700r4 swap? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

grizz · 03-13-2006, 07:48 PM

will putting in a 700r4 in place of a 350 3 speed help with fuel mileage. what would the best rear end ratio be. would i need to replace the trany mount crossmember.thanks

**pjmoreland** · 03-13-2006, 09:53 PM

I replaced my TH350 with a 700R4 and went from about 12MPG to about 16MPG with my 350 V8. I have 3.73 gears. Personally I wouldn't go any lower than that because it would really start to bog the engine when going up hills in OD. 4.11s probably wouldn't be too bad. My '00 Tahoe has 3.42 gears, and is pretty sluggish to get going. It gets 18 MPG or so on the highway, so I guess it's a tradeoff, but of course it's a 5.3 with fuel injection, so I'm sure it's not all because of the gears.

03-13-2006, 07:48 PM	#1
grizz Registered User Join Date: Jan 2006 Location: indiana Posts: 14	700r4 swap? will putting in a 700r4 in place of a 350 3 speed help with fuel mileage. what would the best rear end ratio be. would i need to replace the trany mount crossmember.thanks

03-13-2006, 09:53 PM	#2
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,052	Re: 700r4 swap? I replaced my TH350 with a 700R4 and went from about 12MPG to about 16MPG with my 350 V8. I have 3.73 gears. Personally I wouldn't go any lower than that because it would really start to bog the engine when going up hills in OD. 4.11s probably wouldn't be too bad. My '00 Tahoe has 3.42 gears, and is pretty sluggish to get going. It gets 18 MPG or so on the highway, so I guess it's a tradeoff, but of course it's a 5.3 with fuel injection, so I'm sure it's not all because of the gears.



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