Tire Diameter!
Thread Starter
Team Owner
Joined: Jul 2001
Posts: 182,997
Likes: 88
From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
Tire Diameter!
Does anyone know what the diameter of the original F70-15 tires on 68 Corvettes was?
I would like to know the diameter because the speedo gears in the transmission are probably
calibrated for the original tire size. Therefore - when using another tire diameter -
your speedo will be slightly off. I would like to calculate exactly by how much the speedo
would be off on my Vette. The diameter of my 255/60R15 tires is 27.1 inches!
Thanks
Olivier
I would like to know the diameter because the speedo gears in the transmission are probably
calibrated for the original tire size. Therefore - when using another tire diameter -
your speedo will be slightly off. I would like to calculate exactly by how much the speedo
would be off on my Vette. The diameter of my 255/60R15 tires is 27.1 inches!
Thanks
Olivier
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Joined: Jun 2001
Posts: 4,004
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From: Severna Park Md
Re: Tire Diameter! (GrandSportC3)
You can also check speedo error by driving between mile markers on the highway. Go ten miles and record the odometer at the begining and end. Do the math and get the results. If it reads 9miles then yur 10% off to slow...ect..
Remember with speedo errors, the margine of error get wider the faster you go. Its still 0 at 0. Are you confused yet?? :lol:
Remember with speedo errors, the margine of error get wider the faster you go. Its still 0 at 0. Are you confused yet?? :lol:
Burning Brakes
Joined: Oct 2001
Posts: 1,185
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From: New Wilmington PA
Re: Tire Diameter! (GrandSportC3)
According to Richard Newton's "How to Restore and Modify your Corvette 1968-82." All 'Vettes prior to '78 came with a tire called GR70 that was 27.1" tall, a specification matched by your current setup.
Drifting
Joined: Sep 2000
Posts: 1,355
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From: Germantown Maryland
Re: Tire Diameter! (Cookwithvette)
According to Richard Newton's "How to Restore and Modify your Corvette 1968-82." All 'Vettes prior to '78 came with a tire called GR70 that was 27.1" tall, a specification matched by your current setup.
:flag
Pro
Joined: Jan 2002
Posts: 534
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From: FL
Re: Tire Diameter! (GrandSportC3)
I would like to calculate exactly by how much the speedo
would be off on my Vette.
would be off on my Vette.
Figure your actual miles per hour by dividing the seconds it takes to go one mile into 3600.
Put another way: 3600 / seconds-per-mile = miles per hour.
If it's close to 60 your speedo is good to go. Going one tooth higher or lower in the speedo gear changes the calibration by about 5%.
-Noel
Thread Starter
Team Owner
Joined: Jul 2001
Posts: 182,997
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From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
Re: Tire Diameter! (Jvette73)
>>You can also check speedo error by driving between mile markers on the
>>highway. Go ten miles and record the odometer at the begining and end.
>>Do the math and get the results. If it reads 9miles then yur 10% off to
>>slow...ect..
>>Remember with speedo errors, the margine of error get wider the faster
>>you go. Its still 0 at 0. Are you confused yet??
I tried to check my speedo already between milemarkers. approx. 10.15 miles on my odometer = 10 1-mile markers, which would mean that my speedo is 1.5% off!
What I am trying to find out with this thread is if those 1.5% result from a 1.5% lower diameter of my current tires or if the odometer is responsible for the inaccuracy (or maybe the gears in the tranny)
Olivier
>>highway. Go ten miles and record the odometer at the begining and end.
>>Do the math and get the results. If it reads 9miles then yur 10% off to
>>slow...ect..
>>Remember with speedo errors, the margine of error get wider the faster
>>you go. Its still 0 at 0. Are you confused yet??
I tried to check my speedo already between milemarkers. approx. 10.15 miles on my odometer = 10 1-mile markers, which would mean that my speedo is 1.5% off!
What I am trying to find out with this thread is if those 1.5% result from a 1.5% lower diameter of my current tires or if the odometer is responsible for the inaccuracy (or maybe the gears in the tranny)
Olivier
Thread Starter
Team Owner
Joined: Jul 2001
Posts: 182,997
Likes: 88
From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
Re: Tire Diameter! (Cookwithvette)
>>According to Richard Newton's "How to Restore and Modify your Corvette
>>1968-82." All 'Vettes prior to '78 came with a tire called GR70 that was
>>27.1" tall, a specification matched by your current setup.
Thank you for the information!!
Olivier
P.S. still wondering about my 1.5% difference (see prevous reply)
>>1968-82." All 'Vettes prior to '78 came with a tire called GR70 that was
>>27.1" tall, a specification matched by your current setup.
Thank you for the information!!
Olivier
P.S. still wondering about my 1.5% difference (see prevous reply)
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Pro
Joined: Jan 2002
Posts: 534
Likes: 0
From: FL
Re: Tire Diameter! (GrandSportC3)
Just guessing, but 1.5% may be well within the design tolerance. Like I said, the speedo gears give about a 5% jump for each size.
I think most folks would be happy to be within 1.5%. My speedo reads about 15% slow, something I plan to do something about one of these days, after I get my new wheels and tires.
-Noel
I think most folks would be happy to be within 1.5%. My speedo reads about 15% slow, something I plan to do something about one of these days, after I get my new wheels and tires.
-Noel
Thread Starter
Team Owner
Joined: Jul 2001
Posts: 182,997
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From: Lakeland, FL
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St. Jude Donor '03, '06, '17
Re: Tire Diameter! (Noel Carboni)
>>Just guessing, but 1.5% may be well within the design tolerance. Like I
>>said, the speedo gears give about a 5% jump for each size.
I think I can't be unsatisfied with my speedo being only 1.5% off....
:)
Olivier
[Modified by GrandSportC3, 8:29 PM 3/13/2002]
>>said, the speedo gears give about a 5% jump for each size.
I think I can't be unsatisfied with my speedo being only 1.5% off....
:)
Olivier
[Modified by GrandSportC3, 8:29 PM 3/13/2002]
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Joined: Jun 2001
Posts: 4,004
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From: Severna Park Md
Re: Tire Diameter! (GrandSportC3)
1.5% error is very close to right on. Thats only .9mph off at 60mph and 1.5mph off at 100mph. Thats so close that even low tire pressure could account for the error.
Thread Starter
Team Owner
Joined: Jul 2001
Posts: 182,997
Likes: 88
From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
Re: Tire Diameter! (Jvette73)
>>1.5% error is very close to right on. Thats only .9mph off at 60mph and
>>1.5mph off at 100mph. Thats so close that even low tire pressure could
>>account for the error.
Replacing the stock speedometer with Autometer Phantom Speedometer made a gret difference in accuracy! The stock speedo was off at least 5 - 6 %!!!
Olivier
>>1.5mph off at 100mph. Thats so close that even low tire pressure could
>>account for the error.
Replacing the stock speedometer with Autometer Phantom Speedometer made a gret difference in accuracy! The stock speedo was off at least 5 - 6 %!!!
Olivier