Speedo is off 5mh faster (Not sure why), After switching from 14 to 15 Options

[msk59]

After I purchased the car the first mod was to get rid of 14 inch steel rims. I purchased 2001 model 15 inch and installed 195/55/R15 Goodyear Kelly sports on them. I checked my speed the other day with GPS and I noticed it is 5 miles OFF at 80 mph. The car show that I am driving 80mph while the GPS shows exactly 75. I am surprised because, the door sticker states that I can either use either 14 inch with ***/70/R14 or upto 205/55/R15.

Is there some correction somewhere I need to do? Just curius.

Thanks

[nics]

Sounds about right. Your new tires have a smaller diameter than the stock size. If you got a 195/60-15 (this is what I am running), it will be faster than the actual reading by 0.3%. Your size right now runs slower by 1.8%

Always use a tire size calculator next time to check how much you'd be off from stock.

[msk59]

Thank You Nics: Actually, I was under the impression if Toyota is allowing two different wheel sizes, they must have done something to fix the issue but I guess I was wrong.

[Langing]

Thank You Nics: Actually, I was under the impression if Toyota is allowing two different wheel sizes, they must have done something to fix the issue but I guess I was wrong.

You know that your car's engine is controlled by the ECU (computer), as well as the ABS, and the tendency in the auto industry over time is to "computerize' more and more functionality. What you were expecting could be done. Namely, an extra input to the computer could be tire circomference, or diameter, and the computer could be set up so that it controls your speedometer, depending on tire size.

Check out the specs of some top of the line, expensive, late model car to see if designs have come that far. If not, go get yourself a patent.