Wheel size affecting speedo?

[EcomotiveMatt]

Hi, I'm thinking of putting 18 inch alloys on my Ecomotive, how much will this affect the speedo and is there a way of adjusting the computer to account for the extra 2 inches?

 

[GrahamFR]

and what happens to the thickness of the tyre...

 

[sixysix]

Wheels get bigger so tyres need to be thinner to keep the same rolling radius.
Fit 225/40/18 to you're new wheels and that's it.

That's assuming that you're new 18" wheels are 7.5 wide.

 

[zondaff]

I believe you just get the speedo reset with the wheel geometry, book in to the dealer.
Your speedo will be at most 2-3 MPH inaccurate

 

[Biker]

As has been said above, you do not have to re-calibrate your speedo, provided you select the correct sized tyres.

Below is a chart showing the difference on speedo reeding between an eighteen inch wheel and a sixteen inch wheel.

As you can see the difference is negligible, at 90mph an error of 0.7mph. The tyres differ only by 0.2mm on the overall diameter, normal tyre wear over the life of the tyre would give you more than that.