4.56 Rear Gears?!?!?!?!?!

[69C10@16]

I was playing around with a rpm-speed calculator this afternoon trying to figure out what the 700r4 would change my crusing rpm to. I typed in 3.73 rear gears with a 1:1 transmission ratio for final gear in the th350. I put in my tire size and all that and what it calculates is all wrong. So i started typing in other rear end gears because it is obvious that a PO was in the differential because there's blue gasket stuff all around the cover. I discovered that the closest rear gearing that matches what my truck actually does is 4.56.

Were these gears ever put out on these trucks or did someone change the gearing in my differential?

If anyone else wants to play around with their own calculator to see if I'm all wrong here's what I have.

Turbo-350
235/75/R15's.
At 55MPH Im running about 2800RPM give or take a few.

[WorkinLonghorn]

I know that some 3/4 ton configurations were available with a 4.57 diff. Not sure about the 1/2 tons. Jack up one wheel,turm the DS by hand 2 1/4 revs and the wheel should turn one rev with 4.57s (open diff).

[whatnow123]

I came up with 4.11 running about 2700 at 55, 4.56 came in at 3000 at 55.

[69C10@16]

4.11's would make more sense to me. The calculator I was using could be off some i suppose. Ill try jacking up the rear end tonight to see if it matches what workinlonghorn said. I'd do it this morning but I'm going to Oreilly's Autofest with a couple of friends. Either way I'm going to want to try to find a 3.73 differential to drop in this thing.

[HavinSomeFun]

I came up with 4:11 also.

[truckster]

I had one with 4.56 from the factory.

[jeffspower]

The old automatic trannys without a lockup torque converter will "skew" calculations since there is no mechanical coupling of the crank-to-driveshaft. Even when engine rpm is way over the converters stall speed, it's not a true 1-1 ratio. Generally, you can figure 200-300 rpm more than what the calculator shows- if you have an old TH350, 400, powerglide, etc.

[69C10@16]

All I know is somethings got to give because I went to springfield today and was running 4000 RPM constant on the Interstate. There's no way that's anywhere near good on the truck.

[Psycho71]

Yeah, that's getting a little excessive on a driver, or just about any street motor.

I bet she's fun in low gear though.

Jay

[bad6772]

i run a 4.56 posi rear ...love it....not very pratical on the freeway but lots of fun light to light driving....when it was all together i ran about 2900 at 55mph.....i think this time around i will run either a 3.42 or 3.73 spool.....?

[slammed1]

With overdrive transmissions and tires in the 31" range 4.11-4.56 gears can actually be used. On my 2002 Ram with 3.92 gear and 4 speed overdrive I swapped to 4.56 and only gained 300 rpm across the entire mph scale,it also helped my lil 287 (4.7) V8 run much much stouter. My 04 Ram has a hemi,3.92 and 5 speed auto and I will also place 4.56 in it. Since the overdrives are so efficent adding lower gears dont cut down as much on your top end and can actually increase fuel mileage. 4.10s with a slightly shorter tire and overdrives work great also.

[69C10@16]

Oh yeah she's real fun in low gear. But I'd give that up anyday for the better gas mileage. I did the BEST burnout I've ever done today in Springfield. But I'd rather not be able to do that than run 4000 RPM on the Interstate when I head home.

[WorkinLonghorn]

I notice that the gear is listed (ref.Truck Data Book) as a 4.57 unless it's a Dana,then it's a 4.56.

[69C10@16]

But the top of that page says C-20. Mines a C-10. Would that make a difference?

[grayw0lf]

Welcome to the club. My C-10 has 4.56 out back on 27" tires. Its painful onthe interstate.