indented circles on each spoke (can't figure out how to attach pic) -

i've got 2 inches of lift.

i was chasing the narrower ones more so for less road noise, better fuel economy and less punctures. all i want from the 33 is more clearance and less revs (and the bonus of a correct speedo)

this is driving me nuts-i've spent most of the day searching and reading stuff from all sorts of places / forums. If i were to go to 12.5s, what is the pick of the bunch for mainly highway and sand driving? i was looking at goodyear silent armour? everyone raves about goodyear MTRs, but i reckon i'm wasting my time buying them as i never drive mud.

i'm leaning towards the BFG 33x10.5 A/T so i can go for the narrower tyre

[juscruisin]

everyone raves about goodyear MTRs, but i reckon i'm wasting my time buying them as i never drive mud.

i'm leaning towards the BFG 33x10.5 A/T so i can go for the narrower tyre

MT/Rs aren't great in mud. Good everywhere, else, though. I wish they did them in 33X10.5.

[vSAHARAx]

My old BFG A/T's wernt the best in the sand as i had to drop pressure alot for them to bag out. Look around for a good H/T.

[flyology]

Was thinking about the fuel economy issue when I was driving home from work last night.

I think that there is probably no difference in fuel economy, and if there was it would be slightly on the better side.

My reasoning for this theory is:

Changing the tyre diameter on your vehicle to a larger diameter means your engine revs less at a specific speed. Your speedo is driven by the drive train, and has not been modified to suit when you changed tyres, so the speedo shows less kilometers/miles.

To perform a more accurate fuel economy test it would be necessary to either have a speedo drive gear calibrated to the correct tyre size, (both 31x10.5x15" and the 33"s), or perform two tests over a set distance.

The new fuel economy reading is probably more realistic using 33's (as someone mentioned, there was two tyre sizes fitted to 60 series, but only one speedo drive gear available) than using 31's which will show more kilometers than what has really been travelled. (hence a "better" fuel economy)

There is a way to work it out, but the link to the drivetrain calculator I had doesn't work anymore, it is Sunday morning, I had to work late last night and basically my brain is overstressed at the moment anyway..................

[Shadow]

Was thinking about the fuel economy issue when I was driving home from work last night.

I think that there is probably no difference in fuel economy, and if there was it would be slightly on the better side.

My reasoning for this theory is:

Changing the tyre diameter on your vehicle to a larger diameter means your engine revs less at a specific speed. Your speedo is driven by the drive train, and has not been modified to suit when you changed tyres, so the speedo shows less kilometers/miles.

To perform a more accurate fuel economy test it would be necessary to either have a speedo drive gear calibrated to the correct tyre size, (both 31x10.5x15" and the 33"s), or perform two tests over a set distance.

The new fuel economy reading is probably more realistic using 33's (as someone mentioned, there was two tyre sizes fitted to 60 series, but only one speedo drive gear available) than using 31's which will show more kilometers than what has really been travelled. (hence a "better" fuel economy)

There is a way to work it out, but the link to the drivetrain calculator I had doesn't work anymore, it is Sunday morning, I had to work late last night and basically my brain is overstressed at the moment anyway..................

Thats all well and good, and if you had a 1000hp big block v8 your fuel economy would get better as you increase tyre size

unfortunately the old 12HT only puts out about 100kw, and its just not enough power to maintain the same efficiency when your accelerating.

Also, by changing the final drive ratio you now have less revs for the same given speed, (as you also pointed out) which means that at 100km/h your engine is no longer in its peak performance band (or further down in its performance figures).

Sure going from 31's to 33's is only a small change, a ratio change of 6%, but its enough to make a difference.

As someone else pointed out, the HJ60/61 came out with 7.50R16 tyres which are closer to 32.2" tall. Most 33" tyres are actually less than 33" so going to a 33" tyre brings the vehicle back closer to OE specs.

However, its no good saying "my vehicle is now closer to OE specs so it should get better economy". The fact is, HJ60/61's doing around town work get better fuel economy on 31" tyres than they do on thier OEM tyre 7.50R16, and thus, will also get less economy on 33" tyres.

So going to 33" tyres will decrease your fuel economy, but not drastically, I would say no more than 1L/100km. (10%?) Ofcourse if you do mostly highway km's at 100km/h then your economy might even go down (although i doubt it).

[flyology]

So if I put 13" rims on my cruiser I will get even better fuel economy? Did toyota have two differently tuned 12-HT engines, one that came factory fitted with 16" rims, and one with 15" rims? (being facetious with these two)

I was mearely pointing out that using the odometer of the vehicle to work out fuel economy is flawed unless it is calibrated to the correct sized tyres..........

Slight changes in gear ration/tyre size can assist, when combined with engine modifications that provide a good torque figure (not horsepower) at cruising RPM and sensible driving habbits. If they didn't why would Marks go to all of the trouble to make overdrive gear sets for transfer cases....

[Shadow]

So if I put 13" rims on my cruiser I will get even better fuel economy? Did toyota have two differently tuned 12-HT engines, one that came factory fitted with 16" rims, and one with 15" rims? (being facetious with these two)

I was mearely pointing out that using the odometer of the vehicle to work out fuel economy is flawed unless it is calibrated to the correct sized tyres..........

Slight changes in gear ration/tyre size can assist, when combined with engine modifications that provide a good torque figure (not horsepower) at cruising RPM and sensible driving habbits. If they didn't why would Marks go to all of the trouble to make overdrive gear sets for transfer cases....

Call it whatever you want.

When i went from 31's to 33's on my Hj60 (2H) my economy got worse not better.

Every other person ive ever heard of that has put on bigger tyres thier fuel economy gets worse. Forget all your hypotheticals and what theoretically should be possible, reality is, fuel economy gets worse.

[rowan]

most people who go from 31s to 33s would go from a 31x10.5 to a 33x12.5-i.e most people are going wider as well as taller- in that case of cource your economy is going to get worse, as you've got a heavier tyre plus more friction / rolling resistance.

But if you went from say a 31x10.5 to a 33x10.5 as i am planning, wouldn't you expect minimal change? -i would expect the lowering of revs to counter the extra weight of a taller tyre. i will test this theory when i put them on and let you all know.

i was looking for a good HT but there are none in this size-anyone know of one?

i'm kind of glad you can't get MTRs in this size, otherwise i would be faced with another tough choice

[flyology]

The difference in fuel economy would be minimal. There seems to be so many people say that fuel economy gets worse with 33's, but no one has actually calculated what their REAL fuel economy is with 31' tyres fitted to their 60 series cruiser.

FACT: if you calculate your fuel economy with 31' tyres and don't take into consideration the speedo error your calculations are flawed from the begining.

FACT: Most 31x10.5x15 tyres are not even 31" tall, they are closer to 29" tall.

FACT: 60 series speedos are more accurate with 33" tyres.

FACT: The mathematical constant π (pye?) represents the ratio of a circle's circumference to its diameter and is commonly used in mathematics, physics, and engineering. pye = 22/7 or 3.14

so, using pye, 1 revolution of a "so called 31 inch tyre" (really 29") is 91.06 inches, where as a 33" tyre (if it is in fact truely 33" in diameter) is 103.62 inches.

FACT: this means that for each revolution of the rear axle, a 31" (really 29") tyre travels 12.56 inches less that a 33" tyre.

FACT: with a standard diff ratio of 4.10, your engine RPM will be 4.1 revolutions to one revolution of the rear tyre in 4th gear. add in the overdrive ration of 0.845 of 5th, and engine RPM in 5th works out to a ration of 3.945 for one turn of the rear tyre.

FACT: 1 inch is = to 25.4mm, there for there is 39.37 inches in a meter, and 39,370inches in a kilometer.

FACT: a true 33" tyre will rotate in the vicinity of 379.94 times over a 1 kilometer distance.

FACT: in 5th gear, with 33" tyres fitted, your engine will rotate 1498.86 times to travel 1 kilometer, so with 33" tyres, at 60kilometers per hour, you engine RPM is close to 1500rpm.

FACT: because there is no way of changing your speedo to suit 31" tyres, (unless you have it re-calibrated, or fit a Marks speedo correction box) you will still see the same speed, at the same RPM with 31" tyres. problem is, you have not travelled the same distance.

FACT: working backwards, we know the engine RPM is 1500. We know the diff and gearbox ratio, and the tyre size. so to work out the true distance travelled, 1500RPM divided by 3.945 (our final drive ration) = 380 rotations of the wheel, or 379.94 as mentioned above. Using pye, our 31" tyres (which are really 29" tyres) have a diameter of 91.06", so 379.94 (wheel revolutions per minute, not engine) x 91.06 (tyre radius) = 34597.33 inches travelled. 34597.33 x 25.4 to work it out in millimeters = 878772.18. or .8787 of a kilometer.

FACT: using 31x10.5x15" tyres on a 60 series cruiser with no speedo correction, means your distance travelled (or odometer reading) is incorrect by approximately 120meters every kilometer. That is, it says you are travelling at 100kph, when really you are travelling at around 89kph.

FACT If you use your unmodified 60 series odometer reading to work out your fuel consumption, you are going to get a figure that is just over 10% better fuel economy that what you are really getting. (you dont really travel 600 kilometers on a tank, you are travelling around 540).

This has been a long winded way to prove, using mathematics, not assumptions or guesswork, that if you put 33" tyres on a 60 series, your fuel economy will seem to change by around 10%, when it is really now a more accurate reading of distance travelled, not more fuel used.


Example, on 31x10.5x15 tyres your speedo said you had travelled 600 (but you haven't because the speedo is wrong, you have only travelled 540) klms, and you had to put 60 litres of fuel in to fill up. so, 10klms per litre.


Now you have 33" tyres fitted, your speedo say you have only travelled 540 klms and you still have to put 60 litres in to fill up, so 9klm per litre.

The only thing that has changed is your speedo now shows the correct distance travelled, where as earlier it gave you a false reading saying you had travelled further than you really have, hence fitting 33"s to a 60 series can give you the impression that your fuel economy suffers.

And last of all, has anyone replaced a 4 speed with a 5 speed gear box in a 60 series and said that their fuel economy has suffered? changing the final drive ratio and fitting larger tyres create the same issues............

If the friction value of a tyre change from 31 - 33 affected fuel economy so much then you would need to start taking into consideration road surface friction values, brake drag, head wind, tail wind, etc etc....

any budding physics students out there??

[Antz]

logical calculations BUT;

Forget all your hypotheticals and what theoretically should be possible, reality is, fuel economy gets worse.

Sorry to say but this is the case.

PS you didnt factor in the energy lost in turning the larger, heavier tyres

[flyology]

I am ready and waiting for someone to show me some sort of theory/evidence how heavier wheels and tyres will make such a huge difference in fuel economy.

Even with each wheel and tyre combo being 10kg heavier you still have to use energy to push you main load (the vehicle itself) of around 2500 - 3500kg

The statement that heavier wheels and tyres make fuel economy suffer mean that it must be expensive when towing a boat or trailer, or even with the family in the car..........

and a 10% reduction in fuel economy is a big reduction. Add 10% to the interest rate of your home loan and tell me it is a slight increase......

Funny how that 10% reduction in economy is similar to the 10% reduction in RPM you get when changing tyres size, or the 10% speedo error that has been there since the vehicle was fitted with "31" inch tyres....

Also, tell me, what takes more to drive, a 25kg 33" tyre and rim, or a 20kg 35" tyre and rim?

[Shadow]

I am ready and waiting for someone to show me some sort of theory/evidence how heavier wheels and tyres will make such a huge difference in fuel economy.

Even with each wheel and tyre combo being 10kg heavier you still have to use energy to push you main load (the vehicle itself) of around 2500 - 3500kg

The statement that heavier wheels and tyres make fuel economy suffer mean that it must be expensive when towing a boat or trailer, or even with the family in the car..........

and a 10% reduction in fuel economy is a big reduction. Add 10% to the interest rate of your home loan and tell me it is a slight increase......

Funny how that 10% reduction in economy is similar to the 10% reduction in RPM you get when changing tyres size, or the 10% speedo error that has been there since the vehicle was fitted with "31" inch tyres....

Also, tell me, what takes more to drive, a 25kg 33" tyre and rim, or a 20kg 35" tyre and rim?

Its not the heavier the tyre. its the increase torque required to get the bigger diametre tyre rolling. Put a 100" tyre on your Hj60 and you car will stall every time because it cant generate the torque to get it rolling(gearing incorrect). The issue is where does this increased torque come from.

Your thaughts on speedo correction are valid and i will try to take this into account when I check my fuel economies from now on, however, my 31" tyres measured 30.5" and my 33" tures measure 32.4", so thats only a ratio of 6%, and yet my fuel eceonomy has changed by at least 10%. So there is at least 4% change in economy. Certainly less than i thaught, but still present.

[cruiser60series]

I'm a second year Engineering student and I've been wondering about this one too so i decided to get some physics textbooks out have a look.

For a single rotation of wheels, obviously a 33inch will go further along the ground than a 31, but for this the engine had to do more work (burn more fuel) with the bigger tyre to cover this distance, but in payment for more work we have traveled a further distance.

There is another factor called Rotational Inertia, which is analogous to normal inertia but in rotational inertia the Radius about the axis of rotation plays a big factor. So regardless of covering distance (imagine the car hoisted up with wheels free to spin), Spinning the bigger wheel at the same rpm will use more fuel just because of a larger radius and mass distribution there within (width plays a factor here too). For these calculations we actually sum masses over radius SQUARED. So if we double the radius (keeping mass the same) of a wheel we quadruple the rotational inertia and the quadruple the force required to turn this wheel over a single rotation. The way to keep this inertia down is to keep the weight down and to the center of the tyre.

note: Inertia is defined as resistance to acceleration

SO it requires more work by the engine to move one wheel's rotation with bigger wheels but as previously mentioned this is beneficial in extra distance traveled.

but it's more work to turn a bigger wheel regardless of the distance traveled due to the increase of mass and radius but more importantly the radius from the center at which this mass is.

I'm probably not very clear so look up "Moments of inertia" on wikipedia

[flyology]

Yes, more fuel required, but how much more??

I had a 1982 FJ45 troopy a few years back. It was fitted with 31x10.5x15 inch wheels, and was horrendous on fuel. I could not drive from Cairns to Townsville sitting on 100kph (by the speedo) on a tank of fuel. I bought new wheels and tyres 235? 75 16's and I could drive to Towsville from Cairns sitting on 100kph (by the speedo and probably more acurate a speed) and have just under 1/4 of a tank left.

Now there is no way in the world a 25 year old 2F engine is going to have more torque than a 12-HT.

Also, I cannot see Toyota fitting 16 inch wheels to a car if it was going to cause such a drastic reduction in fuel economy.

Plus, you may need to burn more fuel to move a larger tyre, but have you taken the gearbox reduction into consideration? Gear reduction multiplys torque, to get rotational inertia going.....

I can see I am going to have to fit my 16 inch wheels, do a run to Innisfail and back, then fit my 15 inch rims and do the same trip, and see how much fuel it takes to refil my cruiser each time..........

[RED60]

Whether the speedo/odometer is correct or not, an increase/decrease in fuel economy will still appear if you travel between the same 2 points by the same route. It just can't be reliably compared to another vehicle not traveling the same route....

[Shadow]

Yes, more fuel required, but how much more??

I had a 1982 FJ45 troopy a few years back. It was fitted with 31x10.5x15 inch wheels, and was horrendous on fuel. I could not drive from Cairns to Townsville sitting on 100kph (by the speedo) on a tank of fuel. I bought new wheels and tyres 235? 75 16's and I could drive to Towsville from Cairns sitting on 100kph (by the speedo and probably more acurate a speed) and have just under 1/4 of a tank left.

Now there is no way in the world a 25 year old 2F engine is going to have more torque than a 12-HT.

Also, I cannot see Toyota fitting 16 inch wheels to a car if it was going to cause such a drastic reduction in fuel economy.

Plus, you may need to burn more fuel to move a larger tyre, but have you taken the gearbox reduction into consideration? Gear reduction multiplys torque, to get rotational inertia going.....

I can see I am going to have to fit my 16 inch wheels, do a run to Innisfail and back, then fit my 15 inch rims and do the same trip, and see how much fuel it takes to refil my cruiser each time..........

the size of the wheel makes very little difference, its the size of the tyere that matters.

A 2F produces about the same power as a 12HT, just at a higher RPM. Cant remember the torque but id say the 2F would be close to the torque of the 12HT aswell, maybe 10% less cause its a petrol?

Your 235/75R16 tyres equate to a 30" tyre (29.8"), so youve changed your tyre size very little. Are you saying going from a 15" rim to a 16" rim made your car more than 25% more efficient?

Gearbox changes the ratio the same amount no matter what tyre youve got on. Therefor it will cancel itself out in any comparisons you draw between the two tyres.

[flyology]

No I am not saying going from a 15 inch to a 16 inch rim improved my fuel economy, I merely stated what change I made in tyres. Tyre diameter or radius makes all the difference. The 15 inch tyres fitted to my troopy were called a 31x10.5x15, however as I pointed out very early on in the discussion, they are not a true 31 inches in diameter, they were 29 inches in diameter. (I actually still have the tyres and rims in the shed)

As for the 16 inch tyres, (they were grandtrek AT2's I think 235 75 16's) I measured them and from memory they were very close to 31 inches in diameter.

The aspect ration of a tyre can vary between manufacturers, and also does not take into consideration differences in tread depth.


As for engine specs:

2F 4230 6cyl Petrol 135bhp@3600 210ft/lb@1800 12 OHV 94x102 7.8:1 comp ratio

12H-T 3980 6cyl TID 135bhp@3500 231ft/lb@2000 12 OHV 91x102 18.6:1 comp ratio

2H 3980 6cyl ID 103bhp@3500 177ft/lb@2000 12 OHV 91x102 20.7:1 comp ratio

So yes, pretty close in Hp and Torque figures for the 2F and 12-HT with the 2H lagging behind.

I brought up the issue of gearbox multiplication as This is how a vehicle overcomes inertia.

As for rotational inertia, to actually move larger heavier wheels and tyres, the increase in fuel usage would be so minimal, it would be insignificant.

Jack the rear of you cruiser up. start it and run it through the gears with 31" tyres, then 33" tyres. With some clutch control I would think it possible to get the wheels moving in probably all gears at idle not using any throttle at all.

I think the biggest hurdle for fitting 33" tyres to a 60 series cruiser is using less RPM and putting the engine below the most efficient power level whilst trying to push a 3 and a half ton piece of steel with the aerodynamics of a brick along a highway at speeds over 100kph.

Speak to anyone that owns a 12-HT in a 60 series and more often than not you will be told that the best fuel economy is achieved at approx 90kph on the highway, whether towing or unloaded. (that works out to about 2000RPM in top gear, where a standard engine produces the best torque)

Maybe 33 inch tyres at too big for a 60 to return the best fuel economy, but I think 31 inch tyres (ones that are truely 31 inches in diameter, and not 29 inches as in my case with the troopy) are too small, and cause the engine to rev excessively, and more revs = more fuel used.

Finally, Rowan dont be worried about your fuel bill increasing substantially by fitting 33 inch tyres. If you work out your fuel economy via the odometer, it will appear to have suffered by around 10%. In real life it may suffer by 2-5%, but this will depend on driving style, engine condition and load.

And with the 2F and 12-HT being similar in power figures, I am confidant if you measure your current tyres you will find they are around 29 inches in diameter (I measured my BFG all terains and they are 29.5) and change to a tyre that is a true 31 or 32 inch tyre, you speedo will be more accurate, and you will have a slight increase in fuel economy. Not to mention the additional savings of less engine wear from higher RPM and servicing your vehicle every 5000 klms by the speedo rather than every 4400klms.....

[foster_the_fat]

I'm guessing those torque figures are lb/ft, as the 2H puts out 240 odd nm and the 12HT 300ish from memory.

[flyology]

Specs came from an American site, so yes you are correct they are ft/lb not nm as I incorrectly added in. (I didn't want to cut and past the whole graph). Have gone back and fixed them up. Here is a link to where the info came from....

http://tlc.off-road.com/tlc/article/art ... ?id=264200


The performance/fuel economy figures down the bottom of the page are interesting, I wonder what sort of wheels were fitted to U.S.? (Did they get diesels? Canadian maybe) spec HJ60's.......

[Shadow]

Specs came from an American site, so yes you are correct they are ft/lb not nm as I incorrectly added in. (I didn't want to cut and past the whole graph). Have gone back and fixed them up. Here is a link to where the info came from....

http://tlc.off-road.com/tlc/article/art ... ?id=264200


The performance/fuel economy figures down the bottom of the page are interesting, I wonder what sort of wheels were fitted to U.S.? (Did they get diesels? Canadian maybe) spec HJ60's.......

diesels wereonly available to commercial buyers in both US and canada. (mines etc).