The Stovebolt Site Forums The Shop Area Engine Shop Rear end Gear

Hey All,
Im runnin a mid 70's rear end (chevy malibu) with a TH350, 15 inch 60 series tires on the back.
Any of you Gear Gurus have a suggestion for a good mileage ring gear that would be good for my truck?
Now I havent checked it, but driving tells me it is geared high, at cruise in drive she is pullin way too many RPM's. at 55 or 60.
I have to be on the fuel steady to cruise.
I have owned a few mid 70's cars and never had one ya had to PUSH down the road with the excelerator.
Im very novice at gear ratio's so ANY help will be great!
soon as I get a helper im going to try to rotate the rear tires and count the drivesaft revolutions as suggested here.
If I understand correctly, somethin down in the low 3's as far as gearing would be best for her to cruise down the road without the motor pushin it correct?
thanks
Tim

tim, higher numbered ratios, say, 4.56, is a low gear, and will give you high rpm's at cruiseing speed, but will launch you off the line better.
a lowered numbered gear ratio, say, 2.79, is called a high gear, and would give you lower rpm's at cruise speeds, but wont launch as good off the line as a 4.11.
but, im sure you are like me, not wanting a race truck and not worried about a hard launch off the line, i too am wanting a nice cruiser that will get good MPG's.
i am gonna do the spinning the wheel test myself today and try to see what ring and pinion i have. i also have a th350, and the few times i was at 60mph i was at 2200 rpm's.
50mph is at 1800 rpm's.
hope this helps

leonard

Gudday Tim,
I think all the info you are chasing might have already been covered in the Hi-Po Shop forum under the heading
"G.M. 10 Bolt rear end question".
A posting from Lindsay (GMCanuck 4/17/08) inludes a link to a GM rear end ratio calculator.......mmmm very handy!
Have a read anyrate......some interesting stuff !
All the best....Doc Bob

Something you guys with the T350 transmissions might consider is to have them converted to a lock up converter if thats possible. I know the later 350's had lock up so it might be an easy swap for transmission shop. Then you could keep your lower gears and have a decent cruise rpm. I went to 3:08 gear ratio with a 250 ci, T350 combination. A 3:23 would have been better, the 3:42 was to high for 65 to 70 mph cruise speed. The 3:08 is about to low, the truck is pretty sluggish around town, I guess I was used to the peppy starts with the 3:42. Another good option might be the 4R200 trans, a four speed overdrive thats about the same size as the T350. Joe

I don't see where it say's what engine you are running. That will be the big factor. The actual diameter of the tires would make a difference also.

Is the TH350 an overdrive tranny?

No. Later versions (350C) have a lockup converter in them to help feul mileage. sometimes that gets mistaken as an extra shift into "overdrive" during driveing. That trans isn't really desireable with a high power engine, as it is considered to be rather weak. A 700R4 would be a better choice.

Radial, we had 3.73 gears in our 72 Nova with a 350 engine,350 tranny, ran about 3000 RPM's when cruising on the highway, switched to 3.08 gears when my son took it to college. The 2 1/2 hour highway drive was to hard on gas and the engine. We put 3:73 gears in our 53 pickup but we are using a 700R4 tranny with overdrive ,the rpm's are around 1500 when cruising at 60 mph . We have a much more powerful 350 in our pickup.

Tim, I'm running the 3:73 under the rear of my 37 with a strong 350 engine and WAS running a 350 tranny. I hated the gas mileage and the engine rpms.
Now, I'm not recommending anything here but just saying what I did and I did it because it was available to repay a debt to me.

I got a 700R4 that is fresh built and put it in my 37. WOW, I don't know the rpm difference as I do not have a tach but I can tell you it is running a lot less rpms. It "seems" the mileage double but I have NOT actually checked it.

I have a friend that has built trannys for many years with his dad and now runs his own business. He builds just about any kind of automatic tranny and also builds them for drag cars and rails.
He recommends the later model (87 up?) 350 tranny with the lock up. I know its been said they are weak but Paul told me to let HIM build me one and I would not have to worry about it.

Tim, right now I am happy as a little pig in a slop bucket with my 700 but I wouldn't pass up a good late model 350.
Note: if you pull the 350 tranny and go to a 700 you will have to move your cross member back about 4 inches and have your drive shaft shortened.
Hope all this helps in your decision making....in the meantime drive it and enjoy it. You SHOULD show off the fine job you did.