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Old 03-26-2012, 04:25 PM
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Improved gas mileage

I will be making several long distance trips this summer and next, and need the best possible gas mileage. With a stock chevy low mileage 350 motor and
TH 350 transmission, what would be the best rearend to install for best performance and mpg?

If I were to change out the transmission, what would be the best transmission/rearend combination to get the best performance and mpg?
Thanks

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Old 03-27-2012, 08:07 AM
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Theres a small trick to this; its not as easy as dialing in a manual transmission in my opinion (but then again Im a stick guy).

You can't just multiply 1st gear by the axle ratio and try to get enough gear multiplication to move the car with a rear low enough (numerically) for economy

You need to take the convertor into account, along with the camshaft of the motor. While it may not apply to your specific application, if you get the rpm's too low in high gear, it will make the engine less efficient.
That being said, you also need to look at the type of axle, you may not be able to drop the ring and pinion to the lowest (numerical) set available, as it may take some different parts to install them in your specific rear axle.

I'd say 3.23:1s are a good compromise for the TH350, for a comfortable 1st gear and keeping the highway revs down.

lightweight steel rims, and normal sized tires will help.

A TH200-4r is the same size as most TH350s, and it has overdrive. I would use a 3.23-3.55 in the rear there, again with steelies and normal tires.
Use a quality converter as well.
Also, a huge oilpan is not your friend.
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Old 03-27-2012, 12:54 PM
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Improved Gas Mileage

Thanks for the info. A bit technical for me, but still helpful.
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Old 03-27-2012, 01:49 PM
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Buy a Chevy Volt

Save gas the whiz bang boom car
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Old 03-27-2012, 02:31 PM
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Gas Mileage

You work for the Obama administration?
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Old 03-27-2012, 02:42 PM
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High gearing such as 2.5, 2.8 will increase mpg on the highway. In certain cases a lower gear such as 3.25, 3.50 gearing will give you better mileage on the street. Since your driving long distance, I'd get the highest gear ratio possible. Your goal is to keep the rpm near 2K rpm at 65 mph. An overdrive transmission will allow you to utilize a lower gearing such as 3.25 and drop your rpm when on the highway. It's cheaper to put some high gears in than the cost of changing to an overdrive transmission. With high gearing, you'll be able to cruise highways, but city driving, you'll engine will lack the git up an go...
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Old 03-27-2012, 04:01 PM
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OD tranny

u can replace the th350 to a 200R.. make sure the tranny tail sections are the same.. short for short.. u'll need lock up converter and u can buy the od switching sys so it'll work.. oh and same drive shaft too...
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Old 03-27-2012, 06:00 PM
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You can also

Install taller tires if they fit you can lessen the gear ratio that way and get better gas mileage. I did that in the 70's with the big Fords and it helped
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Old 03-27-2012, 07:22 PM
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Gas Mileage

Thanks guys, appreciate the info.
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