Help! Trying to calibrate my speedometer. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Botietruck · 05-11-2009, 09:32 PM

I need some input here. In an earlier thread I made mention that my speedometer was off about 10 mph. The cruise on my wife's van was set at 60 mph and my truck was running 50-52 mph. I pulled the speedo gear out of the trans. tonight and it is a 24 tooth gear. I'm thinking I should get a gear with more teeth. The parts book lists the next higher number as a 25 tooth gear, and the next one above that gear has 34 teeth. Am I thinking correctly in that I will need a gear with more than 24 teeth? I believe the 34 tooth gear would be too much. Any thoughts??

Jim_PA · 05-11-2009, 09:45 PM

http://www.tciauto.com/Products/Tech...eedo_gears.asp

If you are lucky, you won't need to change the gear inside the tailshaft

Brad · 05-12-2009, 01:08 PM

Typically 1 tooth increase equates to about 3mph. Therefore you'd need a 27 tooth gear by my calculations and since you say your only option is the 25 tooth gear, it sounds like you're going to need an adapter or change out the drive gear in the tranny.

Which style gear do you need? I may have something that works.

69GMCLonghorn · 05-12-2009, 03:10 PM

Just a couple things to add to the chaos...

Make sure to check your numbers at 2 or 3 different speeds before making the change. You should be off by the same percentage at different speeds. I have a situation with mine where it's reading slow at 40 and fast at 70... I still don't know what's up. Just make sure before you do it.

Also, some speedos were not "zero'd" properly, meaning you might be off by 6 MPH at 30 and at 60, instead of a consistent percentage. The speedo needle can somehow be re-zeroed to correct for this. Don't ask me how.

Botietruck · 05-12-2009, 09:20 PM

Thanks guys for the info.
Brad, I'll get a pic of mine tomorrow and post it.

rlb67 · 05-12-2009, 10:24 PM

I believe that you would want to go with a smaller tooth driven gear to make the speedometer cable spin faster.

**DANTIP** · 05-13-2009, 06:38 AM

Rather than base your adjustment on another vehicles speedo, you should drive your truck on a highway that has mile markers. Keep your speed constant and time how many seconds it takes to go one mile. Then divide 3600 by the number of seconds it takes to travel 1 mile and that is your true speed.

Example: If you maintain your truck speedo at 55 and it takes you 61 seconds to go 1 mile your true speed is 59 mph (3600/61). You can then divide 59/55 to determine the % your speedo is off (59/55 = 1.072 or 7.2% off). Do this at different speeds and use this info to determine the best speedo gear.

**LockDoc** · 05-13-2009, 07:22 AM

-
Everything you ever wanted to know about speedometer gears....

http://www.transmissioncenter.net/sp...n_______va.htm

LockDoc

mrein3 · 05-13-2009, 07:43 AM

Quote:

Originally Posted by DANTIP

Rather than base your adjustment on another vehicles speedo, you should drive your truck on a highway that has mile markers. Keep your speed constant and time how many seconds it takes to go one mile. Then divide 3600 by the number of seconds it takes to travel 1 mile and that is your true speed.

Example: If you maintain your truck speedo at 55 and it takes you 61 seconds to go 1 mile your true speed is 59 mph (3600/61). You can then divide 59/55 to determine the % your speedo is off (59/55 = 1.072 or 7.2% off). Do this at different speeds and use this info to determine the best speedo gear.

Good tip.
Even easier is to beg/borrow/steal a hand-held GPS unit. Relatively speaking a GPS unit is about as accurate as you can get. Have a passenger along and write down speedo vs. GPS ground speed. If the number is consistantly off by the same percentage, the speedo gears in the transmission are the problem. If that is the case the tci link or many others will help you pick the correct gears to purchase.

Botietruck · 05-13-2009, 06:03 PM

rlb67, that makes sense. I believe you are on to something there. I'll give it a shot. The gear that came out of my non overdrive trans., which is the one that was in the truck originally, has a 22 tooth gear. I'll try that and see how it does.
Again guys, thanks for the tips/ideas. That's what makes this board so good.

That reminds me...I believe my memebership dues are... "due" .

rambler · 05-13-2009, 06:28 PM

agree on the GPS,, my daily driver is off, and its a 2004!

69RAT · 05-14-2009, 08:56 AM

Be sure your odometer is off also.... My odo is correct but speedo is off 5mph. One of these days I need to move the needle-to much of a pain!

68gmsee · 05-14-2009, 09:54 AM

Quote:

Originally Posted by Botietruck

rlb67, that makes sense. I believe you are on to something there. I'll give it a shot. The gear that came out of my non overdrive trans., which is the one that was in the truck originally, has a 22 tooth gear. I'll try that and see how it does.
Again guys, thanks for the tips/ideas. That's what makes this board so good.

Let us know what you find out on driven gear...
fewer teeth= higher speedometer reading
less teeth=higher speed speedometer reading

I've been trying to figure this one out and, I believe, asked the question in one of my previous posts but got no response.

68gmsee · 05-14-2009, 10:09 AM

I was fairly sure that's what I had done to my 68 to increase the speedometer reading but just wasn't sure.

I did find the following info from the following site:

Adding teeth on the DRIVEN gear will reduce the speedometer reading (cable runs slower). A one-tooth change results in about a 5% change in the reading. Likewise Subtracting teeth on the DRIVEN gear will increase speedometer readings by 5%.
http://www.stl-vettes.com/65Vette/GearCalc.htm

So, it seems rlb67 is right less teeth=higher reading....

05-11-2009, 09:32 PM	#1
Botietruck '71 chevy s.bed/s.side Join Date: Mar 2005 Location: Chesapeake, VA Posts: 1,571	Help! Trying to calibrate my speedometer. I need some input here. In an earlier thread I made mention that my speedometer was off about 10 mph. The cruise on my wife's van was set at 60 mph and my truck was running 50-52 mph. I pulled the speedo gear out of the trans. tonight and it is a 24 tooth gear. I'm thinking I should get a gear with more teeth. The parts book lists the next higher number as a 25 tooth gear, and the next one above that gear has 34 teeth. Am I thinking correctly in that I will need a gear with more than 24 teeth? I believe the 34 tooth gear would be too much. Any thoughts?? __________________ STILL PLAYS WITH CARS...AND TRUCKS!! Parents! Be good to your kids...they get to choose your nursing home! 250 I-6 with a 3 speed Saginaw and Borg Warner Overdrive...She's got overdrive!

05-11-2009, 09:45 PM	#2
Jim_PA Registered User Join Date: May 2005 Location: PA Posts: 2,696	Re: Help! Trying to calibrate my speedometer. http://www.tciauto.com/Products/Tech...eedo_gears.asp If you are lucky, you won't need to change the gear inside the tailshaft

05-12-2009, 01:08 PM	#3
Brad Out of the carpool lane. Join Date: Jan 2000 Location: Clark Co, WA Posts: 5,673	Re: Help! Trying to calibrate my speedometer. Typically 1 tooth increase equates to about 3mph. Therefore you'd need a 27 tooth gear by my calculations and since you say your only option is the 25 tooth gear, it sounds like you're going to need an adapter or change out the drive gear in the tranny. Which style gear do you need? I may have something that works. __________________ 1968 C-10 SWB, 5.7 Vortec/700R4/3.73 posi, Torch Red 1968 Camaro, 250/Powerglide, all original (No, I'm not gonna drop a 350 in it!...Jeez!) 2000 Honda VFR in the faster yellow! 2008 Husqvarna TE-610 1967 C-10 SWB 'Six Appeal'-Gone but not forgotten... Last edited by Brad; 05-12-2009 at 01:13 PM.

05-12-2009, 03:10 PM	#4
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: Help! Trying to calibrate my speedometer. Just a couple things to add to the chaos... Make sure to check your numbers at 2 or 3 different speeds before making the change. You should be off by the same percentage at different speeds. I have a situation with mine where it's reading slow at 40 and fast at 70... I still don't know what's up. Just make sure before you do it. Also, some speedos were not "zero'd" properly, meaning you might be off by 6 MPH at 30 and at 60, instead of a consistent percentage. The speedo needle can somehow be re-zeroed to correct for this. Don't ask me how.

05-12-2009, 09:20 PM	#5
Botietruck '71 chevy s.bed/s.side Join Date: Mar 2005 Location: Chesapeake, VA Posts: 1,571	Re: Help! Trying to calibrate my speedometer. Thanks guys for the info. Brad, I'll get a pic of mine tomorrow and post it. __________________ STILL PLAYS WITH CARS...AND TRUCKS!! Parents! Be good to your kids...they get to choose your nursing home! 250 I-6 with a 3 speed Saginaw and Borg Warner Overdrive...She's got overdrive!

05-12-2009, 10:24 PM	#6
rlb67 Registered User Join Date: Jan 2008 Location: Brighton, TN Posts: 42	Re: Help! Trying to calibrate my speedometer. I believe that you would want to go with a smaller tooth driven gear to make the speedometer cable spin faster. __________________ Randy, West TN 67 Fleetside SWB L6 3 speed on column 84 Feetside SWB L6 3 speed on column yes, I like column shifts 99 Dodge 4X4 Ext. Cab

05-13-2009, 06:38 AM	#7
DANTIP Registered User Join Date: Sep 2001 Location: Fort Wayne, Indiana, USA Posts: 1,882	Re: Help! Trying to calibrate my speedometer. Rather than base your adjustment on another vehicles speedo, you should drive your truck on a highway that has mile markers. Keep your speed constant and time how many seconds it takes to go one mile. Then divide 3600 by the number of seconds it takes to travel 1 mile and that is your true speed. Example: If you maintain your truck speedo at 55 and it takes you 61 seconds to go 1 mile your true speed is 59 mph (3600/61). You can then divide 59/55 to determine the % your speedo is off (59/55 = 1.072 or 7.2% off). Do this at different speeds and use this info to determine the best speedo gear.

05-13-2009, 07:22 AM	#8
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,416	Re: Help! Trying to calibrate my speedometer. - Everything you ever wanted to know about speedometer gears.... http://www.transmissioncenter.net/sp...n_______va.htm LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

05-13-2009, 06:03 PM	#10
Botietruck '71 chevy s.bed/s.side Join Date: Mar 2005 Location: Chesapeake, VA Posts: 1,571	Re: Help! Trying to calibrate my speedometer. rlb67, that makes sense. I believe you are on to something there. I'll give it a shot. The gear that came out of my non overdrive trans., which is the one that was in the truck originally, has a 22 tooth gear. I'll try that and see how it does. Again guys, thanks for the tips/ideas. That's what makes this board so good. That reminds me...I believe my memebership dues are... "due" . __________________ STILL PLAYS WITH CARS...AND TRUCKS!! Parents! Be good to your kids...they get to choose your nursing home! 250 I-6 with a 3 speed Saginaw and Borg Warner Overdrive...She's got overdrive!

05-13-2009, 06:28 PM	#11
rambler MOVE OVER Join Date: Jun 2002 Location: Warrensburg, MO Posts: 5,470	Re: Help! Trying to calibrate my speedometer. agree on the GPS,, my daily driver is off, and its a 2004! __________________ No matter where you are..... There you are... Buckaroo Banzi

05-14-2009, 08:56 AM	#12
69RAT Registered User Join Date: Apr 2006 Location: hopeless in Washington Posts: 105	Re: Help! Trying to calibrate my speedometer. Be sure your odometer is off also.... My odo is correct but speedo is off 5mph. One of these days I need to move the needle-to much of a pain! __________________ '69 Chevy CST#'s 396/TH400, A/C

05-14-2009, 10:09 AM	#14
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Help! Trying to calibrate my speedometer. I was fairly sure that's what I had done to my 68 to increase the speedometer reading but just wasn't sure. I did find the following info from the following site: Adding teeth on the DRIVEN gear will reduce the speedometer reading (cable runs slower). A one-tooth change results in about a 5% change in the reading. Likewise Subtracting teeth on the DRIVEN gear will increase speedometer readings by 5%. http://www.stl-vettes.com/65Vette/GearCalc.htm So, it seems rlb67 is right less teeth=higher reading....



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