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- - 700r4 install (https://67-72chevytrucks.com/vboard/showthread.php?t=266800)

Re: 700r4 install

I use a spreadsheet to calculate engine spd vs. RPM for various combinations of trannys and rear ends, but I never checked 4.11 with a 700r4. Should be OK, seeing as how it's not that much taller than a 3.73. Your cruising RPM at 70-75 would probably be around 2500 RPM or so, which isn't great but is a lot better than your 3-spd. I don't know how it would effect your first gear. Give me your gear ratios and I'll do some calculations.

If you want to stay with the 3-spd, you'd probably be fine with a 3.73 or even a 3.42 depending on your gear ratios. Of course, a synchro would be nice too...

Quote:

Originally Posted by Simbone47 (Post 2451004)

Not sure how gas mileage became the topic. That was never the issue or I wouldn't be driving an old truck. It's about usability. I'd like to be able to operate on the highway. Again, I worry that a simple rear end change will affect my low speed performance, since I don't always have access to downshift to first, this will create a different problem for me. Anyway guys thanks for all the comments.
Chris

Re: 700r4 install

Quote:

Originally Posted by Simbone47 (Post 2451004)

Sorry, I went back and read your original post. I guess all the talk about down sizing your fleet, highway speeds and rpms just put me in the mind set of fuel mileage.

Re: 700r4 install

Sounds great, but... I'm not going to consider an under/overdrive in earnest unless it shifts into and out of overdrive automatically in every gear. I have an automatic and I want to keep it automatic! :) Plus, the 700r4's OD gear ratio (0.70) is lower than gear vendor's "3rd-over" (0.78) which is better for cruising, cruiserman :lol:

Quote:

Originally Posted by cruiserman (Post 2451189)

Add an overdrive to your TH350.

Re: 700r4 install

My 2 cents:

If you don't mind the manual, perhaps a 4-or 5 speed with overdrive would be the better choice over a TH700. Fewer mods to install.

Also, there's the Gear Vendors bolt on Overdrive option for about the same price as you were quoted for the TH700 upgrade. That'll give you a lower low and a higher high.

I agree with the member above that suggested a 3.08 as the cheaper option. I'd go that route 1st if you're serious about making this a DD. After swapping to it, if your truck won't take off in 1st with a 3.08 then there's something wrong with the engine, the carb or the ignition. The good news is you'd still have lots of cash left over to fix it.

Finally, taller tires and rims (16" wheels with LT tires?) will tame that 4.11 gear a little. The taller they are, the lower the engine revs will be at any given speed. I experimented with a 61 Apache when I was a teen and ran 16" military tread tires on it that were about 36" tall. It tamed my 3.90 rear gear to the point that on one occasion I was able to get 25 MPG at 55 MPH. With small street tires, the monster got 10 MPG no matter how you drove it.