Rear end gears

[sb69chevy]

I have a 350ci and a powerglide, and I drive this on the highway on a daily basis. What would be the best rear gear to run w/ this setup that would give the best mpg, etc. My speedometer is fast...@ 70 I'm really running around 62-65 and turning approx 3000 rpm. I avg around 12-13 mpg as it is right now. I know it may not get better w/ the 2 spd, but just thought I'd run it by the experts. Thanks.

sb

[GREASEMONKEY72]

i have the stock th350 with 3.07 gears and turn almost the same rpms as you if i remember right

[sb69chevy]

I have no idea what's in the rear now. PO said he thought they were 4:11s. I haven't pulled the cover to check them.

[basketcase]

A 3.36 should run close to 3000 rpm at an actual 70 mph. A tad more with an automatic and no lock up converter. If you are running 3000 at a real 62-65 you probably have a 3.73 rear gear. Use this chart to help you decide what gears you want depending on what rpm at highway speed you are comfortable with.

Chart

Tom

[GREASEMONKEY72]

just remember to not go to low so your converter isnt slipping all the time and heating up

[Longhorn Man]

here is a non lock up TH350 (same final gear as the powerglide) with 3.07 gears, and a 28 inch tall tires.

[71chevylowrider]

What the heck? I was told by the po that my truck had 3.73's in it, I measured my tires & their about 27" but at 60MPH, my tach is at 3,500 RPM's, I've been thinking of going to a 700R4 but my th350 is built up & has a shift kit & shifts great! According to that chart I would have 4.56 gears, geez if thats true, no wonder it feels so quick & fast around town!

[mnunn454]

Once you find out what rear gear ratio you're running then prescribing the best transmission to use will be fairly simple.

Advantage of the Powerglide is it's almost indestructible. Disadvantages are it's lack of an overdrive and lack of a decent 1st gear which helps to get a heavy vehicle moving off the line.

The TH350s and 400's offer a better takeoff gear, but no overdrive. The TH400 is as strong as the PG but the TH350 isn't unless it's intentionally built that way with HD parts.

The best trannys ratio-wise are the TH200, and TH700s because of the 4 forward gears and the lockup and the overdrive, but they're generally not as strong as the PG and are a little less tolerant of overheating. They also require the throttle valve cable system to be set up properly, or the innerds will get damaged. The TH700 is longer than the PG and will therefore require a shorter driveshaft and a moved or special crossmebber.

You don't want to run a steep (numerically low) rear end ratio with an overdrive tranny, because your engine will "lug" at highway speeds in overdrive. However if you find out you have a Ring and Pinion ratio in the 3.42 to 3.73 range, that's usually ideal for an O/D tranny unless you're going to run super tall tires.

Let us know what you've got.

How?, you say?

Sometimes there's a metal tag on the cover bolts, and sometimes the casting numbers on the rear housing can be desciphered (assuming the PO hasn't changed them). But the easiest way IMO is to place the truck on a flat surface, in neutral, engine off, and make a (white mark on the driveshaft front to back). Then have someone push the truck very slowly forward iin a straight line, one full rotation of the rear tires (valve stem at 6:00 position to valve stem at 6:00 again), while you count the number of driveshaft rotations. 3.1 turns would be close to a 3.07 ratio. 3-1/4 would be about 3.23 and 3-1/2 could be either a 3.42 or 3.55. 3-3/4 is very likely a 3.73. A tad over 4 turns is most likely a 4.11.

The most accurate way is messy, but pull the rear cover and count the ring gear teeth, then divide that by the number of pinion gear teeth. This has the added advantage of installing fresh lube and a gasket, and to do a visual inspection of all the parts.

Good luck.

[sb69chevy]

Thanks guys. Whenever I get the time, I'll pull the cover and count them. Like you said wouldn't hurt to lube and inspect anyways. Besides, I don't have anyone to help me push. How much lube does it need and what weight do you recommend using. Sorry I'm new at this...as if it's not obvious or anything.

Also, if I stayed w/ the powerglide, is there a rear gear that would be the best setup for the highway. Generally speaking.

[cgoodwin]

munn454,

are you saying with a th350 a 307 rear gear is a good mileage gear and the motor won't bog down on the highway?

or would it be better to go to a 342 rear gear?

Just looking for more input. thanks