Thread: Larger tyres - odometer reading affected?

Justin,
Have a look here:
http://www.miata.net/garage/tirecalc.html

According to that, when you were doing 100kph on your old tyres, you're now doing 105.2kph on the new ones.

You'd have to add 5.2% onto the odometer reading to be accurate.

Juz

As it's most likely that your speedo and odometer were not exactly accurate to start with (can be anything up to 10% out, ussually reads fast) even with the calculations from the miata website you still probably won't get an exact figure. If you can get a GPS you could work out your speed and compare it with the speedo and work out the difference.

As an example on my D1 with 225/75's which are a bit bigger that standard when the speedo shows 100kph I'm actually doing about 97kph.

Iain

Originally posted by Juz
Hi all,
I have fitted 245/75 Cooper STs to my TD5 MY03 Disco. I want to get an accurate fuel economy figure, but do not know how much "extra" to allow for odomoter figure as opposed to actual kms travelled.
I may be wrong - but the odometer reading should be under what has actually been travelled?
and if so, what percentage should i add?
any info would be much appreciated.
cheers,
Juz

Isn't there a 5km odometer check on the Waterfall Exprressway - that is near Loftus? isn't it - set cruise control at an indicated 120kph (watch for cops) and simply record distance and time elapsed - work out errors. If correct 5km will take 2.5mins - hence the maths is easy if you do either 120kph or 60 kph (too slow on expressway).

Even easier if you have a GPS - instant readouts

Hi guys,
thanks for the response. might give the freeway speedo test a workout as well.
cheers,
Juz

Originally posted by garrycol


Isn't there a 5km odometer check on the Waterfall Exprressway - that is near Loftus? isn't it - set cruise control at an indicated 120kph (watch for cops) and simply record distance and time elapsed - work out errors. If correct 5km will take 2.5mins - hence the maths is easy if you do either 120kph or 60 kph (too slow on expressway).

Even easier if you have a GPS - instant readouts

100 klms in 1 hour divided by 5klms = 1/20 of 1 hour or 3 mins. (and its legal).

Trev.

With 245.75x16 on my 92 Range Rover the speedo is now 100% accurate according to a GPS comparison, but my odometer is 5-6% pessimistic.
Go figgure.
This implies that with 205 s my speedo was 105% and the odometer 100%, which sort of agrees with the fact that automakers must have a speedo which is 100-110% accuracy. It cannot be under so they are conservative.
Regards Philip A

Originally posted by PhilipA
With 245.75x16 on my 92 Range Rover the speedo is now 100% accurate according to a GPS comparison, but my odometer is 5-6% pessimistic.
Go figgure.
This implies that with 205 s my speedo was 105% and the odometer 100%, which sort of agrees with the fact that automakers must have a speedo which is 100-110% accuracy. It cannot be under so they are conservative.
Regards Philip A

My GPS speed with standard wheel diameter is 7km/hr less than shown on the speedo (less below about 60 kph) but the odometer is pretty much spot on as is the test on 5 km marked sections on some freeways.

So the speedo must be "geared" down but the odometer reads revolutions direct :?:

If this is the case then the odometer will under read the distance travelled by the percentage difference of the circumference of the larger wheel/tyre versus the standard wheel/tyre

Juz

When I went up to the 245/75 16 my speedo became correct but my odometer was 5% out. 5km road side check and odometer read approx 4.75km

Speedo was checked against the ones on roadworks and at Wodonga on the Hume hwy. Checked later by GPS.

Funny how one side is accurate and the other is incorrect

Danny