How much off is my speedo with 18" wheels

I got 18x8 and 18x9.5

225/40/18 and 255/35/18


Also how does this affect the gear ratio?

 

Your speedo and odo is NOT off at all. It's NOT the size of the rim that affects the speedo or gearing. It's the size of the tire.
Your OEM stock rims/tires have an overall diameter of 24.9".
Your front tires have an overall diameter of 25.1", however, it's not the front tires that affect the speedo or gearing. It's the rear because that's where the speedo takes its readings from and that's where your drive wheels are.
Your rear tires have an overall diameter of 25.0". A difference of 0.1" is negligible.

BTW, this should be in the wheel/tire forum.

 

oh ok. so my MPG calculations won't be off to badly with the larger wheels. Thansk Xviper!

 

More things impact MPG than just tire diameter. Yes it will not be off too badly but going with larger -read heavier- rims will hurt your gas mileage. Don't expect it to be the same as before but not off by much.

 

The MPG is going to be affected by the weight of the wheel and tire and the rolling resistance of the tires. Like xviper stated the diameter of the wheels do not change anything. As long as the correct size tires are used you should be good.

 

All this time i thought it's the front wheel thats taking its reading from.

 

So what do you do to counteract the difference if there is a significant difference between OEM and the aftermarket wheels you purchase?

 

It's NOT the wheels. It's the tires! If your tires are vastly different in diameter, get a Yellow Box for speedo compensation.

 

I meant to say tires. Yellow box it is.