Rear end gearing suggestions?

[davecarney2]

Running a 6.0 from an SS Silverado (345 HP) with a 4L80E. Longbed 1959. Looking for good pickup but still get descent gas mileage for daily. Suggestions on what the best gear to run in rear end please.

[Wrenchbender Ret]

If you want to keep the original rear end, 3:38 gears will work very good. They ate readily available. If you are changing the complete axle 3:08 ratio would be good.
You have plenty of power & torque in that engine. If you are working it hard like pulling a trailer or large load you can drop the lever down a notch & still cruise down the highway.
George

[davecarney2]

Sorry should have said it has a narrowed Ford 9 inch in it.

[mongocanfly]

what size tires?
i like mine running about 2000rpm at 65mph
heres a rpm calculator thatll help you decide...http://www.csgnetwork.com/multirpmcalc.html

[davecarney2]

275/60 15

[rpmerf]

Your tire has a 28" diameter.
Your trans has a 0.696 OD ratio.
I do not know what exact ratios you have available, so I will throw some numbers out there.

3.08: 1416 RPM@55 mph, 1673 RPM@65 mph, 1931 RPM@75 mph
3.25: 1494 RPM@55 mph, 1766 RPM@65 mph, 2037 RPM@75 mph
3.38: 1554 RPM@55 mph, 1836 RPM@65 mph, 2119 RPM@75 mph
3.50: 1609 RPM@55 mph, 1901 RPM@65 mph, 2194 RPM@75 mph
3.73: 1715 RPM@55 mph, 2026 RPM@65 mph, 2338 RPM@75 mph
4.10: 1885 RPM@55 mph, 2227 RPM@65 mph, 2570 RPM@75 mph

The speed you cruise at on your daily drive is going to be a factor.

[mongocanfly]

for fun Id be looking at 3.73s... for cruising maybe 3.50s

[Captainfab]

I would go with the 3.73's and certainly no higher than 3.55's.

[truckster]

How far do you drive on an average day, and what's your average speed?

[Hollow65]

Originally Posted by Captainfab

I would go with the 3.73's and certainly no higher than 3.55's.

This, you have od.

[davecarney2]

Just getting it on the road. Rear was built a few years ago. Somewhere around a 3.00 gearing. 3 times around on the driveshaft vs 1 time around on tire. Still working on getting gauges to work so not sure on rpm vs speed yet. Have only driven it a few times so far. Plan on some long hauls to shows and driving to work is all backroads. Have a small camper to pull and wanna hit up the rod rod power tour.

[toolboxchev]

With the 6.0 you will have plenty of power to run anything in the range your looking at.

Now if you want some get up and go, towing rpm range I would be seriously looking toward those 3.73's.

Keeping in the power band of torque is what your looking for in my thinking.

[davecarney2]

looks like I need to choose between a 3.73 and a 3.50. With that being said since it seems to have a 3.00 in it now aside from not being happy with the acceleration is gas mileage going to be worse with the 3.00 since engines not in the optimum range? If I can run it as is for a while I can complete other things but if it’s bogging it down eating more gas I’ll move it up on the list for level of importance to change the ratio. Since none of my gauges work i haven’t been able to calculate fuel mileage yet. Heavy truck big engine I know it won’t be a fuel miser. But with the intent to drive it a lot wanna get best I can at least while having some fun at the same time.

[rpmerf]

Numbers with 3.00 rear end:

3rd: 1981 RPM@55 mph, 2342 RPM@65 mph, 2702 RPM@75 mph
OD: 1379 RPM@55 mph, 1630 RPM@65 mph, 1880 RPM@75 mph

I found the stock tire size and rear ratio for a stock Silverado SS. 275/55R20 (31.91") and a 3.73 for RWD or 4.10 for AWD. Here is the calculation for the 3.73 rear.
1504 RPM@55 mph, 1778 RPM@65 mph, 2051 RPM@75 mph

I would aim for at least the stock numbers, a little higher RPM would be preferable for acceleration. I think you will be fine running the 3.00 for a while. Might leave it in 3rd if you are towing a camper, or if you need more acceleration on back roads.

[davecarney2]

Thanks all huge help. Thanks for all the suggestions and information!!!