new wheels/tires throw off speedometer

[bakos24]

hey guys, getting my new wheels and tires hopfully Thursday. question is will they through off my speedometer. they are 18x9. I got 245/40 in the front and 275/35 in the back. I went off tire rack thinking they should be ok. thanks for the help.

 

[4.6_02_Black_GT]

you will be off only very slightly since stock is 245/45/17 you will be at 275/35/18 so you will close no more than a 1-2 mph off. Your actual speed will be ever so slightly slower than the speedo says. The circumfirance of you new wheels is like a few tenths smaller in diameter than the stock wheels.


If you need it to be perfect just get a speed cal from Dallas Mustang for $99 bucks.

 

[super302]

it will be off by a very small amount, i wouldn't worry about it at all

 

[HoustonGT]

Yup, like previouisly stated, as long as the overall height isnt off by alot, you'll be fine. With an 18" rim, 99% of people run a 35 sidewall to keep the size as close as possible, but im sure the 40 wont throw it off by more than 2mph.