The Stovebolt Site Forums The Shop Area Driveline 1972 Rear end gearing

I have a '72 1/2 ton Chevy. What factory rear gear ratios were offered for this truck and does this rear end have a specific name? It has a 307 w/th350 transmission.

My plans are to update to a 350 and 700r4 someday and figure I should look for the best ratio for the daily driver routine in this work truck for best gas mileage.

Thanks, Jeff

The cover on the differential determines how it is identified
eg: 10 bolt
12 bolt
1/2 ton most likely is a 10 bolt.A 3:08 ratio wouldn't be bad

Thanks, guess I'll go peek at it when my wife gets back from work with it.

Jeff

I think it would be a 12 bolt cover. Probably the 3:08 gear in it.

My 1970 Blazer has 3:08's in it. I have owned 6 blazers, 4 pickups, 2 panels, 1 burb in the 67-72 style and had 3:08, 3:73, 4:11, 4:56 3/4 tons 5:17 duelly 1 ton. Your rear is a 12 bolt.

Best gear ratio is determined by the type of work and the tire size.

A general guideline would be the following

3.73:1 with 28 inch tires
4.11:1 with 31 inch tires
4.57:1 with 35 inch tires

These numbers can be tweaked for a bit for more economy or more work.

Run the numbers on a gear calculator to see what works best for you. I have had several different gear ratios and tire combinations on my truck. Less rpm at a given speed does not necessarily translate to better gas mileage. It often can translate to less than stellar performance with no mileage increase. My truck did great with 3.70 gears and 28 inch tires. When I changed the tires to a 31 inch tire, the 3.70 gears weren't enough. The truck just didn't have any guts. Milage increased from about 14 mpg to 14.5 mpg but performance decreased significantly.

John,

What engine are you running with your guidelines?

The 307 has done great so far, espescially after adding the Rochester Q-jet and throwing out the old stock carb. The Q-jet is from a 350 and can cause the engine to stumble if you stomp it to the floor from idle due to the difference in CI. I've been driving it that way for a few years now and seldom do I have a need to open the secondaries on it. It gets good mileage, between 14-16 mpg for all around driving and about the same carrying a 1K pounds of material around town. Over 1,500 pnds and fuel economy definately suffers. I think with the added addition of CI from a 350 plus the 700r4, the combo will be great.

I believe it has 29-30" tires on it.

Thanks, Jeff

The engine isn't really important...

I am running a torky Pontiac motor... 389 with a 1/2 inch stroker crank. 447 inches total with a 5 speed od transmission.

Some interesting engine facts for my application a 59 gmc 1/2 ton longbed.

When I got the truck it got 10 mpg with 3.9 gears, a 336 v8 and sm420 transmission. (28 inch tires)

I quickly changed the transmission to a 5 speed with .73 OD ratio and changed the rear gears to 3.38:1. Mileage increased to about 12 mpg. (still using 28 inch tires) Engine noise was significantly reduced at highway speeds.

336 v8 got tired so replaced with current motor 1959 pontiac bored and stroked to 447 inches. New motor destroyed the 3.38 rear in less than 30 miles. Replaced rear with a 3.70:1 ford 9 inch rear. 14 mpg (still using 28 inch tires)

changed rims and tires to 31X10.5 tires. Mileage increased to 14.5 mpg. Performance was sluggish in all gears. The taller tires effectively changed the 3.70 ratio to 3.34 ratio. 4.11 gears would cancel out the taller tires putting the truck back to where it was with 28 inch tires and 3.70 gears.

I then changed the rear ratio to 4.30:1 instead of 4.11 gears. The difference is about 100 rpm at 70 mph. Mileage decreased significantly to about 12 mpg.

Why such a drastic loss of mpg? The higher profile of the truck with the taller 31 inch tires actually creates more of a drag at highway speeds. The drag is caused by the air movement under the truck.

Taller tires will lessen rpm at freeway speed, but can also rob mileage by increased rolling resistance and cause other drag variables.

Anyway... look at new cars or trucks and compare what gear ratios are being ran with tire size. That will likely be around the guidelines above. Again... the numbers can be changed a bit. taller tires or higher gears can be substituted to move towards economy or power.

My pontiac likes lower rpm around 2200 for cruising. Your sbc might actually like higher or lower rpm. I think cruising at 65 mph, rpm shouldn't be lower than 1800 or higher than 2600. Again those numbers aren't set in stone and don't take every possible variable into consideration. A very light vehicle might be happy at 1600 rpm.

My mileage experience is based on carbuerated vehicles.

I've looked and have not found.... a web site or calibration site where you plug in your ratio's and tires and it will tell you of your final gear.....say a 700r4 with a 3.90, or similar.

Does anything like that exist??

is this what you are looking for


http://www.onlineconversion.com/bigger_tires.htm

Yes it is. Thank you,Shoer. I made it a favorite so I won't loose it.

I did the same thing the first time I seen it, it has come in handy a time or 2