Anybody notice change in speedometer?

[AttiS550]

I am about to go from my stock PP wheels and tires (tire size at least) to the Project6gr wheels and bigger tires. I am going from a 255/40/19 F 275/40/19 R to a 285/35/19 F 325/30/19 R. The tires are Conti Ext DW.

I have read about speedometers being off after changing the over all diameter of wheels and tires. I believe my rear tires will be a difference in size of 12.5 mm after doing a little math. with the same size wheel I think this means the rears will be about an inch smaller overall from top to bottom. Will this significantly throw off my speedometer or any other reading for that matter? If yes is there a way to fix this issue when and if I experience it?

Thanks in advance!

:cheers:

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[plc268]

Your front tires are actually taller on that setup... 26.85 inches front, 26.67 rear. Just thought to mention that.

Stock PP wheels are 27 front, 27.66 rear. So you're losing an inch of height in the rear. It'll actually make your speedometer read faster than what you're going.

 

[lemers]

And the difference should be more noticeable the faster you are going

 

[plc268]

It's roughly a 3.5% change. So your speed should read ~3.5% higher.

At 30 mph indicated speed, you'll actually be going 28.95, but at 70 indicated mph you'll be going 67.5. This is something you can verify by setting your cruise control and using a gps speedo (your phone should work for this), and see what speed your car maintains.

Either way, it's not a huge deal. If you go with larger tires, you'll have the opposite speedometer discrepancy, and that can get you in trouble if you're not aware of it (due to the fact you're travelling faster than what is indicated).

 

[lemers]

It's roughly a 3.5% change. So your speed should read ~3.5% higher.

At 30 mph indicated speed, you'll actually be going 28.95, but at 70 indicated mph you'll be going 67.5. This is something you can verify by setting your cruise control and using a gps speedo (your phone should work for this), and see what speed your car maintains.

Either way, it's not a huge deal. If you go with larger tires, you'll have the opposite speedometer discrepancy, and that can get you in trouble if you're not aware of it (due to the fact you're travelling faster than what is indicated).

And if you use that as an excuse to a police officer, they can also write a ticket for "improper equipment"

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