Gearing for a heavy GU on 35's

[Northside 4x4]

Would have been interesting to see the engine work be completed first.
As you would be able to see where it is going to make its peak torque with the turbo you choose, you could have easily figured out which ratio would have you sitting around peak torque for your towing needs.
Should be a good match with 4.3's and a turbo upgrade though. Moving that peak torque back to 1600-1700rpm and holding it for longer makes a huge difference in the way the car drives.

[love ke70]

trailers 3.5 tonne rating, and everythings built over that, we just rated it at 3.5 to save having to go air brakes, and there wasnt any point rating it higher. and hey, 2 pallets of bricks on the back of my ute was worse than the dirt in the trailer. its all about how you drive it

im just making the point, get the motor right, and mines not off its tits, its just a well sorted motor, and you wont have to worry about the 10% loss in gearing.

my tray alone weighs over 400kg, plus i have full carpenters tools, what most people put in a dual axle trailer. and the only time i complain about my gearing is when i drive it from 1000 rpm in second with the trailer on, like when you come to an intersection and dont quite get it back to first

so have you already done the gear swap, or are you going to take the common sense alternative and make the motor what you want it to be first, then decide if you need the gearing change?

[BIGDAVET86]

The turbo setup i will be getting is from United injection and its a modified TD05 turbo. I dont know all the specs on the turbo itself, but peak torque around 1800 rpm and nice and flat till 3300 rpm apparently. Not modifying the internals of the pump, just adjusting fueling and timing to suit. Replacing factory intercooler with a cross country top mount air to air, with a second scoop on top. They also change the airbox lid and a few other bit and pieces.

I have driven a 06 GU wagon on 33's with std gearing with this exact setup and it was nice and ballsy, but completey unloaded, and thought the gearing was perfect, a touch taller but not singing like a bird to do 125km/h. But not doghy and un responsive like it is now off boost, taking a long time to build that boost. I like it a little shorter, so you can wind it up in fourth and fly past the quads and tripples on Gt Northern Hwy.

Trying to combine the better responce formt he new setup with the right gearing to get the ute to perform how i want it to. Its trial and error, but i think i have got it right

Cheers
Dave

[blowngq]

I think you have chosen the right gearing. The correct calcs give me 4.5:1 gearing to suit 35's. this would give you the factory settings for revs & speedo as if on 31's. So 4.375:1 is close to 4.4:1 so you are a tad under. The 4.625:1 will be to high for your needs. The 4.3's should give you another 300 rpm on what you have now. The 4.6's would give you around 600. So for me I'll be going from 2100 rpm @100 to 2400 rpm on 4.3's. the 4.6's would give me 2700 rpm @ 100. The 4.6's would be great for a weekend warrior only but not so for touring. give us a report on your findings once fitted cheers.

[thehanko]

I think you have chosen the right gearing. The correct calcs give me 4.5:1 gearing to suit 35's. this would give you the factory settings for revs & speedo as if on 31's. So 4.375:1 is close to 4.4:1 so you are a tad under. The 4.625:1 will be to high for your needs. The 4.3's should give you another 300 rpm on what you have now. The 4.6's would give you around 600. So for me I'll be going from 2100 rpm @100 to 2400 rpm on 4.3's. the 4.6's would give me 2700 rpm @ 100. The 4.6's would be great for a weekend warrior only but not so for touring. give us a report on your findings once fitted cheers.

talking stock gearing:

Out of curiosity how did you come to the 'correct' calcs of 4.5? my calculations are change in diameter is in exact ratio of its change in circumference, therefore (% change in circumference is = % change in diameter) 35/31 = 1.13 (% change x std diffs = needed value of new diffs) 1.13 x 4.1 = 4.63 so 4.625:1

For your rpm calculations i'm also confused, 4.3s are as 6.5% gearing reduction (4.3/4.1 = 106.5%) give or take a small amount 6.5% of 2100 is 130 additional RPM not 300

If those theory's are flawed i'm interested to here how. genuinely.



Big dave for what your after 4.3's make sense based on your drive of the wagon (not stock gearing).

Just for interests sake to compare to the wagon you drove (based on 3.9 as im told is the gearing stock)

33 / 31 = 106.5% increase in gearing.

your 35's are a 113% increase in gearing but your stock diff gears are 5% lower meaning his wagon is how yours would be give or take 1% with your current gears and tyres. so currently his and your gearing are comparable - but his engine is not.

so 4.3's will give you a little lower gearing than his set up - which should work well for what you want / expect for the laden needs. it wont be back to stock gearing AFAIK but your engine wont be stock herbs either.

[blowngq]

I think you have chosen the right gearing. The correct calcs give me 4.5:1 gearing to suit 35's. this would give you the factory settings for revs & speedo as if on 31's. So 4.375:1 is close to 4.4:1 so you are a tad under. The 4.625:1 will be to high for your needs. The 4.3's should give you another 300 rpm on what you have now. The 4.6's would give you around 600. So for me I'll be going from 2100 rpm @100 to 2400 rpm on 4.3's. the 4.6's would give me 2700 rpm @ 100. The 4.6's would be great for a weekend warrior only but not so for touring. give us a report on your findings once fitted cheers.

talking stock gearing:

Out of curiosity how did you come to the 'correct' calcs of 4.5? my calculations are change in diameter is in exact ratio of its change in circumference, therefore (% change in circumference is = % change in diameter) 35/31 = 1.13 (% change x std diffs = needed value of new diffs) 1.13 x 4.1 = 4.63 so 4.625:1

For your rpm calculations i'm also confused, 4.3s are as 6.5% gearing reduction (4.3/4.1 = 106.5%) give or take a small amount 6.5% of 2100 is 130 additional RPM not 300

If those theory's are flawed i'm interested to here how. genuinely.



Big dave for what your after 4.3's make sense based on your drive of the wagon (not stock gearing).

Just for interests sake to compare to the wagon you drove (based on 3.9 as im told is the gearing stock)

33 / 31 = 106.5% increase in gearing.

your 35's are a 113% increase in gearing but your stock diff gears are 5% lower meaning his wagon is how yours would be give or take 1% with your current gears and tyres. so currently his and your gearing are comparable - but his engine is not.

so 4.3's will give you a little lower gearing than his set up - which should work well for what you want / expect for the laden needs. it wont be back to stock gearing AFAIK but your engine wont be stock herbs either.

31" tyre = 787.4mm
35" tyre = 889mm
difference 101.6mm
11.43% difference
4.11 x 11.43% = .469773
4.11 + .469773 = 4.579

Depends on what calc you use but seems to me that you may be right with your calcs. I had to dig out some of my old archives to find a calc that i used. Think my memory is shot.

this is what i came up with this time

RPM=

mph x ratio of diff x ratio of gear x 336
---------------------------------------------------------
tire diameter


So 4.11:1 on 31's would have 2328 rpm
on 35's 2062

35's with 4.375 = 2195 rpm
35's with 4.625 = 2320 rpm

So I wonder why some blokes say they are revving around 26-2800 on 35's with 4.625's

[gu town]

So I wonder why some blokes say they are revving around 26-2800 on 35's with 4.625's

how many 35's actually measure 35'???

[blowngq]

So I wonder why some blokes say they are revving around 26-2800 on 35's with 4.625's

how many 35's actually measure 35'???

Even if the tyre measures in at 34.5"s on 4.625's the rpm would only be 2353 rpm.

[REV_310]

depends also how bald they . i've 4.375's on 35s td42t its alright on 35's its very good on the road but i would go for tc gears if i wanted to play with low range but for fuel economy and speed i would use 4.3's. 4.6 would be too high and you would be stuck doing 100 cause your engine would be revving its C!@# out. and because you cant switch diff gears from inside the cab.

[blowngq]

depends also how bald they . i've 4.375's on 35s td42t its alright on 35's its very good on the road but i would go for tc gears if i wanted to play with low range but for fuel economy and speed i would use 4.3's. 4.6 would be too high and you would be stuck doing 100 cause your engine would be revving its C!@# out. and because you cant switch diff gears from inside the cab.

What revs are you sitting on at 100 kph? According to the formula 2400 rpm would be where it would sit with 4.625 with 35's & that's far from revving to high. With the formula you should be sitting on approx 2200 rpm with 4.375 on 35's.

I'm using .836 for 5th gear.

As for the op I still think he's better off with the 4.3's as if he tow's a lot he should stay in 4th & with 4.6's then yea it might wear thin in no time.

I'm just interested to know as I'm planning a ratio swap soon & mines comfortable sitting on 2400 rpm. which is 4th gear atm with 4.11's & 35's.

[REV_310]

i smash though gears but not long till i'm in 5th on a straight

[thehanko]

i smash though gears but not long till i'm in 5th on a straight

this means what exactly?

[thehanko]

31" tyre = 787.4mm
35" tyre = 889mm
difference 101.6mm
11.43% difference
4.11 x 11.43% = .469773
4.11 + .469773 = 4.579

ok thats where we are different. you worked out the increased gearing as a % of the new tyre and applied the % change of new tyre with the ratio of the old diffs. if comparing apples with apples the % needs to be both from the origional or both from the final, cant use both in the one calc.


i.e based on % change from stock
tyre calc
101.6 / 787.4 = 12.9%

diff cal
so 4.11 x 1.129 = 4.64 ratio

or
based on % change from final set up (but need to know final ratio to do this)
tyre calc
101.6 / 889 = 11.4%

diff calc
11.4% x 4.64 = 0.528
and 4.64 - 0.528 = 4.11

or 4.64 x 0.886 = 4.11

Just wanted to look into the maths of it as, as we have found it does effect builds.


simple formula to use - can be rearranged as needed to calculate what ever part people want to know. n means new o means old. tyre size is in diameter inches or mm is fine as long as both are the same.
its simple only and doesnt take into consideration wanting a specific change in gearing (if you want that pm and ill alter it).


ratio n/ ratio o = tyre n / tyre o

in this case
ratio n / 4.11 = 35 / 31

rearranged to find new diff ratio
ratio n = ratio old (tyre n / tyre o)
ratio n = 4.11 (35/31)
= 4.11 x 1.129
=4.64
so 4.64:1 or as close as possible is the new ratio required to return the gearing to stock.


tyre n = tyre o (ratio n / ratio o)

note solve whats inside the brackets first for anyone who is unsure.

Sorry to the OP for high jacking - I think you got what you needed and thought it might help future searchers.

[thehanko]

Blown gq thats a handy formula for gearing and rpm and speed. good to have thanks. just need to know 5th gear ratio and your set.

as for rpm v speed.

I would say there is alot of vague info out there from some people go off the speedo which is inaccurate... more inaccurate if they have modified the drive line at all with tyre or gearing.

then others go off their gps but some dont state that they use their gps,

so we have 4 variables, gps speedo, car speedo, and gearing and tyres. with so many variables and with information coming in from many who don't understand it its hard to trust any of it.

and lets not forget how many people make comments like after the turbo was fitted the tacho sat 300rpm lower at 100kph... all in all I would ignore most of it

I prefer maths as if you put in the right data you will get the right information. i.e if i change from 4.11 to 4.64 gears my car will rev 12.8% more. doesn't then matter what speed i'm talking about. when maths goes a bit wrong is when tyres actually measure 31.6 instead of 32 etc etc but its only minor.

[BIGDAVET86]

I have had the 4.3's fitted and they are perfect. Just drove 600km and it was exactly what i wanted. Havent done any engine mods yet. Sits on 120 on the speedo using 18l/100km. Nice and pokey off the line, but not too short, 1st is still needed, and sits at hwy speeds better. Overtakes well. Heading down the beach in a week or so. Time will tell. Also got new clutch; safari tuff clutch. Spot on. Whole package has transformed the ute.

Cheers
Dave

[love ke70]

18/100, in a diesel?
thats not good...

[gu town]

I have searched but cant really get a definate answer on what im chasing.

I have a 04 GU coil cab on 35's with std gearing and std turbo setup. I am about the upgrade the turbo and intercooler hoping for about 110kw at the tyres. I am setup fairly heavy with 315l of fuel, steel tray with steel roofed/canvas canopy. I also tow a 1T boat. I live in the Pilbara and do a fair bit of highway/long dirt roads and want to get the whole show a bit more sorted. Without the boat into i like to do some decent wheeling and i am really feeling the pain on the beach at the moment, just lacks the punch down low.


Cheers
Dave

I have had the 4.3's fitted and they are perfect. Just drove 600km and it was exactly what i wanted. Havent done any engine mods yet. Sits on 120 on the speedo using 18l/100km. Nice and pokey off the line, but not too short, 1st is still needed, and sits at hwy speeds better. Overtakes well. Heading down the beach in a week or so. Time will tell. Also got new clutch; safari tuff clutch. Spot on. Whole package has transformed the ute.

Cheers
Dave

18/100, in a diesel?
thats not good...

not great... but from my experience only a little above avg. I'm sure that when it gets tuned up those figures will come down.


Glad you're happy with it Dave.

[turbo gu]

Glad your happy with 4.3s!

i am guessing 18l/100 is when towing and loaded!

[love ke70]

get some Dgas on that shit

edit: turbo it first if you havnt yet, and get a laminova on it, im running day to day 3.2 tonne, 12L/100 then add a bit of gas. with the gas once i correct kms, economy doesnt change between 35's and 31's, so should be similar with you running 4.3 with 35s. gotta love torque...

[BIGDAVET86]

Thats not towing but loaded up. Was in a bit of a rush so was sitting on 120km/h on the speedo. Overtaking everything, boot in up the hills. Not on nice long flat hwy up north, down to Ravensthorpe, so few more corners and hills. Speedo would be out abou 8% im guessing giving me a cruising speed of 130km/h Not too bad i reckon. Still had legs to overtake too. Hits 140 overtaking no dramas. Im still waiting to get the turbo and intercooler done, damn tax bill is raping me for the next little bit, and going into business for myself so $$$ getting tight

Cheers
Dave

[bushy555]

> Hits 140 overtaking no dramas

You do realise that GU's were never designed to go this fast?

[chunks]

According to the speedo they are!

[love ke70]

> Hits 140 overtaking no dramas

You do realise that GU's were never designed to go this fast?

when your overtaking a road train you dont wanna be doing less in my experience...

[chunks]

Overtaking triples in my 60 with a lazy 2H turbo was always interesting

[gu town]

> Hits 140 overtaking no dramas

You do realise that GU's were never designed to go this fast?

when your overtaking a road train you dont wanna be doing less in my experience...

x58947565087346098349874!

[BIGDAVET86]

well 140 on the speedo should be up round the 150-155 mark i reckon so thats enough for me. As for how fast to overtake, its all a matter of timing. Sitting on their arse then trying to floor it and go round is not the go. Sit a good distance back and you will have better vision and a run up so to speak, when you overtake. If its that dicey, just call him up on ch40 and ask if he can give you a shout when its all clear.

Unfortunately i wont be geting a turbo upgrade for a while now. Gotta buy a new work ute instead.

Cheers
Dave

[gu town]

well 140 on the speedo should be up round the 150-155 mark i reckon so thats enough for me. As for how fast to overtake, its all a matter of timing. Sitting on their arse then trying to floor it and go round is not the go. Sit a good distance back and you will have better vision and a run up so to speak, when you overtake. If its that dicey, just call him up on ch40 and ask if he can give you a shout when its all clear.

Unfortunately i wont be geting a turbo upgrade for a while now. Gotta buy a new work ute instead.

Cheers
Dave

your speedo should be pretty well on the mark now shouldn't it?