Hey guys, I am in the process of ordering new parts to overhaul my 7.5" 10 bolt rear end. It's off from a '79 chevy malibu wagon, it featured 14" wheels and th200. I did some search and found out the gear ratio should be 2.29. looks worthless with OD trans. I will have a 700 R4 installed: 3.06, 1.63, 1.00, 0.70 gears. What gear ratio should I choose ? My engine is a crate 350 goodwrench, hugger headers, edelbrock intake and carb. TALL bias. I'll never race the car, just looking for the best mileage and highway use. Any expert advice ?
do you have any s10 4x4 trucks in france? they come with 3.42 or 3.73 gears and are about the same size as that one. I have a 350/700r4 and 3.42 gears with 205/70r14 tires. It drives just right.
Use a rear end ratio that will put the motor RPMs half way between the maximum torque and the maximum horsepower at the speed you plan on driving the most. At that speed most motors achieves the maximum efficiency.
The 2.29s are definetely gonna be too high with any O.D. tranny. There are gonna be many guys telling you that little peanut 7.5 read isn't going to hold up behind anything stronger than a 267 sbc. Welp, I used that exact rear end complete with the 2.29 ratio behind a very strong 406 sbc with a 12 inch clutch and a 4 speed saginaw in a 3300 lb , '50 chevy sedan delivery. I beat on that thing a LOT. I wanted to see just how long the 10 bolt would last. Great news is I ran an old vega 4 banger tranny..had either 3.50 or 3.40 first gear, depending on who you talked to. It worked perfect with the 2.29s and the low-end grunt of that 406 and I ended up replacing the reverse idler in the tranny but the rear never broke. I sold the car to a guy who beat on it even harder than I did and he finally blew the tranny but that puny rear end never broke and that was with 285/70R 15s....9 inches of tread width on each side. Why not run a 350 turbo and keep the 2.29s? Will be great on the hiway with a 28-29 inch tire and a 350 motor. That way you sidestep the hassles and expense of runnning a 700R. If it's just a street car you won't need the low first gear of a 700R. Plus, the 350 is lighter and less expensive to build and buy...just sayin.
To answer your question I'd go with a very popular chevy rearend ratio 3.55 or 3.73. Also most of the cars from GM stock came with 3.42's when the 700r was used and you can get an S-10 rear with that ratio cheap, the yards are full of them. Just check the width as the 2wd and 4wd are different width, pick the one that fits you needs. Frank
Gear, Ring and Pinion, 3.42:1 Ratio, GM, 7.5 in./7.625 in., 10-Bolt, This is a good all around Ratio Below are other gear set avalible. Click of ratio for a link to summit 3.73:1 4.56:1 4.10:1 3.42:1 3.23:1 3.08:1 2.73:1
With a 383 stroker SBC, 700R4, and 8.2 10 bolt w/ 3.08 gears in a 70 chevelle I was able able to get 16 mpg, cruise, and still have plenty of fun hot rodding. I upgraded to 3.73 because now I drag race the car but if I was keeping it a cruiser I would have left the 3.08's.
I run a 700R4 ( SBC ) in front of a 3.50 and 29 inch tall tires. Works great and gets decent mileage. I have a 3.73 for my current project ... but anything in between 3.40 and 3.73 should work great with your combination.
Thanks a lot for all the input guys ! I'll stay with the 700r4 as I allready bought it a long time ago, new from TCI with the lockup kit and all the accessories... Unfortunately, theres no way to find a 350 turbo or stock s-10 rear end at the local yards, so I'll stick with a new ring and pinion. 3.42 seems like a good compomise, my car has triangulated 4 link, bags and bias, I think I'll probably never race it, do burn outs or donuts... picking 8.5" rearend for this use would be worthless; even some beaten up rides hold very well with the 7.5"... as long as you dont full throttle shift or burnout too often The crucial thing is Mileage (I'd like to drive the car often and as far as possible this season, gas prices here are rediculous. We usually drive 70-80 mph on the freeway so better have an adapted final gear ratio). The summit site has a cool gear ratio calculator. I dont really know what RPM would be relevant for highway use. Goodwrench 290hp has peak torque at around 3500 RPM, looks a bit high ? On a side note: Speedway website warns that to change gear ratio on those 7.5" rear ends, you need to buy a spacer to stay with the 2 carrier setup.
Long ago I had a '78 El Camino SS that came from the factory with a 350/th350/7.5 posi with 2.56 gears. I went through 2 th350's in the time I owned it but never destroyed the 7.5. With those gears it loved to cruise in the 70-80 mph range.
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