For the mathematicians - ZR-1 Net Registry Forums

Funracer · 05-07-2013

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 · 05-07-2013

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

XfireZ51 · 05-07-2013

Quote:

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.

efnfast · 05-07-2013

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=- · 05-07-2013

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 · 05-07-2013

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 · 05-07-2013

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

XfireZ51 · 05-07-2013

Looking at the Nova Resource site , my calc is

(15 tooth drive gear x 20.2)/ 27"= 2.79

Chart show use of 42 tooth green gear.

scottfab · 05-07-2013

Quote:

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!

-=Jeff=- · 05-07-2013

Dom,

Looks to be avail

GM PN 25513048

05-07-2013	#1
Funracer Join Date: Oct 2011 Location: Shalimar, FL Posts: 192	For the mathematicians 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?

05-07-2013	#2
efnfast Join Date: Nov 2011 Location: Dunbarton NH Posts: 7,540	Re: For the mathematicians No, because the diameter of the tires will be about the same, you don't ride on the rims.

05-07-2013	#4
efnfast Join Date: Nov 2011 Location: Dunbarton NH Posts: 7,540	Re: For the mathematicians 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?

05-07-2013	#5
-=Jeff=- Join Date: Jul 2007 Location: Bartlett, IL Posts: 7,156	Re: For the mathematicians 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) __________________ 1990 Corvette ZR-1 #1051 Watson Headers (2" Primary) - Flowmaster Cats - Borla Catback Late Model IH - Plenum Coilovers - 4.10s Shelby Series 1 - Wilwood Brakes Custom Interior NCM Lifetime Member #978

05-07-2013	#6
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,683	Re: For the mathematicians 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.

05-07-2013	#8
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,683	Re: For the mathematicians Looking at the Nova Resource site , my calc is (15 tooth drive gear x 20.2)/ 27"= 2.79 Chart show use of 42 tooth green gear.

05-07-2013	#10
-=Jeff=- Join Date: Jul 2007 Location: Bartlett, IL Posts: 7,156	Re: For the mathematicians Dom, Looks to be avail GM PN 25513048 __________________ 1990 Corvette ZR-1 #1051 Watson Headers (2" Primary) - Flowmaster Cats - Borla Catback Late Model IH - Plenum Coilovers - 4.10s Shelby Series 1 - Wilwood Brakes Custom Interior NCM Lifetime Member #978