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#1 |
Registered User
Join Date: Aug 2004
Location: Idaho
Posts: 640
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Help fixing my speedometer
My speedometer reads high, about 5 miles too fast when at freeway speeds but not so drastic at slower speeds. I installed a 700r4 over the summer, and if I remember correctly I installed a red speedo gear set. I'm not sure what needs to be changed, but I could sure use some help. Is there a way to know just what gears I need, instead of ordering them one at a time and checking accuracy of the guage with a GPS? My tires are 265/75/16 with a 3.73 rear end... if anyone knows a formula for this.
I was kind of excited thinking the good gas milage I was getting, but then it hit me the other day that my odometer is also reading too high... ![]() |
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#2 |
Registered User
Join Date: May 2003
Location: Shreveport LA
Posts: 3,170
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Re: Help fixing my speedometer
Count the teeth on your driven gear (the one hooked to the cable), and figure you'll need a new one with one or two more teeth than what you have.
If it's reading 75 when you're going 70, multiply the tooth count by 75/70, and if it comes out a whole number (or pretty close), get a gear with that many teeth. If it's not too close to a whole number, I'd prefer one that's rounded to the smaller number, because it will read a little fast, rather than a little slow, which might get my lead-footed arse in trouble. If you want to get more exact, http://tciauto.com/tech_info/speedo_gears.htm might help. If not, at least you can calculate your tire diameter there & take it to http://novaresource.org/speedo.htm and plug it into the formula to find the speedometer gear ratio you need. Take that to the ratio chart & see what combo of gears will get you as close as possible. TCI sells gears, and they also have a chart with GM part numbers if you want to buy locally. |
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#3 |
Msgt USAF Ret
![]() ![]() Join Date: Jan 2005
Location: Kalamazoo, Michigan
Posts: 8,714
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Re: Help fixing my speedometer
Some more info here. Click on speedometer gears if the left menu and surf down to the 700r4
http://www.tciauto.com/tech_info/speedo_gears.htm
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VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative. Last edited by VetteVet; 01-17-2006 at 10:53 PM. |
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#4 |
Senior Member
![]() ![]() Join Date: Jan 2005
Location: CA
Posts: 5,527
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Re: Help fixing my speedometer
Your tire diameter should be around 31.65 inches. The TCI website says that with a red driving gear (17 teeth), you'd need a 40.378 tooth driven gear.
Black driven gear (40 teeth) = GM part number 1362048 (TCI part number 880028) Here's another good link: http://www.73-87.com/7387garage/drivetrain/speedo.htm |
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