Speedo calibration? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

WinDancer · 06-09-2007, 11:19 AM

How do you calibrate the spedo to actual speed?
TIA

Rokcrln · 06-09-2007, 11:25 AM

I took mine to a shop in Sacramento to have this done. Only to find out that they can not do mine with just a new spedo gear so they had to add on an extra box with the right calibration. Then we found out my speedo was bad because with the odometer at 60 mph the speedo needle was at 72mph. I had another speedo at home but it is 20mph off so I bought a new one from ECE for about $90 and now I just need to go get it all calibrated.

But to answer your question, depending how far off it is you can change out the gear at the end of your speedo cable were it goes into the trany. GM has about 6 different one (all color coded) for different factory wheel and gear set ups. If you can not do it this way then you will also need to have a grear box set up with the right calibration for your set up.

Kevin
LFD Inc.

cdowns · 06-09-2007, 11:45 AM

a speedo shop is best but it is possible to come fairly close by going thru a measured mile with a stopwatch/ or ride around residential areas looking for those signs that say "you are going xxxmph"

Jayss109 · 06-09-2007, 02:09 PM

option one: go with what was said above

option 2: get an electronic one and never worry about it again.

I have found that the city "your speed" signs are wrong, atleast in my area. I have 2 right by my house, and they both read 2-4mph fast. Checked with several vehicles.

my two cents

Jason

fastom · 06-09-2007, 04:08 PM

Use a GPS, thats how I got mine close.

fastom · 06-09-2007, 04:10 PM

I re-read the question and thought about what I just answered. I have the electric speedo, I used a GPS to get the correct speed and then adjusted the speed sender.

Rod · 06-09-2007, 06:19 PM

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

http://www.novaresource.org/speedo.htm

these might help the diys

GMC Jim · 06-10-2007, 07:41 AM

Rockcrin--- The link that Rod posted ( novaresource ) has the info which you need...I believe. Several years ago, I changed rear axle (different Ratio)and also installed a lockup 350 transmission in a '79 Cad Seville, Went to a local transmission shop; they looked up the specs on a chart similar to the nova resorce site's charts and they had the 2 speedometer gears I needed

piecesparts · 06-10-2007, 09:19 AM

In most cases, the GM dealer has a listing of different trannys and the needed "Drive" and "Driven Gears that is in most combinations for certain rear end setups. This info is in most old shop parts books. If nothing else get the parts catalog for the Chevy truck 82 to 87 and the info is in that book--I have copies of those pages here. That info would mean if you have 350 tranny and 3.73 gears then the speedo gears in the tranny would be nu8mber XXXXX and nymber YYYYYY and so on. That is where I started on my 84 GMC. Then I took my stopwatch and actually timed myself going down the highway at 60 MPH and calculated what speed I was actually going (example 60 mph should take 60 seconds). (The noveasource web has all of this) I also used several mile markers to get the most accurate one in length, then I measured what my odometer actually counted off in a mile distance (example the odometer counted off 9 tenths in a mile) From that I setup my speedo to be the most acurate around the 70 MPH mark, since the majority of my driving is at that speed. I am now about .05 on the odometer off at 70 MPH now. A speedo is not accurate in all areas of it's indication, it will be off different amounts at different speeds. Find a spot that is where you use it the most and work for that speed. The error is not linear in measurement.

I got mine to adjust in by putting in the right "drive", "driven", and speedo gear "adapter guide" for the driven gear to get close. The info I got from the GM dealer out of the old parts book was invaluable in this endeavor. Speedo shops are not easy to find in Kansas. I went to a transmission shop friend of mine and got all of my parts, he had them on his shelf.

byrd · 06-10-2007, 11:14 AM

If you find that you need to have the actual speedo calibrated and not the gears in your tranny, then contact this place they can take care of it for you.

CAPCO
4614 Bittersweet Rd.
Louisville, Ky
40218
502-966-5086

06-09-2007, 11:19 AM	#1
WinDancer The Original HotRod KnifeMaker Join Date: Jan 2007 Location: Tenino, Washington Posts: 811	Speedo calibration? How do you calibrate the spedo to actual speed? TIA __________________ Dave 69 GMC SWB 2WD 454 Turbo 400

06-09-2007, 11:25 AM	#2
Rokcrln Senior Member Join Date: Jul 2005 Location: Winters Ca. 95694 Posts: 4,843	Re: Spedo calibration? I took mine to a shop in Sacramento to have this done. Only to find out that they can not do mine with just a new spedo gear so they had to add on an extra box with the right calibration. Then we found out my speedo was bad because with the odometer at 60 mph the speedo needle was at 72mph. I had another speedo at home but it is 20mph off so I bought a new one from ECE for about $90 and now I just need to go get it all calibrated. But to answer your question, depending how far off it is you can change out the gear at the end of your speedo cable were it goes into the trany. GM has about 6 different one (all color coded) for different factory wheel and gear set ups. If you can not do it this way then you will also need to have a grear box set up with the right calibration for your set up. Kevin LFD Inc.

06-09-2007, 11:45 AM	#3
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: Spedo calibration? a speedo shop is best but it is possible to come fairly close by going thru a measured mile with a stopwatch/ or ride around residential areas looking for those signs that say "you are going xxxmph" __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

06-09-2007, 02:09 PM	#4
Jayss109 Active Member Join Date: May 2005 Location: San Jose, CA Posts: 235	Re: Spedo calibration? option one: go with what was said above option 2: get an electronic one and never worry about it again. I have found that the city "your speed" signs are wrong, atleast in my area. I have 2 right by my house, and they both read 2-4mph fast. Checked with several vehicles. my two cents Jason __________________ Cadillac Powered

06-09-2007, 04:08 PM	#5
fastom That's right, it's Copper!! Join Date: May 2007 Location: Bakersfield, Central California Posts: 558	Re: Spedo calibration? Use a GPS, thats how I got mine close.

06-09-2007, 04:10 PM	#6
fastom That's right, it's Copper!! Join Date: May 2007 Location: Bakersfield, Central California Posts: 558	Re: Spedo calibration? I re-read the question and thought about what I just answered. I have the electric speedo, I used a GPS to get the correct speed and then adjusted the speed sender.

06-09-2007, 06:19 PM	#7
Rod Join Date: Dec 1999 Location: Eastern - Manitoba Canada, Winnipeg Posts: 4,369	Re: Spedo calibration? http://www.transmissioncenter.net/sp...n_______va.htm http://www.novaresource.org/speedo.htm these might help the diys __________________ Senior active founding member. 70 3/4 ton GMC Suburban 4x4 250/4 spd. 71 1/2 ton Suburban Chevy 4x4, 350/350 72 Chev 1 ton tow truck 402BB w/Holmes 440 wrecker on propane 2005 FLSTSCI Harley Springer 85 FXSB Harley Lowrider 72 Triumph 650 Bonneville 5spd "Poor people don't have hobbies" Quote from wife.

06-10-2007, 07:41 AM	#8
GMC Jim Senior Member Join Date: Dec 2002 Location: Portland, Oregon Posts: 901	Re: Spedo calibration? Rockcrin--- The link that Rod posted ( novaresource ) has the info which you need...I believe. Several years ago, I changed rear axle (different Ratio)and also installed a lockup 350 transmission in a '79 Cad Seville, Went to a local transmission shop; they looked up the specs on a chart similar to the nova resorce site's charts and they had the 2 speedometer gears I needed __________________ '71 GMC;fleetside;PB/PS/AC/CC/402 Eng./Custom Paint/110,000act miles/3 fuel tanks(52gal).

06-10-2007, 09:19 AM	#9
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: Spedo calibration? In most cases, the GM dealer has a listing of different trannys and the needed "Drive" and "Driven Gears that is in most combinations for certain rear end setups. This info is in most old shop parts books. If nothing else get the parts catalog for the Chevy truck 82 to 87 and the info is in that book--I have copies of those pages here. That info would mean if you have 350 tranny and 3.73 gears then the speedo gears in the tranny would be nu8mber XXXXX and nymber YYYYYY and so on. That is where I started on my 84 GMC. Then I took my stopwatch and actually timed myself going down the highway at 60 MPH and calculated what speed I was actually going (example 60 mph should take 60 seconds). (The noveasource web has all of this) I also used several mile markers to get the most accurate one in length, then I measured what my odometer actually counted off in a mile distance (example the odometer counted off 9 tenths in a mile) From that I setup my speedo to be the most acurate around the 70 MPH mark, since the majority of my driving is at that speed. I am now about .05 on the odometer off at 70 MPH now. A speedo is not accurate in all areas of it's indication, it will be off different amounts at different speeds. Find a spot that is where you use it the most and work for that speed. The error is not linear in measurement. I got mine to adjust in by putting in the right "drive", "driven", and speedo gear "adapter guide" for the driven gear to get close. The info I got from the GM dealer out of the old parts book was invaluable in this endeavor. Speedo shops are not easy to find in Kansas. I went to a transmission shop friend of mine and got all of my parts, he had them on his shelf. Last edited by piecesparts; 06-10-2007 at 09:25 AM.

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