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abell75 · 02-17-2011, 05:55 PM

To answer the question:

Yes, your speedometer will be wrong. (I know this math is not quite right, but it's close). It will read LOW by a percentage approximately equal to the increase in circumference of your tires: 2/14= 14%

So when going 57, you will see 50 on your gage
When going 114, you will see 100

Like others have said, there are many topics posted already. Opinions are mixed; some say the added rotational inertia cancels out the improved final drive ratio, others have positive results.

like almighty bmw stated, you need the circumference of the TIRE not the wheel, to make the calculation.

02-17-2011, 05:55 PM	#4 (permalink)
abell75 EcoModding Lurker Join Date: Nov 2010 Location: St. Louis Posts: 38 Grand Prix - '02 Pontiac Grand Prix GP 90 day: 24.01 mpg (US) Thanks: 3 Thanked 0 Times in 0 Posts	To answer the question: Yes, your speedometer will be wrong. (I know this math is not quite right, but it's close). It will read LOW by a percentage approximately equal to the increase in circumference of your tires: 2/14= 14% So when going 57, you will see 50 on your gage When going 114, you will see 100 Like others have said, there are many topics posted already. Opinions are mixed; some say the added rotational inertia cancels out the improved final drive ratio, others have positive results. like almighty bmw stated, you need the circumference of the TIRE not the wheel, to make the calculation. Last edited by abell75; 02-17-2011 at 05:56 PM.. Reason: the TIRE, not the WHEEL.