Speedometer off by 10mph - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tjvending · 02-09-2009, 02:35 PM

What do I need to do to get my speedometer to read correctly? Right now its off about 10MPH over. Is there a certain color of gear to make this correction? I'm not sure of the gear size in the rear, Its a stock 350 with 350trans.

Andy4639 · 02-09-2009, 03:15 PM

You need to know the rear gear and the gear on the tail shaft of the transmission and the speedo gear on the side of transmission. This will help even though it's directed at 700-R4 it's basicly the same.
http://67-72chevytrucks.com/vboard/s...d.php?t=322481

If you can find a transmission place that can calibrate it for you as I did.

Wrenchbender Ret · 02-09-2009, 03:24 PM

Remove the driven gear from the holder behind the speedo cable. The number of teeth is writen on it or count them. Get a gear with about 2 more teeth on it & put it in. Then if it seems close compare the odometer with the mile markers on the freeway. If you are not near a freeway mark off a mile with a car that you know is right. Like the first telophone pole to the tree in front of the Joneses. If the 10ths of mile dont match change the gear till you get it real close.

Longhorn Man · 02-09-2009, 04:45 PM

10 MPH at what speed?
50MPH is a good speed to get your numbers off of. My burb reads 5MPH slow at 50 MPH, so i know it is 10% off. When i carry the tranny to the rebuild guy, I'll tell him to speed it up by 10% and it'll be done.

68gmsee · 02-09-2009, 04:49 PM

I had a similar problem with my 3 speed. I would be going around 55 but showing about 65 on the speedometer.

Depending on what you mean by "10mph over" you can also go to the dealer and get a different "toothed" driven gear. That's what I did and it worked pretty close. The gears are not that expensive.

You can do a search on speedometer gears on this site. There's been a lot of posts regarding this same problem.

Try this site to give you some info on the type of gears you might need. I'm sure that if you were to call them they could fix you up.

http://www.transmissioncenter.org/GM...eter_Gears.htm

69GMCLonghorn · 02-09-2009, 08:11 PM

I ran into a problem with mine that may also be something to look at. These guys are right; find out what your off by at 30 and 60 MPH. The reason for this is you want to see if it's the same PERCENT off at both speeds. If it's off by the same MPH, not percent, then you need to have the needle properly reset to zero. There is a speedo shop out here that does it, but I just opted for a new speedo since I had it out.

jimfulco · 02-10-2009, 03:00 AM

http://novaresource.org/speedo.htm has a formula that will get you where you need to be. You'll need your tire diameter and rear axle ratio to plug into the formula to get your required speedo gear ratio, and there's a chart that shows what gears you need to get as close as you can to that ratio. Gears are available online or at a Chevy dealer, maybe at a trans shop, too.

tjvending · 02-11-2009, 08:17 PM

I work on my Speedometer today, I pulled out the gear and counted the number of teeth, it has 19. So I got the count and put it back in. I took it for a test ride to make sure that the speedometer was working and not leaking any fluid. When I started off I notice the speedometer was working normal, so I took it down the local 2way HW and its seems to be working fine now. Go figure, I guess who ever had it out last time didn't put in back in correctly or something. Any how it now works. Thanks for all the input, another problem solved. TJ

02-09-2009, 02:35 PM	#1
tjvending Registered User Join Date: Oct 2007 Location: rockville, md Posts: 257	Speedometer off by 10mph What do I need to do to get my speedometer to read correctly? Right now its off about 10MPH over. Is there a certain color of gear to make this correction? I'm not sure of the gear size in the rear, Its a stock 350 with 350trans. Last edited by tjvending; 02-09-2009 at 02:36 PM.

02-09-2009, 03:15 PM	#2
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,945	Re: Speedometer off by 10mph You need to know the rear gear and the gear on the tail shaft of the transmission and the speedo gear on the side of transmission. This will help even though it's directed at 700-R4 it's basicly the same. http://67-72chevytrucks.com/vboard/s...d.php?t=322481 If you can find a transmission place that can calibrate it for you as I did. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

02-09-2009, 03:24 PM	#3
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,228	Re: Speedometer off by 10mph Remove the driven gear from the holder behind the speedo cable. The number of teeth is writen on it or count them. Get a gear with about 2 more teeth on it & put it in. Then if it seems close compare the odometer with the mile markers on the freeway. If you are not near a freeway mark off a mile with a car that you know is right. Like the first telophone pole to the tree in front of the Joneses. If the 10ths of mile dont match change the gear till you get it real close.

02-09-2009, 04:45 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Speedometer off by 10mph 10 MPH at what speed? 50MPH is a good speed to get your numbers off of. My burb reads 5MPH slow at 50 MPH, so i know it is 10% off. When i carry the tranny to the rebuild guy, I'll tell him to speed it up by 10% and it'll be done.

02-09-2009, 04:49 PM	#5
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Speedometer off by 10mph I had a similar problem with my 3 speed. I would be going around 55 but showing about 65 on the speedometer. Depending on what you mean by "10mph over" you can also go to the dealer and get a different "toothed" driven gear. That's what I did and it worked pretty close. The gears are not that expensive. You can do a search on speedometer gears on this site. There's been a lot of posts regarding this same problem. Try this site to give you some info on the type of gears you might need. I'm sure that if you were to call them they could fix you up. http://www.transmissioncenter.org/GM...eter_Gears.htm

02-09-2009, 08:11 PM	#6
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: Speedometer off by 10mph I ran into a problem with mine that may also be something to look at. These guys are right; find out what your off by at 30 and 60 MPH. The reason for this is you want to see if it's the same PERCENT off at both speeds. If it's off by the same MPH, not percent, then you need to have the needle properly reset to zero. There is a speedo shop out here that does it, but I just opted for a new speedo since I had it out.

02-10-2009, 03:00 AM	#7
jimfulco Registered User Join Date: May 2003 Location: Shreveport LA Posts: 3,170	Re: Speedometer off by 10mph http://novaresource.org/speedo.htm has a formula that will get you where you need to be. You'll need your tire diameter and rear axle ratio to plug into the formula to get your required speedo gear ratio, and there's a chart that shows what gears you need to get as close as you can to that ratio. Gears are available online or at a Chevy dealer, maybe at a trans shop, too.

02-11-2009, 08:17 PM	#8
tjvending Registered User Join Date: Oct 2007 Location: rockville, md Posts: 257	Re: Speedometer off by 10mph I work on my Speedometer today, I pulled out the gear and counted the number of teeth, it has 19. So I got the count and put it back in. I took it for a test ride to make sure that the speedometer was working and not leaking any fluid. When I started off I notice the speedometer was working normal, so I took it down the local 2way HW and its seems to be working fine now. Go figure, I guess who ever had it out last time didn't put in back in correctly or something. Any how it now works. Thanks for all the input, another problem solved. TJ



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