For the mathematicians

[Funracer]

I have 4.10s and never corrected the speedo. If I go to 18 or 19"
rear wheels that would make my speedo nearly correct again, right?

[efnfast]

No, because the diameter of the tires will be about the same, you don't ride on the rims.

[XfireZ51]

Originally Posted by efnfast

No, because the diameter of the tires will be about the same, you don't ride on the rims.

Steve,

The 19" tires are about 1+" taller than stock 27 v 25.7". I have 3.73s and am using the stock speedo gears in the trans. My speedo error is ~3mph at 70 w the speedo reading lower than the GPS. 4.10s may put you right there w the taller tires.

[scottfab]

Originally Posted by efnfast

No, because the diameter of the tires will be about the same, you don't ride on the rims.

Correct. Diameter of tires is the determining factor.
However, while we don't ride on the rims normally
sometimes when you take a $@(($ corner wrong
you DO ride on the #@#( rim.
$#)$( it.
ugh!

[efnfast]

Originally Posted by scottfab

Correct. Diameter of tires is the determining factor.
However, while we don't ride on the rims normally
sometimes when you take a $@(($ corner wrong
you DO ride on the #@#( rim.
$#)$( it.
ugh!

Well, that sounds like you're speaking from experience

[scottfab]

Originally Posted by efnfast

Well, that sounds like you're speaking from experience

Yup and is why I now have two sets of wheels.
Happen 16yrs ago when I first went from a C3 to a C4.
Something about turning radius
There's nothing like the sound or rim on top of curb to
get your attention.

[efnfast]

Ultimately it is all based on the rolling diameter of the tire regardless of rim size. Well, plus all the other ratios.

I'm also running 3.73's with stock tires and rims. I'm assuming I have speedo correction. Mine is off about 3 mph at 70. So when the speedo said 140, I like to say I was doin' a buck 45.

Dom, your 19's are 1" taller. By choice? Or is that the lowest profile available?

[-=Jeff=-]

with my 18" and a 4.10 I used the Speedo correction gears from Haibeck and my speedo seems to be within 1 mph or so (Have not checked it lately but easy enough to do)

[XfireZ51]

Steve,

Didn't choose a particularly taller tire. The Nittos I have are ones that a good number of Z06 owners use when replacing their RF tires. What rolling radius is and how it compares to stock size tires I am not certain. We just replaced the driven gear from a Purple 43 tooth to a Green 42 tooth. Probably 41 teeth would be closer although not sure whether one is available. At this point, it's close enough for me when it comes to daily use and highway cruise. When I first got the car, speedo was 10mph lower than actual.

[XfireZ51]

Steve,

Didn't choose a particularly taller tire. The Nittos I have are ones that a good number of Z06 owners use when replacing their RF tires. What rolling radius is and how it compares to stock size tires I am not certain. We just replaced the driven gear from a Purple 43 tooth to a Green 42 tooth. Probably 41 teeth would be closer although not sure whether one is available. At this point, it's close enough for me when it comes to daily use and highway cruise. When I first got the car, speedo was 10mph lower than actual.

BTW, here's a good resource for this calc. We used it to determine if my green gear would work.

http://www.novaresource.org/speedo.htm