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

GMC_Aaron · 11-05-2002, 03:16 AM

I don't know squat about tranny's, so I was wondering how to recalibrate the speedo. I know my 72 GMC Super Custom is running about 7mph faster than what the speedo reads. I am running 255 70 R15 tires on my truck and I know that has something to do with it. How do I compensate?

**GMCPaul** · 11-05-2002, 05:53 AM

If you pull the current speedometer driven gear out of your transmission you can change it out and by adding or subtracting 1 tooth to the new gear you buy you change the speedometer reading up or down by approx. 5 mph. The new speedometer driven gears are available at your local GM dealer. By adding a tooth you increase the speedometer reading by subtracting a tooth you decrease it.
GMCPaul

slam33 · 11-05-2002, 10:46 AM

http://www.tciauto.com/tech_info/speedo_gears.htm

melloyello · 11-05-2002, 11:03 AM

take to a tranny shop that specializes in calibrating should only cost 50-100 depending how much they wanna rip ya off thats still cheaper than alot of speeding tickets....I got a ticket last friday for doing 10 over when my speedo said i was doing the limit....well thats 86 bucks to the cops and not put towards my truck

.........just my 2 coppers here

Reumster · 11-05-2002, 01:17 PM

Not everyone knows that AutoMeter makes a new electric speedo cableless speedo. It's very easy to calabrate go 60 on the speedo, hit a button on the back when you pass a mile marker, and once again when you hit the next mile marker, done perfectly accurate, and if you change gears or tires, you can recalibrate with the touch of a button. Nice

**MikeB** · 11-05-2002, 02:33 PM

For finer calibration, you can also go up or down a tooth or two on the "driving" gear -- the ring gear that's mounted on the tailshaft. Obviously, this requires dropping the driveshaft and pulling the tailshaft housing, but I did both the gears on my TH-350 in a couple of hours.

bobs409 · 11-06-2002, 09:22 AM

Here's a good page to help figure out which gears you'll need:

http://www.chevelles.com/shop/speedo.html

Although it's for Chevelles, it's the same for trucks too.

11-05-2002, 03:16 AM	#1
GMC_Aaron Registered User Join Date: Jul 2002 Location: Ventura, Ca. Posts: 87	Speedo Calibration I don't know squat about tranny's, so I was wondering how to recalibrate the speedo. I know my 72 GMC Super Custom is running about 7mph faster than what the speedo reads. I am running 255 70 R15 tires on my truck and I know that has something to do with it. How do I compensate? __________________ Aaron Ventura, Ca. 1972 GMC Super Custom SB350/TH400

11-05-2002, 05:53 AM	#2
GMCPaul Senior Member Join Date: Mar 2000 Location: Rockville,In. USA Posts: 3,722	If you pull the current speedometer driven gear out of your transmission you can change it out and by adding or subtracting 1 tooth to the new gear you buy you change the speedometer reading up or down by approx. 5 mph. The new speedometer driven gears are available at your local GM dealer. By adding a tooth you increase the speedometer reading by subtracting a tooth you decrease it. GMCPaul __________________ Secure Online Catalog for-GMCPauls Truck Parts Email- GMCPAULS@GMAIL.COM

11-05-2002, 10:46 AM	#3
slam33 Registered User Join Date: Aug 2001 Location: Long Beach, Ca Posts: 1,432	http://www.tciauto.com/tech_info/speedo_gears.htm __________________ 71 LWB 350/350still working on it but it's going tp be sweet. www.geocities.com/stevemau/slam33page.html " TARGET=_blank>http://www.geocities.com/stevemau/slam33page.html?1004806705410 </A>

11-05-2002, 11:03 AM	#4
melloyello Registered User Join Date: Nov 2002 Location: Spokane WA Posts: 66	take to a tranny shop that specializes in calibrating should only cost 50-100 depending how much they wanna rip ya off thats still cheaper than alot of speeding tickets....I got a ticket last friday for doing 10 over when my speedo said i was doing the limit....well thats 86 bucks to the cops and not put towards my truck .........just my 2 coppers here __________________ 67 C/20 camper special 350

11-05-2002, 01:17 PM	#5
Reumster L.E.D. Freak Join Date: Jan 2002 Location: Polson MT Posts: 1,358	Not everyone knows that AutoMeter makes a new electric speedo cableless speedo. It's very easy to calabrate go 60 on the speedo, hit a button on the back when you pass a mile marker, and once again when you hit the next mile marker, done perfectly accurate, and if you change gears or tires, you can recalibrate with the touch of a button. Nice __________________ Specializing in Custom L.E.D. inserts. http://www.ccled.com/ Contact me @ blazer2wd1972@hotmail.com (406)675-8082 or Joel @ jmcwaters@eps-corp.net 1972 2wd Blazer, 350/700R4/3:73 posi, err & Custom L.E.D. inserts! LOL

11-05-2002, 02:33 PM	#6
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,663	For finer calibration, you can also go up or down a tooth or two on the "driving" gear -- the ring gear that's mounted on the tailshaft. Obviously, this requires dropping the driveshaft and pulling the tailshaft housing, but I did both the gears on my TH-350 in a couple of hours. __________________ Mike 1969 C10 LWB -- owned for 36 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- converted from 250-six to roller cam 350, Vortec heads -- sold 1981 C10 Silverado LWB, 305, TH350C -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!

11-06-2002, 09:22 AM	#7
bobs409 1969 Custom 30 wrecker Join Date: Sep 2001 Location: Pottsville, Pa Posts: 2,611	Here's a good page to help figure out which gears you'll need: http://www.chevelles.com/shop/speedo.html Although it's for Chevelles, it's the same for trucks too. __________________ Bob 63 Impala 283/PG/3.36 (under restoration) 66 Impala 327/PG/3.36 66 Chevelle SS 409/M22/3.55 69 Chevelle 307/PG/3.08 (future restoration) 69 C10 short stepside 454/TH350/3.73 69 Custom 30 tow truck 350/4 spd/4.10 (resto done, CRUISIN TIME!) 71 Cheyenne 20 fleetside 350/4 spd/4.10 71 Chevelle 6/PG/3.08 72 Chevelle 454/M21/4.10



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