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Old 10-21-2005, 12:48 PM
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axle swap

I have a 70 suburban with a 350sb, a torque(gear)splitter with an sm465 and a 3.42, 12 bolt chevy rear end. At 65mph I'm cruising about 1800 RPM's. I'm thinking of swapping the rear end with some 3.08 gears to lower the rpm's a little more with the hopes of better gas mileage. Is there a rear axle that would be a direct swap with drum brakes? Also my sub has coil springs.

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Old 10-22-2005, 08:09 AM
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Quote:
Originally Posted by jose a roman jr
I have a 70 suburban with a 350sb, a torque(gear)splitter with an sm465 and a 3.42, 12 bolt chevy rear end. At 65mph I'm cruising about 1800 RPM's. I'm thinking of swapping the rear end with some 3.08 gears to lower the rpm's a little more with the hopes of better gas mileage. Is there a rear axle that would be a direct swap with drum brakes? Also my sub has coil springs.

Since the truck has a coil spring rear suspension , you need to find a matching diff from a similar truck / burban. Or find a diff with the proper width then weld the needed mounting pads onto the new diff.


Or have the gears changed

At 1800 rpm's at 65 mph , I would not want a carburetor equipped engine to spin much lower RPM. Vacuum would fall off , the carb would richen up and mileage may get worse
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Old 10-22-2005, 10:34 AM
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Quote:
Originally Posted by Crosley

At 1800 rpm's at 65 mph , I would not want a carburetor equipped engine to spin much lower RPM. Vacuum would fall off , the carb would richen up and mileage may get worse
If everything were perfect you might gain 1/2 mpg.
I agree about the carb. You'd have to go to annular boosters, etc. to get the thing to atomize at the low velocity.

The best bet would be to install a SFI from some donor car. Street and Performance in Mena AR can help sort it out.

Guessing that you could do it used for $ 800..... For the extra 3 mpg, what it would cost will buy a lot of gasoline.
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Old 10-24-2005, 07:18 AM
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Quote:
Originally Posted by jose a roman jr
I have a 70 suburban with a 350sb, a torque(gear)splitter with an sm465 and a 3.42, 12 bolt chevy rear end. At 65mph I'm cruising about 1800 RPM's. I'm thinking of swapping the rear end with some 3.08 gears to lower the rpm's a little more with the hopes of better gas mileage. Is there a rear axle that would be a direct swap with drum brakes? Also my sub has coil springs.
My 1988 Suburban is running 235/75/15 tires, 3.42 gears, and a TH-700R4 transmission with a .70 overdrive gear ratio, at 60 MPH my RPM's are around 1700 giving me a mileage of 14 MPG but my driving habits of this thing are far and few between as I am doing work on it constantly, (currently replacing rear brakes) I gassed up 2 weeks ago and only have 70 miles on it since then.

I plan on trading off to a different axle next year with a ratio of either 3.73 or 4.10 and jumping up to a 235/85/16 tire. Mathematically the 3.73 gear and the taller tires will only change the existing combination by a measly .9% at the speedometer, but the 4.10 gears will raise my RPM's more into the engine RPM range where the powerband would net me evne more mileage.

Like myself, you are running a more than 3-ton vehicle, and mileage is going to be more difficult to find than in any other vehicle, I am lucky enough to already have fuel injection on mine but I am waiting for a few things that are supposed to raise my mileage only slightly, I will be doing a test to see how they perform and if they are "as advertised" or not.
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