young

i was reading merc enthusiast and there is an article that mentions that the cars speedometer is tied to the size of the weels. and although they do not specify the exact increase in size, they say that larger wheels can affect the speedometer by upto <b>10%</b>. i think that is quite a bit. but are they talking about going from 16" wheels to 19" wheels? or 16" to 17"? (i'd assume the former more).

i also assume that when the c7 package was added, the speedometer is recalibrated to the 17" wheels that come w/ it rather than the standard 16" wheels.

on a sort of related topic; urged on by bigbaddad's speedometer pic of 135mph, i thought i'd try to push the car a little on route 1. after i got to +100mph, and really zipping by other cars, i reconsidered trying for 135mph. later on, i realized i had my snow tires on also...

tommy

Correct plus sizing should limit the amount of speedo error. Now, if there are people who are deviating from a proper plus size, then you will get some error, but will get a convenient excuse for the fuzz as a byproduct.

Flashman

Changing the total wheel & tire size WILL mess up your speedometer.

However, increasing your wheel size can be offset by decreasing the tire thickness. This is what happens with the C7 wheel package and most wheel upgrades. The tire itself gets thinner. The total wheel & tire size remains the same. Neat trick that.

Bos

Here's a screen shot of a spreadsheet I worked out a while ago for the various popular tire plus sizes for the Coupe. You can see that as you go up in wheel size the overall height of the tire changes, but it's still a minimal percentage change from stock. Therefore, the speedometer reading difference would be negligible.

avlis

Here is one of the best tire size calculators I've used. Download a zipped version from here:

http://pages.prodigy.net/bronco77/misc/tiresz.zip

It allows you to compare 2 different tire/wheel on one screen and shows you the speedo error between the two. Try it, you'll like it!

mdp c230k

It's the circumferance of the total wheel/tire combo that matters as far as the speedo is concerned. As long as the circumferances are the same between different setups there will be no speedo error. Always shoot for the same number or you can run into gear ratio problems. Larger rolling diameters need more torque to turn, putting pressure on the drivetrain. You'll get a higher top speed but slower off the line. Smaller gets you faster but lower top speed. Also, the shorter the sidewall the less give, causing the ride to be harsh and jumpy over bumps. Yeah, it'll handle great on smooth surfaces but it is hell on old twisty roads with bad surfaces. IMHO stick to 17" for the best of both worlds.

Circ=((rim size/2 x 25.4) + (aspec/100 x width))^2 x 3.14

BrabusCClass

Does it really make a big difference? I went up to 110 with my friend and every 5 or 10 miles I asked him what his speedo said and it was very accurate. I did not notice any difference before and after my rims were changed. I went from 16" to 18" but my tire size stayed 225. 225 50 16 to 225 40 18. Maybe thats why. Hmm.

Rick

The important factor that plays into this is wheel circumfrance. True "Plus" size calculations take this consideration by reducing aspect ratio (sidewall height) in proportion to the wheel size and width. A 205/55-16 is the same rolling circumfrance as a 225/50-16(+1)...and so on.

Tire Rack has an excellent calculator....as does BFGoodrich and Michelin. Check out their websites...and happy motoring!