Speedometer Calibration

[draino76]

I have a 79 gmc 1/2 ton full time 4x4. I just bought a 4" lift and 285-75-16 tires for it. I would like the speedometer to at least be close, how do I adjust it? The transfer case has a gear driven "step up" gear box on the side that drives the speedometer cable. My local GM shop says I'm S.O.L. , but they do not always seem to very helpfull on anything that is 20+ years old. Any Ideas?

Thanks
Wayne

[tgh117]

Check out this site. http://www.miata.net/garage/tirecalc.html

You can put in your current size tire, and what you will be going to, and it will give you an appoximate value of what your speedometer reading will change to. On my truck I run 33x12.5 and I am about 3-4 mph off.

Hope it helps.

[willett]

you can change up the gear in your t case that dreives the speedo cable.

i had alot of trouble at the dealership with my rig about it until they eventually put me on the phone with one of the older mechanics. i thin khe said try one with 2-3 less teeth at first.

mine is out about 10km's with 35's

[rfmaster]

I use portable GPS navigator to establish actual vehicle speed. It is best to get speedo vs. GPS difference at highway speeds - 60 to 70 Mph. Once you know which way you need to adjust your speedo take a look at your current combination of drive and driven gears used in your transmission speedometer - these are color coded. The link below will provide additional information
http://www.tciauto.com/Products/Comp...eter_gears.asp

//RF

[dswancutt]

The adaptor should have the ratio marked on it. On my 78 k10, it is just a straight 1:1. If it does not have the ration, see if it has the GM part number and I can look up what ratio it is.

[piecesparts]

My suggestion is to figure out what exactly is your rear end gear ratio, the final transmission output ratio, and the tire size that you want to run. The 278/75/16 tires that you have now are a 32.42" tall tire and you can figure what RPMs are from that and reverse to what speed you are going. Also, make a speed chart from a calculator and a stopwatch using the fact that 60 MPH should take 60 seconds to drive. Example 60 MPH X 60 SECONDS = 3600. Now run a mile a divide the seconds of that mile into the number 3600 and you have your present speed. (By the way, this chart is what the Highway Patrol Units have in their cars---been there proved that) That will tell you how far off from real time you are on your speedo. The speedometer drive gear and driven gear CAN be changed to make the speedo go up or down. The speed multiplier on the side of the tranny can be changed and GM used to have those available, however many tranny shops have them or can get them. A long time ago, I went to my local Chevy Dealer and copied an old parts book that listed ALL of their different tranny gear sizes and associated rear end ratios, but they are on the web now and are easy to find.



Tire size calculator site: http://www.discounttire.com/dtcs/infoTireMath.dos

[Malo83]

My quik fix was to go down the highway have my better half drive our new 08 Malibu and check my speed at 60,65,70 against the Malibu, I was 5mph off, when she was at 60 I was 65 now I know i'm 5mph faster

[Pyrotechnic]

Is it possible that speedometers wear out after an amount of time and need to be recalibrated ? I know there's a spring preloaded against the needle, but I'm thinking that the spring could lose tension over time and give ridiculous readings, even if your speedometer gears are correct ?

[87c10slammer]

thats when you get a jumping speedo..

[blackdiesel]

You need to get a speedo adaptor ( where the cable gos to the t-case) for a truck that has higher gears than you have. seens your running less RPM and the same MPH it's the same as if you swaped out gears in the rear end.

[Jeepwm69]

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

Look about two pages down and this place sells the adapters. I've got to get one ordered for Dad's 83, but have to get time to figure out which one I need first.

[draino76]

Hey, thanks for all of the good ideas, I thought for I may have to resort to an aftermarket electronic speedo. I have a soft spot for the factory gauges.

[Red_69]

Isn't it cool to see how many people know of solutions or at least where to look after you talk to your local lazy@$$ parts counter guy and he says you're screwed?

[piecesparts]

You get what you pay for SOMETIMES!