Sam Khoury

I used the following link to compare tire sizes and the affect on the speedometer reading.

http://www.miata.net/garage/tirecalc.html

Now here's my question. I have a 2002 GS300 SportDesign where the stock tire size was 225/55R16 but on all other 2002 GS300s the stock tire size was 215/60R16. If Lexus did not recalibrate the speedometer on the GS300 SportDesign when they switched to the 225/55R16 tires, it will be reading 1.6% too fast. Not that I'm doubting Lexus, but do you think they recalibrated the speedometers for the SportDesign?

rominl

iTrader: (4)

ummm.... i doubt..... from what i have heard, speedo themselves are already having around 2% deviation to begin with?

bitkahuna

iTrader: (20)

Lexus does not compensate for different tire sizes. The factory 16s and 17s are quite different in size, ergo, different speedo/odo results.

Sam Khoury

Oh really, that's surprising. I didn't realize that on my 1999 GS300 when I went from the 215/60R16 stock 16 inch tires to the the 235/45R17 stock 17, I made the speedometer read 3.2% too fast. That's quite a large difference.

The good thing is that if I go with the 245/45R17 on the stock 17" GS400/430 wheels like I'm thinking about doing, it's only going to make the speedometer read 0.2% too fast when compared to the 225/55R16 tires I have on the stock 16s now. That's of course assuming it was calibrated correctly to those 16 inch tires. If the speedometer was calibrated to the non SportDesign 215/60R16 tires, then it will be 1.8% too fast if I switch to the 245/45R17 tires.

Can I ask the dealer to recalibrate the speedometer or will they charge me an arm and a leg to do it?

///MDex

Other way around bub.

Bigger wheels & tires spin more slowly.

I wouldnb't bother with it really. I don't think the mph is guaged off the wheels anyway. We've had this discussion before, but I think it's the ABS sensor.

Sam Khoury

Dex, my first statement about the speedometer reading 3.2% faster was correct. Obviously if you put the wheels on the ground without tires, the 17s will travel farther than 16s but once you add tires, the overall circumference can actually be larger on the 16s than the 17s depending on the profile of the tires on each one. And it's the overall circumference of the tire (not the wheel) that affects the speedometer. The circumference actually gets smaller going from 215/60R16 to 235/45R17. If you goto that web page in my first post and enter the tire sizes you'll see that the circumference of the 215/60R16 tires is 82.2 inches and the 235/45R17 tires is 79.6 inches, a reduction of 2.6inches. Therefore each revolution on the 17inch tires you are traveling 2.6 inches less distance, 3.2% less (2.6 inches/82.2 inch circumference of the 16s). This means your speedometer is reading 3.2% faster because it calculates that you are traveling farther than you really are in a given amount of time since the calibration is off.

rominl

iTrader: (4)

yeah dex you took it the other way. your point of larger wheels needs larger moment of inertia to move is one thing, but speed is another. the speed on the car is caculated by the number of revs the axle does in certain time. therefore with a wheel of larger diameter you will traval longer distance. therefore if you put on a wheels with smaller diameter, your speedo might read 65 but you are actually doing 62, and if you put on larger diameter wheels, speedo reads 65 and you might be doing 70.

this all applies when you get new wheels and tires. that's why for my car when i got my 19 i SHOULD stay with the specific profiles of tires i am using to match the overall diameter

bitkahuna

iTrader: (20)

One thing to consider, the "3.2%" diff doesn't mean the 16's were 'right' for the speedo. Maybe Lexus split the diff. for calibration, meaning if you drive the 16's the speedo is 1.6% too slow and if you drive the 17's it's 1.6% too fast. No biggie.

You can compare two sizes at once on my own calculator page by clicking here.

rominl

iTrader: (4)

or lexus actually calibrated the cars for 16" and 17" differently when the car came out of the fatory?

IS0LD0UT

iTrader: (1)

Any other comments on this? Check my other post https://www.clublexus.com/forums/sho...d.php?t=154867

The lexus dealer installed 215/60R16 blizzaks for the winter, my factory tires were the 235/45R17. A tire calc shows the 215/60R16 being 3.18% bigger in diameter. Would a dealer install tires that far out of OEM spec, without being able to calibrate the ECU? I just put my summer 235s back on and am concerned that the car might be racking up some extra miles.

Avenger

Here's a link to a very simple Tire Size Calculator that shows MPH difference after changing your OEM tire size. http://www.miata.net/garage/tirecalc.html

sakataj

iTrader: (182)

u know this thread was from 2002? ATLEAST u searched for your first post and didnt make a new thread