I was thinking of replacing my stock 16" wheels and tires with some 17" or 18" ones. Will I have to do anything after the upgrade in order to have the speedometer be accurate? Or do I have to pick lower profile tires so that the circumference stays the same as the stock sets.

Thanks,
Ken

 

You will want to stay near syock diam anyway since not much taller will put you into the lower spring mount on the strut.

But if you did go any taller or shorter the ECU needs to be adjusted for accuracy. I had a buddy with an 06 who swapped on 18s and said he lost 5-7 MPG avg. I finally told him he wasn't getting credit for all his miles so the numerator was too low in the calc and misrepresenting MPG.

 

Here is a better site for checking speeometer variations when changing tire and or wheel sizes. I use it a lot. It will give you the differences at many different speeds. You don't need to enter the "Measured Circumference" just the wheel size, tire width and aspect ratio. It is an easy tool to use.

http://www.web-cars.com/math/tiresize.html

 

I like that site better also, thanks for the link. It reaffirmed that a235/40R18 is a good swap.

 

I put too tall tires on....makes my warranty last longer!

 

I'm keeping my eye on you vinny.

 

Just make sure you put the old ones back on before taking it in for service...

 

Will the dealer adjust the ECU once you go to bigger wheels? I would like my mpg to be as accurate as possible.

 

There is no adjusting a ECM for tire size on an HHR.

 

The best thing for you to do if your wanting to keep everything accurate.

GO to the Tire Rack web site and choose you tires by Diameter and Revolutions per mile. IF you keep them close it will keep your dash accurate.

I looked up the spec on my 18's on the SS and matched my winter tires to the sames spec only on a 16 inch wheel. I am off 2 Rev Per Mile from the factory tire. That is about as yout going to get.

Also same tire sizes will vary from MFG to MFG. A 215 55 16 from Firestone is not the same diameter as a BFG or Goodyear. They will be close but not exact.