Speedometer calibration/adjustment... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Army_Dude · 01-12-2006, 01:35 PM

I was lucky enough to score a used speedo from a board member after I broke mine. I got it in last night and when I drove it today I notice it is WAY off. It jumps up to about 20 mph almost instantly and it reads 70 to 75 mph when going down a road I know I travel 50 mph at best. Is there a way to adjust/calibrate or do you just live with what ya got? OR>>>>Is it possible I did something wrong on the install, it seemed pretty straightforward but hey I had to ask (ya never know). Thanks!

Slonaker · 01-13-2006, 12:20 AM

As far as I know, the speedometers are all calibrated the same (indicated mph per 1000 rpm of the cable...), and adjustments are made with the speedo gear at the transmission. I believe they are even standardized industry-wide.

Slonaker

Army_Dude · 01-13-2006, 01:46 AM

HHmmmm, I doubt that is in the haynes. Gotta go look

hotrodhomi · 01-13-2006, 02:56 AM

you need a new speedo gear in your tranny. they are about 5 bucks at the dealer. Cheap. you will need to know rear end ratio. tire size. tranny type, your mother favorite pizza topping.. the usual. good luck. They also have guys that specialize in speedo calibrations. They run your truck on a dyno type machine and make adjustments form there. Cost unknown.

jimfulco · 01-13-2006, 04:35 AM

Did your old one read right? If it did, the new one should read the same, unless you changed something else (transmissions, speedo gears, or rear end gears).

Maybe it's related to the one in my '79. At a steady 75mph on cruise, I've seen it read anywhere between 60 and 95mph, and holding steady at those speeds, at least for a few minutes. Sometimes if you hit a bump just right it will jump to some other speed for a while. I thought about buying a used one, but I figured I already had a used one.

Army_Dude · 01-13-2006, 10:57 AM

Yeah, the other one worked ok. So I know it has to be the speedo. I'm going to check my connection. Thanks.

**Captkaos** · 01-13-2006, 11:47 AM

The speedo in the head units are all the same. As indicated above the correction is made in the transmission with the proper gears.

It is basically done like this:
http://www.73-87.com/7387garage/drivetrain/spdgears.htm

Slonaker · 01-13-2006, 12:11 PM

When I got a new gear for my speedo cable a couple of years ago (it had a burr, was chewing up the seals and leaking at the bullet), the shop told me that the gears are color coded. Mine was a kind of grayish-purple color. They got me one the same color, and it had the right number of teeth. That might help if you try to get another one. BTW, it was only a dollar or two at a place that provided transmission parts to local rebuilders.

Slonaker

Army_Dude · 01-13-2006, 01:36 PM

but what I'm not getting is how could it be the gear at the transmission side?All I did was pull my speedo, broke it while pulling it, and replaced it with another.

greg0380 · 01-13-2006, 02:04 PM

probably not the gear. let me know if you need a speedo, i have a couple from 85-86 trucks (85mph)

hotrodhomi · 01-13-2006, 04:52 PM

does the new speedo have the same span?

gearhead49 · 01-13-2006, 08:31 PM

I doubt it is the tranny gears, I had a simular problem with mine. Ya might try taking the one part you need out of the new unit and putting it in the old one !

hotrodhomi · 01-13-2006, 09:01 PM

Could you elaborate?

01-12-2006, 01:35 PM	#1
Army_Dude Registered User Join Date: Dec 2005 Location: Fayetteville, NC Posts: 187	Speedometer calibration/adjustment... I was lucky enough to score a used speedo from a board member after I broke mine. I got it in last night and when I drove it today I notice it is WAY off. It jumps up to about 20 mph almost instantly and it reads 70 to 75 mph when going down a road I know I travel 50 mph at best. Is there a way to adjust/calibrate or do you just live with what ya got? OR>>>>Is it possible I did something wrong on the install, it seemed pretty straightforward but hey I had to ask (ya never know). Thanks!

01-13-2006, 12:20 AM	#2
Slonaker Insert Witty Text Here Join Date: Oct 2002 Location: San Antonio, TX Posts: 3,415	Re: Speedometer calibration/adjustment... As far as I know, the speedometers are all calibrated the same (indicated mph per 1000 rpm of the cable...), and adjustments are made with the speedo gear at the transmission. I believe they are even standardized industry-wide. Slonaker __________________ '86 Chevy C10 (Sold 04/19/13 ) Stock '01 Silverado

01-13-2006, 01:46 AM	#3
Army_Dude Registered User Join Date: Dec 2005 Location: Fayetteville, NC Posts: 187	Re: Speedometer calibration/adjustment... HHmmmm, I doubt that is in the haynes. Gotta go look

01-13-2006, 02:56 AM	#4
hotrodhomi Registered User Join Date: Apr 2004 Location: Sacramento Posts: 3,737	Re: Speedometer calibration/adjustment... you need a new speedo gear in your tranny. they are about 5 bucks at the dealer. Cheap. you will need to know rear end ratio. tire size. tranny type, your mother favorite pizza topping.. the usual. good luck. They also have guys that specialize in speedo calibrations. They run your truck on a dyno type machine and make adjustments form there. Cost unknown. __________________ '72 2wd blazer, bagged w/ watts link & EDC '72 C10 "Lowered Farm Truck" STOLEN 5-18-11 '66 impala ht Last edited by hotrodhomi; 01-13-2006 at 02:56 AM.

01-13-2006, 04:35 AM	#5
jimfulco Registered User Join Date: May 2003 Location: Shreveport LA Posts: 3,170	Re: Speedometer calibration/adjustment... Did your old one read right? If it did, the new one should read the same, unless you changed something else (transmissions, speedo gears, or rear end gears). Maybe it's related to the one in my '79. At a steady 75mph on cruise, I've seen it read anywhere between 60 and 95mph, and holding steady at those speeds, at least for a few minutes. Sometimes if you hit a bump just right it will jump to some other speed for a while. I thought about buying a used one, but I figured I already had a used one.

01-13-2006, 10:57 AM	#6
Army_Dude Registered User Join Date: Dec 2005 Location: Fayetteville, NC Posts: 187	Same as before Yeah, the other one worked ok. So I know it has to be the speedo. I'm going to check my connection. Thanks.

01-13-2006, 11:47 AM	#7
Captkaos www.73-87chevytrucks.com Join Date: Dec 1999 Location: Birmingham, AL Posts: 6,935	Re: Speedometer calibration/adjustment... The speedo in the head units are all the same. As indicated above the correction is made in the transmission with the proper gears. It is basically done like this: http://www.73-87.com/7387garage/drivetrain/spdgears.htm __________________ Chris Lucas 1973 Chevy C-10 1978 GMC Jimmy (2WD) - SOLD 1987 R10 twin turbo LS 1991 R3500 SRW CrewCab 1985 K5 diesel swapped project 1989 K5 2WD conversion w/ Vette susp Project Captkaos Customs 73-87chevytrucks.com

01-13-2006, 12:11 PM	#8
Slonaker Insert Witty Text Here Join Date: Oct 2002 Location: San Antonio, TX Posts: 3,415	Re: Speedometer calibration/adjustment... When I got a new gear for my speedo cable a couple of years ago (it had a burr, was chewing up the seals and leaking at the bullet), the shop told me that the gears are color coded. Mine was a kind of grayish-purple color. They got me one the same color, and it had the right number of teeth. That might help if you try to get another one. BTW, it was only a dollar or two at a place that provided transmission parts to local rebuilders. Slonaker __________________ '86 Chevy C10 (Sold 04/19/13 ) Stock '01 Silverado

01-13-2006, 01:36 PM	#9
Army_Dude Registered User Join Date: Dec 2005 Location: Fayetteville, NC Posts: 187	Re: Speedometer calibration/adjustment... but what I'm not getting is how could it be the gear at the transmission side?All I did was pull my speedo, broke it while pulling it, and replaced it with another.

01-13-2006, 02:04 PM	#10
greg0380 Registered User Join Date: Dec 2003 Location: CT Posts: 454	Re: Speedometer calibration/adjustment... probably not the gear. let me know if you need a speedo, i have a couple from 85-86 trucks (85mph) __________________ 1985 GMC Sierra 1500, 1/2 ton, 2WD, LWB, TBI 305 5 speed, 3.42 rear 2007 GMC Sierra 1500, xtended cab, 4X4, flexfuel 5.3L 1999 BMW 528i

01-13-2006, 04:52 PM	#11
hotrodhomi Registered User Join Date: Apr 2004 Location: Sacramento Posts: 3,737	Re: Speedometer calibration/adjustment... does the new speedo have the same span? __________________ '72 2wd blazer, bagged w/ watts link & EDC '72 C10 "Lowered Farm Truck" STOLEN 5-18-11 '66 impala ht

01-13-2006, 08:31 PM	#12
gearhead49 Registered User Join Date: Dec 2005 Location: wa Posts: 2	Re: Speedometer calibration/adjustment... I doubt it is the tranny gears, I had a simular problem with mine. Ya might try taking the one part you need out of the new unit and putting it in the old one !

01-13-2006, 09:01 PM	#13
hotrodhomi Registered User Join Date: Apr 2004 Location: Sacramento Posts: 3,737	Re: Speedometer calibration/adjustment... Could you elaborate? __________________ '72 2wd blazer, bagged w/ watts link & EDC '72 C10 "Lowered Farm Truck" STOLEN 5-18-11 '66 impala ht



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