If I increase my tire size how much will the spedo be offset?

And;

Ive got the orig 265/70 R17s. Whats the next size up that will make a difference in my RPMs at 70 mph?

 

Here you go try this link for your question.

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

Tony

 

Thats cool. But does the spedometer adjust with a new tire size? Or is there a way to calibrate?

 

It can be recalibrated in software in the ECM. The dealer can do it, or you can buy one of several programmers that can do it.

 

Discount tire has a calculator on thier site. Lots of others do,too. On a 3.73
rear, next size up is 285, not enough difference for anything,practically. A
295 or, a 33.5" tire, replaced where a 265 was is about -5 on the speedo.
(If you show 65 your actual speed will be about 70)

 

How big a tire can the dealers reprogram for? 35s?

 

I have 315's and the dealer reprogramed for them....you need to provide the dealer with the tire revolutions per mile which you can get from the tire manufacture (my Cooper's are 606 rpm).....the dealer enters that number into the pinion factor with the DRB II tool.....speedometer is dead on accurate now....checked it with radar......takes about five minutes to reprogram.

 

On an 06 the dealer can only program up to the 'Power Wagon' size, which is 285/70R17.
And as far as I know, someone please correct me if I am wrong, None of the aftermarket downloaders can correct the FACTORY speedometer display. The BullyDog Outlook can display your actual road speed, but it DOES NOT alter or correct the in dash speedometer display. Again this, pertains only to the 2006.

 

Superchips corrected in dash. Pretty sure smarty will also.