Outer Limits 4x4 Board

G'day all, can anybody please explain to me the difference in tyre sizes eg:- the standard size for my cruiser should be 265/ 75 R-15 112S. But my cruiser is wearing 33x12.50 R-15 108S. Now I read the standard size from the sticker on the doorframe, and the 33 etc I read from the tyre whilst kneeling down beside it. I've already given these sizes to my son for his comment and he says I've made a mistake in my readings, so I've just been out to the car again armed with a bright torch and checked that I was right the first time. My reason for wanting this info is to find out if my speedo is now giving a wrong reading. I drove steadily at 95KPH and my wife following in her car claimed she had to be doing 105/110 KPH to keep up with me. Someones speedo is telling porky pies and I reckon its mine due to the difference in tyre sizes. Any idea's, thanks in advance...Al

Yeah....it would be your larger tyres causing at least a 10% error in your speedo reading.
The easiest way to confirm this would be to check it with a GPS....the only true way to measure your speed !

Usually for every inch larger in tyre diameter size, you get a 10% error in your speedo !

I'm running 35" tyres on my Feroza which came out with 29" tyres as standard. When my speedo says I'm doing 84kph, I'm really doing 'bout 105kph. There are adapters you can buy which alter the speedo to suit the tyre size that your running......not sure where you'd get one from though

Marks 4wd Adaptors sell the speedo correction gearboxes.

Ferozius wrote:Yeah....it would be your larger tyres causing at least a 10% error in your speedo reading.
The easiest way to confirm this would be to check it with a GPS....the only true way to measure your speed !

Usually for every inch larger in tyre diameter size, you get a 10% error in your speedo !

I'm running 35" tyres on my Feroza which came out with 29" tyres as standard. When my speedo says I'm doing 84kph, I'm really doing 'bout 105kph. There are adapters you can buy which alter the speedo to suit the tyre size that your running......not sure where you'd get one from though

i dont need a GPS to tell me how fast i am going

you are on drugs there is a simple math formula

or the redneck way (my preference)
follow along beside a mate and screams out the speed he is going ( a stocker/rental/or company car is best as they are normally newer and more accurate, and you dont care if they rub a bit at speed!) you look at that and mark down or remember the 60kph, 80kph, and 100kph, these are the main ones.

the mathematical formula will give you a percentage that you will be out travelling at a given speed.

keeping in with my theory of no secret squirrel

the formula is:

1-(x/y)x100 = percentage change in speedometer reading

(i think) do not quote me or send police fines to me YMMV!!!!!

bj on roids wrote:

Ferozius wrote:Yeah....it would be your larger tyres causing at least a 10% error in your speedo reading.
The easiest way to confirm this would be to check it with a GPS....the only true way to measure your speed !

Usually for every inch larger in tyre diameter size, you get a 10% error in your speedo !

I'm running 35" tyres on my Feroza which came out with 29" tyres as standard. When my speedo says I'm doing 84kph, I'm really doing 'bout 105kph. There are adapters you can buy which alter the speedo to suit the tyre size that your running......not sure where you'd get one from though

i dont need a GPS to tell me how fast i am going

you are on drugs there is a simple math formula

or the redneck way (my preference)
follow along beside a mate and screams out the speed he is going ( a stocker/rental/or company car is best as they are normally newer and more accurate, and you dont care if they rub a bit at speed!) you look at that and mark down or remember the 60kph, 80kph, and 100kph, these are the main ones.

haha this is exactly what me and my mates do too!

works wonders

thats the same way i worked mine

Follow cars through speed cameras. If the flash doesn't go off, you can be pretty sure of you speed (just remember your indicated speed in relation to your actual speed)

Ferozius wrote:Yeah....it would be your larger tyres causing at least a 10% error in your speedo reading.
The easiest way to confirm this would be to check it with a GPS....the only true way to measure your speed !

Usually for every inch larger in tyre diameter size, you get a 10% error in your speedo !

man that is such a rash generalisation you truly have no idea do you!

Thats a little harsh BJ...

Can you buy an alternate speedo drive gear, i went from 265-16s to 285-16s on my GU change the speedo drive cog and speedo is now within 2kph of the GPS - cost $60..

perhaps a little explanation

1-(x/y)x100 = percentage change in speedometer reading

i have elaborated and you yourself can prove my theory, its not some half assed illiterate crap trap spewed forth from an unknown or unrespectable source.

1-(x/y)x100 = percentage change in speedometer reading.

here is the working out and the breakdown, in my mind:

x being the original tyre size
y being the new tyre size

1-(31"/35")x100 = 11.43% to the nearest hundredth of a percent, change in increasing your rolling tyre diameter by 4" not 10% for every inch as has been suggested

do the math, and cmawwwn, give it up!!

where (") = inches

YMMV
WMAB

BUNDERA wrote:Thats a little harsh BJ...

harsh but true and fair.

I do not like innacurate generalisations, unless it is me making them.

There is a place in ringwood you can get the conversion done. I had it done on my Vitara, it cost $70. Its pretty much spot on. The place is called 'Ringwood Speedo service or Ringwood speedometer service'. On the corner of Heatherdale and whitehorse road. Just work out first if your going to keep that same size tyre or are you going to change again.
Chris.

MAN, that was a little harsh :o

must have had my rags