The Stovebolt Site Forums The Shop Area Engine Shop rear gearing advice

New guy here. I recently found this great site and have already learned quite a bit about restoring my grandfather's farm truck, which is a 66 C-10 LWD w/250. My first step will be to change the 4.11 gearing in the back. It has the 4 spd HD Muncie trans and the 12 bolt rear. It is not very highway friendly and can get it rebuilt locally for about as much as getting used one. I was thinking of going all the way down to the 3.08. Is this too low, any other suggestions? I have 7.00/15 tires and eventually plan to get offset rims with 235/70. Any advice would be appreciated. Dennis

Yes, I can help. Even though my truck is a '52 GMC, I have done the wheel and tire upgrade on my truck, using 8-lug year 2000 GMC chrome 16" wheels. The bolt pattern was the same, but I did need to have 1 1/4" spacers made for them. The tire shop I bought the wheels from was able to provide me with the spacers the same morning. The spacers are made to bolt onto the original hubs, and then they have their own set of studs that the wheel then fits over with lug nuts. It works very well and I have had no problem with them. I believe my tires are modern Uniroyal all-season steel belted radials in 235/70R-16 size, and they handle at least twice as good as the no-longer-available school bus tires on split rims that I used to have. It was an excellent upgrade.

About your taller gear ratio, it depends on your engine. The 3.08 will give you much better highway cruising, but if you have an inline six engine, it may take you all day to get up to highway speeds. That's obviously an exageration, but the point is, with enough torque, the 3.08 gears will feel fine. You may almost need big block torque to be able to turn those gears good enough at lower rpms.
I have 3.08 gears on a 1968 Firebird with a super torquey 465 cube Pontiac engine, and they feel very good on that car. But even with a small block Chevy, I don't think there would be enough natural torque to make the 3.08's feel good.

You might be better off compromising to a 3.55, or maybe even a 3.73. And my personal opinion is that considering the age of the rear diff, it is well worth it to have it gone through and rebuilt. If you trade for another old rear end to get the gear ratio, that diff too could require rebuilding, and then you're in it for more money. Better to do it right the first time.

...And it wouldn't be a bad idea to get a C-clip eliminator kit while you're at it. You could do without it, but if the C-clips ever fail, there is nothing stopping the axle, with the wheel, from pulling out of the differential and passing you on the highway. I have seen this happen before. In all other respects, the Chevy 12-bolt is an excellent rear end.

Thanks Hot Rod Dad! I was wondering what driving in the city would be like as well. The 4.11 does ok below 45 mph, but I live in rural W.Neb attm and do quite a bit of hwy driving. With the 3.73 I won't need a new case in the swap, however will there be a noticable difference at highway speeds. Not worried about 75 mph but would like to go 65 mph with out revving to high. Maybe 3.4X to the 3.55 would be the best? BTW I grew up in Lakewood, say hi to the mountains for me! Regards, Dennis

I run a 3.08 gear with a 250 and turbo 350 automatic, 235/75 R15 radial. My truck is a 37 and with two people weighs 3490lbs. It runs out very well and will cruise at 65 all day long. It is slower getting to full speed now, it did have a 3.42 gear, but what I lost in time I made up in rpm. The engine sounds great and I don't have to worry about over stressing it! After all, the 250 isn't a real power house anyway, but it does have plenty of low rpm torque, and thats what it takes to pull a heavy truck. Joe

Read as much as possible b4 you spend the $$$ to change anything on grand-paw's truck.
Do a search -
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi
AS mentioned in this reference is for the T5. But you could go to 'smokemup' http://www.smokemup.com/auto_math/mph_range2.php

and input your 4 speed muncie and see your outcome. I'll bet the 3.08:1 would be just fine. Cause you really only have three choices - 3.08, 3.40, 3.73 besides your 4.11 and your 4 speed starts out in 2nd anyway... and you live in a fairly flat environ (vs. california)
you are better off just rebuilding your 12 bolt. Many of us only have non-rebuildable tube rear ends, always worth replacing them.
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=010333;p=1#000000

Borg Warner T5 Transmission and Engine RPM VS. SPEED RATIOS, 3.08~3.43~3.73~4.11:1 REAR END RATIOS.

Speed vs. RPM For Tire and Gear Ratio
A real accurate way to determine your tire diameter is to draw a line on the bottom of your tire, then draw a line on the ground which lines up with the line you just drew on your tire. Then move the car until the car makes one full rotation of the tire and mark the ground where the line on the tire is at the bottom again. Now measure the length between the two lines and divide by pie (3.14) and you'll have your tire diameter to use on the calculator (don't forget to use inches). This is the preffered method to use in determining your actual tire diameter. Tire sized 28.5'' to a typical modern radial Chevy standard size rim. If yours is taller then add MPH.

The columns are a bit off but they really do line up properly. My spreedsheet did not line up as I thought. This is purely for your information. Accurate to +/- 4%.
Enjoy

CHEVROLET T-5 Borg Warner Transmission Ratios:
1st: 2.95 2nd: 1.94 3rd: 1.34 4th: 1 5th: 0.74
Tire Diameter: 28.5 inches
Rear Gear Ratio: 3.08:1
READ MPH ACROSS ROW /READ RPM DOWN the FIRST COLUMN, then LINE UP THE SPEED IN EA. CORRESPONDING CROSS COLUMN.
RPM 1st 2nd 3rd 4th 5th 6th
1000 9 14 20 26 36 0
1500 13 20 30 40 54 0
2000 18 27 39 53 71 0
2500 22 34 49 66 89 0
3000 27 41 59 79 107 0
3500 31 48 69 92 125 0
4000 36 54 79 106 143 0

Thanks Jim and Joe for the replies. Great info. I have done alot of searching on this last few hours, links and threads but did not see much about the swap with c-10s, mostly 3100s. Is it still as 'easy'?

I recently changed the 3.42's in my 1992 GMC shortbed to 3.08's and I am extremely pleased, my gas mileage went up 1 1/2 to 2 Miles per gallon and driveability is improved. I have the 350 engine with over 310,000 miles on it, kind of tired but works quite well. I would recommend the 3.08's as a great driving gear especially if you do much freeway driving.

I have a 235 with a "modern" 3 speed (70's) syncro trans - kept the "on the tree" shifter which i love and also have an 89 blazer rear end (to keep the 6 lug original wheels) the rear end is a 2.73 ratio and I love it-- I am on the interstate a lot and go 65 and 70 with ease--
I went to one of those sites that calculate the wheel/tire ht. and rpms and speed to decide what I wanted...any way works great for me! I get around 20 mpg which I think can be improved some...

Raiderhater, the mountains say hi back...

Actually, we're leaving the Denver area. The "imported" people here are so unbelievably rude in their SUV's that we've wanted to leave for a long time.

We're New Mexico bound.
About Smokemup.com, I think you have to register and pay some nominal fee now to use their automath service, which I have always found to be excellent. It used to be free. Anyhow, it is a nice way to help determine what highway gears you want, based on the desired rpm at a certain speed.

If you have a granny-geared SM420, a 3.08 might actually make the steep first gear more usable. Then 4th will feel like your overdrive.
But anything taller than 4.11 will help.

I have a 3.08 rear in my '55 1st GMC Suburban. Engine is the GMC 270 and transmission is the ever popular SM420.

I find it very drivable as long as it is empty. Hook a trailer on behind it and it does not have enough grunt to feel happy going down the road.

If you want a good highway friendly driver, and do not expect to haul much of a load, I think the 3.08 is a pretty good choice.

If you want to pull a heavy trailer you have the wrong motor to start with...

Thanks again all to replied. I have had too much time to research this and now I am leaning toward putting in a T5 5 speed. Seems like I could keep the rear gearing for now and still get overdrive. Then eventually could regear the backend to something like 3.73 or the 3.55 if really needed. Seems like I could make either swap for about the same dollar amount, or cheaper if a reasonable tranny could be found and a mechanic could help me out on the weekend. Volunteers?

Yo Hot Rod Dad, the granny first is useable! I used it in a parade and it worked great - actual walking speed idling my 292. And its also been helpful when pulling heavy things. But you are right, most of the time you don't use it, and if you are not really working the truck...