dsdfan

There are many stock tyre/rim options for the IS250, resulting in varying rear wheel diameters:

205/55 R16

225/45 R17 (AWD)

245/45 R17

255/40 R18 (Sports)

These various diamaters vary by more than 3%. Therefore the speedometer and odometer for all these options can't be all accurate.

Does anyone know which rim/tyre diameter the IS250 speedometer/odometer is calibrated for?

This leads to the next question: Which rim/tyre diameter is the gear ratios optimally configured for?

I asked the dealer who investigated and later reported that the readings are taken from the transmission, and therefore are accurate and immune to any variations in tyre diameter. This does not make sense to me however.

Or is each IS250 calibrated at the factory according to which wheel/rim combination it is fitted with? I doubt Lexus varies the gear ratios for each car according to the diameter of the rim/tyre fitted at the factory!

Thanks for the advice.

Turkoman

I think the overall tire diameter is very similar between a 225/45/17 and a 225/40/18 for example so there may not be any need to calibrate differently unless you are using an odd size. The overall dimansion stays the same (at least with the OEMs) as the rim size gets bigger and tire gets thinner. I think when I last looked at it the difference was like 0.1 inch.

dsdfan

Thanks for your comment Turkoman. You quoted the respective tyre/rim sizes of the stock 17" and 18" front wheels. You are right! Indeed, 205/55 R16, 225/45 r17 and 225/30 r 18 have similar diameters that vary within 1%

HOWEVER, being a RWD car (except for AWD versions,) the speedometer, odometer and gear ratios are linked to the rear wheels. The IS250 has four rear wheel size stock options, as listed:

205/55 R16

225/45 R17 (AWD)

245/45 R17

255/40 R18 (Sports)

These four diameters vary by more than 3%, which exceeds an acceptable margin of accuracy. Which is the diameter that will give the most accurate speedometer reading? which diameter was used in testing and design in determining the most optimal gear ratio?

jjwalker

Go here to calculate tire/wheel sizes.

http://nepasea.net/pages/tiresize.html

Bichon

I have the 18" OEM wheels, and have noticed that the speedometer reads high, e.g., I'll drive by a police radar speed check trailer and it will say your speed is 37 MPH, while my speedo shows 40.

With my Audi, the speedo and those trailers used to agree.

t0e

iTrader: (1)

Hey! I have seen some of the same issues when I pass by a radar check point. It is either To High Or Too low. Might be a Radar issue. -paul

TripleL

Let me state right up front that I don't know much about tire and wheel sizes, but the OPs question has spurred this thought that I hope will assist.

Being as the speedo and the tach are electronic, is there a setting that a dealer could adjust??

So for example if the overall tire and wheel diameter is 12.49" on stock and your new combination is 12.88" why couldn't they just change it in the car's system?

mkaresh

I suspect that if the rolling diameters differ by more than a couple percent they use multiple speedometer calibrations. They cannot systematically produce cars with inaccurate odometers.

IS350_PHIL

iTrader: (2)

I would agree with TripleL. It seems to me, in this age of rolling computers, that someone would be able to program the cars brain and tell it what diameter tire you have and it would adjust from the stock baseline...

dsdfan

They can't make it too easy to access the odometer calibration though. It would be open to abuse.