The Stovebolt Site Forums The Shop Area Driveline Figuring out my 71 C-10 250c.i. 4spd. Differential ratio.

Well I've had this 1/2 ton for 3 weeks and got the brakes fixed, fortunately.

Anyway I put new 215-70-15R tires on it, all 4. Also I put on a Tach, set for 6 cyl. setting.

So I took it out on the Interstate and found the speedometer was a bit off. Like I have to indicate about 67-68mph to do a mile in 60 seconds, averaged over 10 miles, steady(as possible). Anyway at that speed it is Tach-ing about 3100 rpms. So I took it up to where I was doing miles in about 51 to 52 seconds, which would be around 70 mph. At that speed I was Tach-ing 3600 to 3700 rpms. The speedometer at that speed was showing 78-79 mph.

Anyway, At 70-ish mph it is really working hard or it sounds that way. Also, for a 153 mile trip, I got only about 11.5 mpg, having been very careful about filling tank both going and coming back. Also, the Odometer runs about 12.5% further than the actual distance I've gone.

No tag or markings on the differential I can find, cleaned and looked. Wondering if it is a 4.11 or 3.73?

Thank y'all for help.

With those figures and the specs from the tire rack, it points to a ratio of around 4.01 ?

Sure looks like it. I did some searching and the only ratios I find in the ball park of 4.?? is 3.73 and 4.11. Hmm, what to try? For my use the truck doesn't really need taht much gearing.

It's a short bed too, bed is about 6-1/2 feet long is all.

Maybe a th 700 r4, with the overdrive? Would the drive shaft have to shortened/lengthened?

215/70R15 @ 775 revs per mile = 3,700 @ 70 with 4.11 gears.

The 700-R4 would make it 2,600RPM @ 70, and a 3.07 rear with your current trans would make it about 2,800.

Another option would be an overdrive manual trans like the T-5.

Thanks 66Sub,

I think you're exactly right on the RPMs. On further familiarizing myself with the truck it has a GM 12 bolt rear end and doing the drive shaft/wheel rotation check several times supports a 4.11 ring and pinion ratio strongly.

I have a working 700-R4 from a neighbors wrecked Chevy truck. I would guess I'd have to have the forward drive shaft section shortened(?) to accomodate the additional length over the Saginaw 4 spd. Of course I'll need the kit for the lock out and such. I guess that's right, never done it before.