Speedometer Affected by Wheel Circumference?
 

I'm aware that if you were to get a tire size where the overall wheel circumference is different than stock, this will cause the speedometer readings to be off depending on the size difference.

My question is: Does this apply to all 4 wheels, or does this apply only to the rear as our car is a rear-wheel drive? Or does one have more of an affect than the other, like the rears more than the fronts?

Most car speedo pickups are off the output shaft of the transmission, thus, the tires on the "powered" wheels determine the speedo reading. On the S, this would be the rear wheels/tires. On FWD cars, it would be the front tires. You can easily witness this if you spin your drive wheels on a slippery surface like snow or sand. Your speedo will climb based on the speed of those wheels and not on how fast the car is moving along the ground.
By comparison, most motorcycles have their speedo pickups on the front wheel.

xviper, thanks for your explanation.

Just out of curiosity. Assumming I went with the stock OEMs in the rears and I went with 205/65-16 in the fronts instead of the OEM size of 205/55-16, would this wheel/tire combinations have an affect on the speedo readings?

No, what you put on the front wheels of this car will not affect the speedo. However, why would you want to use such a "tall" tire? This would give you a less rigid sidewall, hence, adversely affecting cornering. It would make the tire flex sideways more than it should when you turn. Not to mention it'll make the car look a bit weird.

205/65-16 is a size I just use to ask the question. I'm currently running 255/40-17 rears and 225/45-17 fronts and considering going to 215/45-17 fronts next time I change tires. I know this size is a bit off of OEM specs, that's why I posted the question. Wasn't sure if this will affect the speedo or not. However, based on your response, it will not which is great!

Thanks for all your help.

Now you've touched upon a subject that I'd like to ask of you.

Next spring, I plan on getting the exact sizes of tires you currently run. Why are you going to 215 fronts? Just curious if there's a reason why I shouldn't go with 225's.

:hello:

Just for reference I checked out the MY04 (& I think it's same for MY05) OEM wheel/tire spec, it calls for 215/45-17 front and 245/40-17 rear (Bridgestone Potenza RE050)...

;)

[QUOTE=xviper,Oct 28 2004, 04:51 AM] Now you've touched upon a subject that I'd like to ask of you.

Next spring, I plan on getting the exact sizes of tires you currently run.

Yes, it does. Thanks.