Larger tyres = inaccurate odometer?

[rushN]

Hey guys

Just wondering if tyres with a larger rolling diameter makes the odometer read inaccurately?

Querying this for service intervals and more specifically for fuel consumption calculations now that the OEM trip computer is now (more) inaccurate due to fittment of larger tyres.

From my calculations, going from 245/65R17 to 265/65R17 means an increase in circumference from 2356mm to 2438mm (82mm increase), which equates to 410 tyre revolutions per km for the new tyre, versus 424 tyre revolutions per km for the original tyre. Does the decrease in revolutions impact the odometer, making it read lower than it should?

Cheers

[King Triton]

rushN

Yes you are correct the odometer will now read less than actual km's travel.
But to work this out properly you need the Rolling Circumference of each size tyre and then convert it into a percentage so it is easier to calculate

If the measurments you have are the rolling circumference than you just
divide
2438 by 2365
=1.034

so your new tyres are 3.4% larger that the originals

so multiply your km's x 3.4% or 1.034 and you will have your correct km's traveled

[rushN]

Thanks heaps King.

Yes, those measurements I give are the rolling diameter of the new tyres, so your calcs are correct.

If what you say is true, then this brings up another query...

...the speedo with the OEM tyres was reading 5km/h below true (based on my GPS). The speedo is now 100% correct per GPS. Does this mean that my odo was incorrect previously, and is now accurate?

I had a thought that the odo was taken from the gearbox and not the front or rear axle, making the tyre size irrelevant to distance measured?

Cheers

[King Triton]

As far as i know the Speedo is out with the factory tyres but the Odometer is correct with factrory tyres.

[Mud16]

Borrow a GPS and see your actual speed and compare,I put 265/75/16 MTZ on mine amd my speedo is really close to actual speed

[rushN]

Borrow a GPS and see your actual speed and compare,I put 265/75/16 MTZ on mine amd my speedo is really close to actual speed

Already done this. Re-read the third post.

[Austblue]

odometer k's travelled is definitely effected by your tyres. If you're concerned you can get an adjusting gadget from Jaycar.

[fraz91]

How hard is it to attach an adjuster to the odometer? mine's now reading 5.2% slower than stock since i put the new rubber on (265/70/R17). worked out the difference today coming back from belanglo forest when the GPS said the trip was 134kms and the trip meter said the car had done 127kms

[Longranger1]

Hey guys

Just wondering if tyres with a larger rolling diameter makes the odometer read inaccurately?

Querying this for service intervals and more specifically for fuel consumption calculations now that the OEM trip computer is now (more) inaccurate due to fittment of larger tyres.

From my calculations, going from 245/65R17 to 265/65R17 means an increase in circumference from 2356mm to 2438mm (82mm increase), which equates to 410 tyre revolutions per km for the new tyre, versus 424 tyre revolutions per km for the original tyre. Does the decrease in revolutions impact the odometer, making it read lower than it should?

Cheers

I wouldn't worry too much about it - it's only about a 3.5% difference. The indicated speed will be more accurate though.

If you service every 7500km's then it will only be out 262.5 km's extra km's. A speedo corrector will sort it out, but you will still be over-reading by 6-7 km/h at 100 km/h actual (instrument error).

Sadly, you can't have both ways unless you remove the speedo and have it recalibrated to read lower (without intercepting the signal).

Essentially, if you correct the speedo with the bigger tyres then the odometer should be accurate. The speedo on the other hand will show the same over-reading error.

[fraz91]

Sadly, you can't have both ways unless you remove the speedo and have it recalibrated to read lower (without intercepting the signal).

Essentially, if you correct the speedo with the bigger tyres then the odometer should be accurate. The speedo on the other hand will show the same over-reading error.

Does this therefore mean that I can't use a speedo corrector to adjust the odometer without changing the indicated speed? is it possible to just intercept the odometer wire and connect the speedo adjuster to that instead of to the speedo?

[fridgie]

Thanks heaps King.

Yes, those measurements I give are the rolling diameter of the new tyres, so your calcs are correct.

If what you say is true, then this brings up another query...

...the speedo with the OEM tyres was reading 5km/h below true (based on my GPS). The speedo is now 100% correct per GPS. Does this mean that my odo was incorrect previously, and is now accurate?

I had a thought that the odo was taken from the gearbox and not the front or rear axle, making the tyre size irrelevant to distance measured?

Cheers

I run 265/75/16's and my speedo is pretty much spot on. My odometer is out by about 50M/km as per the roadside 5km odometer check signs

[Homer]

Why do you have an issue with the odometer?
It's reading on the good side for re-sale and it's not out by enough to cause you to mis judge a refuel.
Good work on resurrecting a topic rather than starting a new one Fraz

[Longranger1]

Sadly, you can't have both ways unless you remove the speedo and have it recalibrated to read lower (without intercepting the signal).

Essentially, if you correct the speedo with the bigger tyres then the odometer should be accurate. The speedo on the other hand will show the same over-reading error.

Does this therefore mean that I can't use a speedo corrector to adjust the odometer without changing the indicated speed? is it possible to just intercept the odometer wire and connect the speedo adjuster to that instead of to the speedo?

The difference is instrument error verses signal error. You have to verify by gps whether the odometer is accurate or not (or known accurate distance markers). It may mean that by using a speedo corrector that if you want the speedo to read spot-on then the odometer may be out and vice-versa. If you intercept the signal with a speedo corrector then the odometer reading could well be out. So a compromise may have to be made.

To intercept the odometer input could be a very tricky exercise I think as I am pretty sure they have the same input signal.

[NowForThe5th]

I seem to recall that someone had a go at this a long time ago but couldn't figure where the signal split on the circuit board. Needs an electronics expert. It did seem feasible, though, to be able to adjust the input variably so that two sets of tyres could both be made to read right on the speedo. As Homer says, the odo is out the right way and the difference is fairly small, so, who cares.

[Snooozy]

for those that are interested, here is the link

viewtopic.php?f=23&t=4383&hilit=speedo#p65449

[cyberhonky]

i use the gps and then change it on the scangauge- 2 speedo's displaying correctly that way

[RHKTriton]

The TAXOMETER is the one thats nice to have right!

[RossyB07]

Didn't need to start another thread on this so i thought I would just add to it.

As I've just changed tyres to 265/70's the speedo is pretty spot on, but obviously the odometer is out.
As detailed above there is a speedo fixing device you can purchase from Jaycar but, is there a way to correct the odmoeter once bigger tyres are fitted so it will read true as when the stock tyres were on??

I realise I can do calculations but I'd honestly rather the odometer be reading true on the actual car.

[josh_cod]

From what I understand and what the posts above say, the two readings for speedo and odomoter are from the same input and changing one will change the other unless you get really involved. If you wanted to fix your odometer then it would mean that the speedo would be out again.

[ultimate]

I haven't done it on a Triton but the Jaycar system works very well on the Hilux. If you wire it into the speed sensor from the back of the gearbox, it corrects speedo and odometer perfectly. Marks adapters also have a unit which works good. It's a fair bit more expensive than the Jaycar one but it comes complete with no need for soldering.

[Snooozy]

http://www.newtriton.net/phpbb/viewtopic.php?f=23&t=4383&hilit=speedo+fix#p65449
I have used the Jaycar unit for about 2 years
My speedo & odometer are spot on