speedometer and tire size dumb question
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
speedometer and tire size dumb question
I have 215/65/R15 on right now which calculates to 26 inches.
I want to put 255/60/R15 on but that calcs out to 27 inches tall.
How do you make sure speedometer is still correct?
What was the stock height?
I want to put 255/60/R15 on but that calcs out to 27 inches tall.
How do you make sure speedometer is still correct?
What was the stock height?
Advanced
Joined: Jul 2004
Posts: 51
Likes: 2
From: Greensboro NC
Not sure what the stock tire size was for your year (but probably wasn't a 'P-metric' size, so you'll have to find the equivalent P-metric)
Using a speedometer gear calulator like:
http://www.700r4.com/speedoCalc/speedocalc.shtml
you could figure out the speedo gear ratios (drive:driven) for the tires you have now, versus the 'stock' size (or else take the speedo cable out of the trans and compare the tooth count to what your new tire sizes require.
If you have a GPS unit or can borrow one, that's a quick way to check the accuracy (or inaccuracy) of your speedo.
//Pancho
Using a speedometer gear calulator like:
http://www.700r4.com/speedoCalc/speedocalc.shtml
you could figure out the speedo gear ratios (drive:driven) for the tires you have now, versus the 'stock' size (or else take the speedo cable out of the trans and compare the tooth count to what your new tire sizes require.
If you have a GPS unit or can borrow one, that's a quick way to check the accuracy (or inaccuracy) of your speedo.
//Pancho
Le Mans Master
Joined: Aug 2003
Posts: 7,720
Likes: 12
From: Chattanooga TN
without a GPS, which would be best, get on the interstate. Drive at steady 60 mph indicated and go 1 mile (between markers). Determine how much time elapsed. Do this a few times and get an average. Say it was 57 sec (you are actually going 63 mph). In that case you would be off 5% slow (60 sec-57 sec=3 sec, 3 sec/60 sec=5%). If you had a 23 tooth driven gear in the tranny, go to a 22 tooth (23x.95=21.85). If the speedo is reading fast, increase teeth. If the speedo is reading slow, decrease teeth. You can also follow the odometer over 1 mile, or 10 would be better, and see how far you are off. Figure the percent off and change driven gears in the tranny. Some combinations are not available, so be sure to ask the GM parts guy when you go to get the new gear.
Melting Slicks
Joined: Apr 1999
Posts: 2,010
Likes: 3
From: Moon Twp. PA USA
Stock diameter was +/- 27.4". I also have 26"dia. tires, same size as you. Had to change stock spedo gear. Used GPS to verify, as my neighbor is a tech junky and has one.
Plan to chang to stock diameter tires this year so stock speedo gear will have to go back in.
Plan to chang to stock diameter tires this year so stock speedo gear will have to go back in.
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
So if I understand correctly, Glenn242, I have a stock speedo so by going to 27" I should be OK which means I've probably been reading an incorrect speedo for a while now. Guess I need to still find out what color my plastic gear is in my M20.
Melting Slicks
Joined: Apr 1999
Posts: 2,010
Likes: 3
From: Moon Twp. PA USA
Originally Posted by Surfer69
So if I understand correctly, Glenn242, I have a stock speedo so by going to 27" I should be OK which means I've probably been reading an incorrect speedo for a while now. Guess I need to still find out what color my plastic gear is in my M20.
