TH350 or 700r4

[bhammer]

My 72 has a TH350 in it now. I have been doing a little research on 700r4's, does it really get better gas mileage putting in a 700r4? Now my truck is getting somewhere between 8-10 mpg... I would love to have better mileage, is it worth the swap?

[cparman]

I am not sure about the mileage, but the engine will turn about 750-1000 rpm less. If you currently have a TH350 in your truck, I would suggest a 200r4 instead of the 700. The 200 will be bolt for bolt, and the driveshaft will not have to be cut. All of that equates to less money spent to get to the same place. AS long as you can get a 200 for the same money as a 700.

[Wrenchbender Ret]

It depends on what the rear end ratio is & what engine you have. Also what kind of driveing you do.
I have a 70 stepside, 350 eng. 3.08 gear ratio, & 700-4R. I get 16 MPG combination & 18 all highway if I take it easy.

[bhammer]

It has a 350 in it. It may be out of tune a bit, but it runs good now. Don't think that would save me all that much as mileage. I am not sure on the rear end. I have no way of knowing without taking it apart. I am assuming it is a 3.73.... It is a daily driver for me and it is mostly city driving. I am just wanting to keep it a daily driver without having to fill up every 2 days...

[bhammer]

It has a 350 in it. It may be out of tune a bit, but it runs good now. Don't think that would save me all that much as mileage. I am not sure on the rear end. I have no way of knowing without taking it apart. I am assuming it is a 3.73.... It is a daily driver for me and it is mostly city driving. I am just wanting to keep it a daily driver without having to fill up every 2 days...

[FRENCHBLUE72]

Id give the 200 a try I know I am going to when my th350 dies...

[mrein3]

Its all about the rpms your turning.
If you drive city like you say, you probably aren't going to save much with a 700r4.
A th350 or th400 will have a couple of hundred rpms wasted because of the slip in the torque converter. If you run a 700r4 aournd town you would probably have to run in its 3rd gear which is 1:1 just like a th350 or th400. However with a lock-up converter your final 1:1 has no slip if you wire it up to lock up in 3rd gear. Your savings will be minimal.

On the highway, going 55mph, I run around 2800 rpms. IF I had a 700r4 its final ratio is .7:1 so knock off 30% or 840 rpms = 1960 rpms. Take off the bit of slip in the th350 converter and you're in the 1600 - 1700 rpm range. On the interstate at 70mph the same 30% reduction is applied to final drive.

Reading this forum you hear of folks running 3.73 rear ends getting quite a jump in mpg. Guys with 3.08 don't do so well and some even LOSE miles per gallon.

Beg, borrow, or steal a tach and make note of where your engine is at the speed you usually drive. Figure out your rear end gear. Then ask that same question here.

[mclairmo]

I am pretty sure you can determine rear end ratio without taking rearend apart. I remember jacking up the rear, painting a line on the drive shaft and tire and spinning the tires while counting the number of times the driveshaft turns. Not exactly sure how to do this anymore or if it works on these trucks.