Speedometer Adjustment(change of ratio)

[Petrella]

Hello guys, i have a 1988 chevy with a 6.2 diesel, and a th700r4 tranny. My truck used to have 3.08 gear ratio with 235, 75r15 tires. i recently changed my axle ratio to 3.73, and my tire size is now 265 75r16 because im getting into the towing. My speedometer is really off! everytime i drive down the road, the speedometer shows that i am driving about 10-15 miles faster than i am actually going. What exactly do i have to do in order fro my speedometer to show the actual speed?

Thanx Anthony

1988 Chevy 2500HD 6.2L Diesel

[ 10-27-2003, 06:36 PM: Message edited by: Petrella ]

[britannic]

First you'll need to know how many teeth your tranny speedometer drive gear has.

Then go to www.4lo.com (http://www.4lo.com/) and calculate the speedo gears you need.

Order a new driven gear and install.

Do be aware that there are two or more types of driven gear housings, which take a set range of different tooth gears. So if your new driven gear falls outside of the existing housing's range, you'll need to get the another housing. You may need to compromise, as it's difficult to get an exact match with certain tire/diff combinations.

An alternative is to get an electronic speedo and fit a configurable electronic speedo drive to the tranny.

There are also aftermarket driven gear gearbox convertors that can help make your ratio more exact.

Yet another method, is to just have the speedometer itself recalibrated to work with the existing gears.

[catmandoo]

don't know much about the speedo on the 700 but the th400 is like brittanic says there is 2 different housings and on the 400 it is cast right on them the number of teeth that will fit that particular housing such as (37,38,39,40) next housing would say (41,42,43,44)on the 400 i believe it is 3 mph per tooth, add teeth to slow down so if you had a 400 you would need 4 or 5 teeth so if that was the case you would end up changing the housing too.

[Petrella]

how would i recalibrate the speedomoter itself?

[britannic]

Originally posted by Petrella:
how would i recalibrate the speedomoter itself? You'd need to take it to a speedo shop that can recalibrate it.

[Petrella]

wow this isnt easy at all. i didnt think it would be this difficult to adjust the speedometer. maybe ill just live with the way it is now

[britannic]

Once I figured out which gear I needed, it was a snap, the whole job took about 20 minutes to R & R the gear in my NP208 T-Case.

I got my wife to follow in her Dmax to check the calibration and we found my truck was within 1 mph of hers - that's plenty good enough considering even the Dmax won't be 100% accurate, what with tire wear, pressure and temperature etc.

[gmctd]

The 'housing' they are speaking of is the 'plug' you will be pulling out of the tailshaft housing to change the ratio. It's about 2" in diameter with an o-ring seal.

If you jack the rear of the truck, the job won't even waste any trans fluid, only whatever is in the gear drive housing - 1/8th cup or so.

Install a new o-ring and shaft seal with the job for leak-free service.

jd

[Petrella]

Ahh, i see, so im only removing the plug in the back of the transmission. So if i go to the dealer and say here is my tranny, here is my ratio, here is my tire size, can they give me the closest one that i am looking for?

[catmandoo]

probably not.it's probably gonna be trial and error.i myself would go and get a gear with 4 or 5 more teeth as you say it's off 12-15mph we used to figure 3 mph per tooth try it if still fast go 6 teeth or whatever depending on how far it's still off.the dealer should be able to tell you if that particular gear will fit in your housing .

[odee]

when I done all the 3/4 ton running gear swap and gear change I went to my buddy at the gm dealer and got a copy of tire size and gear ratio for a given year truck/sub. It told me the drive gear color and the driven gear. Worked out really good for me. Some of the ratio and tire combo needed an external speed reducer/multiplier that the chart told you about.