Can someone help. We went for nice drive today in my 1967 Red Convertible. Also met a nice police officer. We went throught his speed trap. When I went through I thought it would be a perfect time to verify my speedometer since I just replaced my tires with new 215/70/15. I pull over next to him and asked him how fast I was going. Accoring to the officer I was going 57 mph. (Problem was it was a 40 mph zone). My speedometer was at least 10 mph less. How can I get it checked and corrected? No ticket but he was a little peeved.
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Re: Speedometer Correction
Those tires shouldnt throw it off at all. The rolling dia. is almost the same as the originals. -
Re: Speedometer Correction
Those tires shouldnt throw it off at all. The rolling dia. is almost the same as the originals.Comment
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Re: Speedometer Correction
The 215/70R-15 has exactly the same revs per mile - 775 - as the OEM 7.75-15 bias ply tires, so the 215/70s will not throw off your speedometer calibration. Suggest you verify the axle ratio by counting turns, and remove the speedometer driven gear to count teeth and note the color. You could also check the speedometer against a late model car to see if it's reasonabley accurate.
Several on this board can tell you if you have the correct driven gear for your installed axle ratio.
DukeComment
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Re: Speedometer Correction
The 215/70R-15 has exactly the same revs per mile - 775 - as the OEM 7.75-15 bias ply tires, so the 215/70s will not throw off your speedometer calibration. Suggest you verify the axle ratio by counting turns, and remove the speedometer driven gear to count teeth and note the color. You could also check the speedometer against a late model car to see if it's reasonabley accurate.
Several on this board can tell you if you have the correct driven gear for your installed axle ratio.
DukeComment
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Re: Speedometer Correction
As a base reference, an original car with a 3:70 rear axle ratio should have a 22 tooth speedometer driven gear. This gear is usually green in color. As the rear axle ratios get higher, the amount of teeth on the driven gear get larger to keep the same accuracy in the speedometer.
I recently switched rear axle ratios and had to change the driven gear. It really is quite easy and basically, after removing a retaining screw and clip, just unplugs from the right side of the transmission.
As a general rule of thumb, if your speedometer is inaccurate, a driven gear with one more tooth will slow it down aproximately 5 mph. Most are still available through Chevy dealers or the Corvette aftermarket companies.Comment
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Re: Speedometer Correction
As a base reference, an original car with a 3:70 rear axle ratio should have a 22 tooth speedometer driven gear. This gear is usually green in color. As the rear axle ratios get higher, the amount of teeth on the driven gear get larger to keep the same accuracy in the speedometer.
I recently switched rear axle ratios and had to change the driven gear. It really is quite easy and basically, after removing a retaining screw and clip, just unplugs from the right side of the transmission.
As a general rule of thumb, if your speedometer is inaccurate, a driven gear with one more tooth will slow it down aproximately 5 mph. Most are still available through Chevy dealers or the Corvette aftermarket companies.Comment
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Re: Speedometer Correction
Thanks. I'll try driving along side my wifes car and verify the speed. Then I check which speedo gear I have. Then it's off to the Chevy dealer for a new speedo gear. If I understand correctly. If the speedometer is reading low by 10 mph I should get a speedo driven gear with two(2) less teeth.Comment
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Re: Speedometer Correction
Thanks. I'll try driving along side my wifes car and verify the speed. Then I check which speedo gear I have. Then it's off to the Chevy dealer for a new speedo gear. If I understand correctly. If the speedometer is reading low by 10 mph I should get a speedo driven gear with two(2) less teeth.Comment
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Re: Speedometer Correction
Ralph --- Try comparison with your wife's car at several speeds; sounds to me like your speedo is out of calibration; 47 mph shown versus 57 mph actual is about 20% off. If you find a 22-tooth (grey plastic, part # 3987922, thx, Joe L.) gear installed, you would need an 18 tooth brown gear to correct that discrepancy. Or, if you have a high numerical ratio differential, and find that you currently have the Orange 25-tooth plastic gear, you could install the Yellow 22-tooth one, but this would only correct 3/4 of your error. The correction rule is by PERCENTAGE or ratio of the old-to-new gear teeth count, which at speeds close to 100 mph works out to 5 mph per tooth change.Comment
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Re: Speedometer Correction
Ralph --- Try comparison with your wife's car at several speeds; sounds to me like your speedo is out of calibration; 47 mph shown versus 57 mph actual is about 20% off. If you find a 22-tooth (grey plastic, part # 3987922, thx, Joe L.) gear installed, you would need an 18 tooth brown gear to correct that discrepancy. Or, if you have a high numerical ratio differential, and find that you currently have the Orange 25-tooth plastic gear, you could install the Yellow 22-tooth one, but this would only correct 3/4 of your error. The correction rule is by PERCENTAGE or ratio of the old-to-new gear teeth count, which at speeds close to 100 mph works out to 5 mph per tooth change.Comment
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Re: Speedometer Correction
When I check the speedo driven gear is their any chance of it falling back into the transmission and causing more problems. Just read a similar post ref. a C1.Comment
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Re: Speedometer Correction
When I check the speedo driven gear is their any chance of it falling back into the transmission and causing more problems. Just read a similar post ref. a C1.Comment
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Re: Speedometer Correction
Wayne is correct. It's the PERCENT error that counts. Pick a reference speed that is convenient, say 60 MPH. Calculate the percent error. If it reads 48 MPH at a true 60, the error is 12/60 or 20 percent. You want the speedometer to go twenty percent faster, so you want a gear with 20 percent FEWER teeth. If the installed driven gear has 24 teeth, a 20 tooth gear would be closest to twenty percent fewer teeth.
Corvette axles with the exception of 3.55 have about a ten percent spread from each other which is why the common driven gears are 18, 20, 22, and 24 teeth for 3.08, 3,36, 3.70, and 4.11 axles respectively. The 3.55 uses a 21 tooth driven gear. All these axle ratios incorporate an 8 tooth drive gear in the geabox
The only oddball is the 4.56 axle. Corvette's that were built with the 4.56 option have a six tooth drive gear in the gearbox and use a 20 tooth driven gear.
DukeComment
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Re: Speedometer Correction
Wayne is correct. It's the PERCENT error that counts. Pick a reference speed that is convenient, say 60 MPH. Calculate the percent error. If it reads 48 MPH at a true 60, the error is 12/60 or 20 percent. You want the speedometer to go twenty percent faster, so you want a gear with 20 percent FEWER teeth. If the installed driven gear has 24 teeth, a 20 tooth gear would be closest to twenty percent fewer teeth.
Corvette axles with the exception of 3.55 have about a ten percent spread from each other which is why the common driven gears are 18, 20, 22, and 24 teeth for 3.08, 3,36, 3.70, and 4.11 axles respectively. The 3.55 uses a 21 tooth driven gear. All these axle ratios incorporate an 8 tooth drive gear in the geabox
The only oddball is the 4.56 axle. Corvette's that were built with the 4.56 option have a six tooth drive gear in the gearbox and use a 20 tooth driven gear.
DukeComment
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