72 k10 swb 4x4 gear ratio?

[Tomahawk30]

Hey guys,

My speedometer has been off (runs 7-10mph slow) since I got the truck, and I'm trying to figure out if I just need to swap out the driven gear to adjust it. The truck has a 4" lift and 33" tires. I have no idea what the gear ratio is... It is a posi rear... Is there a certain gear ratio these came with, or should I just do the driveshaft spin count trick? Otherwise I'll just pull the cover this weekend and look for the codes. Just thought I might be able to save a little trouble by asking here. Thanks guys!

[Tomahawk30]

Whoops... Got in a hurry... Should have indicated that it is a th350 transmission...

[Blazer1970]

If the truck originally came with a 350 engine, it came with 3.07s. If it came with a 307 or a 6 cyl., it came with 3.73s. Any other ratio would be listed on the SPID.

[Bishops Trucks]

You could always change the little nylon speedomiter gear in the trans. They go up or down in 6 mph increments I believe.

[raycow]

On some of these tire swaps you will be outside the range of factory speedo gears that are available. In that case, you can use a ratio adapter, which is a miniature gearbox that attaches between the cable and the transfer case.

However, you first need to determine the exact amount of error. The easiest way is to borrow a GPS and note your actual speed at an indicated 60 mph (or some other speed that is easy to maintain for long enough to get a GPS reading).

Divide the GPS speed by your speedometer speed. That will give you a number like 1.xx. Now pull your speedo gear, count the teeth, and divide the tooth count by the 1.xx figure you got earlier. That will give you (to the nearest whole number) the number of teeth you need on the new gear. If your friendly local GM parts dept says no such gear, that's when you need the ratio adapter.

Ray

[Tomahawk30]

Good deal, thanks Ray! I actually ran a speedometer app on my Droid phone this morning on the way to work. It was showing right at 60mph when the speedometer was reading 42 - 43, so it is way off at the higher speeds.

The engine is the original 350. I don't see anything different indicated on the SPID. There's a pic of the SPID in the album linked in my signature.

[Tomahawk30]

BTW, what was the original tire size on these trucks? I'm sure 33's are bigger than stock, but how much bigger?

[Blazer1970]

Originally Posted by Tomahawk30

BTW, what was the original tire size on these trucks? I'm sure 33's are bigger than stock, but how much bigger?

You can assume that they were about 29 inches, but it will not be exact. Tough to tell any closer.

[Tomahawk30]

Thanks Tim. I'd just never really researched what the stock size was...

[Tomahawk30]

Wow... Used the calculator on this page to see what the MPH adjustment would be from 29's to 33's, and it is nowhere near the difference that I'm seeing with my speedometer.

http://www.offroaders.com/info/tech-...peedoConverter

Maybe my speedometer is having issues? It doesn't immediately move the needle when I start rolling, and at a certain point it will jump up to about 15mph. Bad gauge?

[Codypaul]

Originally Posted by Tomahawk30

Good deal, thanks Ray! I actually ran a speedometer app on my Droid phone this morning on the way to work. It was showing right at 60mph when the speedometer was reading 42 - 43, so it is way off at the higher speeds.

The engine is the original 350. I don't see anything different indicated on the SPID. There's a pic of the SPID in the album linked in my signature.

i did the same thing. have a piece of paper written what the speed ='s versus what the speedo says lol.

[Tomahawk30]

That's one way to do it Cody!