Help fixing my speedometer - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Jakes-66-K10 · 01-17-2006, 05:48 PM

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...

jimfulco · 01-17-2006, 10:44 PM

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.

**VetteVet** · 01-17-2006, 10:52 PM

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

**pjmoreland** · 01-18-2006, 02:16 AM

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

01-17-2006, 05:48 PM	#1
Jakes-66-K10 Registered User Join Date: Aug 2004 Location: Idaho Posts: 640	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...

01-17-2006, 10:44 PM	#2
jimfulco Registered User Join Date: May 2003 Location: Shreveport LA Posts: 3,170	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.

01-17-2006, 10:52 PM	#3
VetteVet Msgt USAF Ret Join Date: Jan 2005 Location: Kalamazoo, Michigan Posts: 8,714	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 __________________ 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.

01-18-2006, 02:16 AM	#4
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,527	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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