I seem to remember this info around before the site melt down, but can't find it.

When is it really needed to re adjust the computer when changing wheel size? If say, I go from 17" to 18 or 19", will I need to reflash or is the change negligible (affect to the speedometer readout)?

 

It has nothing to do with the wheel size. It's all about the tire size. The goal is to get the diameter of you new tires as close to the stock diameter as possible. IIRC, it's 27.2"

 

Of course you are correct. So at what overall diameter would a flash be needed if you go larger?

 

There's no exact number. It just depends on how much speedo error you're willing to put up with. Personally, I would stick with +/- 0.3". I think the 255/45/18's are .01" smaller.
What sizes are you looking at. Are you trying to run a larger tire to fill the wheel well?

EDIT:: Oops. Typo. The .01" should've been 0.1".

 

what about 295/45/20

 

a 295-45-20 is 30.5 inches tall and i wouldnt think that would fit
275-35-20 = 27.6"
285-30-20 = 26.7"
335-30-20 = 27.9"
345-25-20 = 26.8"

those should all work if your trying to stay with as wide a tire as you possably can

 

Looking at 18inch wheel, but haven't decided on the rubber. Might go a bit bigger tire in back than front.

Looks as if most any average tire sizes will be negligible to the speedo.

Thanks for the help! I will eventually get a tuner, but probably not until later unless i finally decide on a CAI....

 

Tire diameters vary with brand even with the same size tires. The stock 17" Pirelli PZero Neros are 27.1 inches tall, according to Tire Rack's data. 255/45/18's of the same brand are 27" tall.

You can always go with a wider tire in back, as long as you compensate for the height with a lower aspect ratio, e.g. 285/40/18 (40 series vs. 45 series for the front) is 26.9" tall (again, using the PZero Nero M&S as the example). This would give you a wider tire on a 1" larger wheel without affecting your speedo.

Check out the Specs link of any tire on Tire Rack, which lists a lot of info about the tire diameters, revs per mile, etc. They also list the tire weights, which can be useful for those looking to lower the unsprung weight of their vehicles.

 

i thought you wanted 20?