better gas milage questions

Are you actually making over 600 ft-lbs at the crankshaft? If not, then the TH400 has not one advantage over a built 700R-4. The build adds initial cost, but the OD WILL pay for itself.

First, you probably can't shift worth a damn, either too much running in 6th, or too much in 5th. That combo is proven campable of 31 mPG at a steady 70 MPH on flat highway with no wind.
Second, if you changed the cam, that would likewise cause a dramatic loss of MPG with stock gearing.
Third, your computer read the exhaust oxygen to maintain a stoichiometric a:f ratio at cruise for emissions reasons. Non-computer carbs can cruise a good bit leaner.
Lastly before I go to work, think how much better both your MPG and your acceleration will be when you swap the stupid TH400 for a good 700R-4.

just from my experience 1, you pay to play, 2 its a freakin dually, there is no good gas mileage..

I'm only making around 400 ft-lbs. but my crew cab dually weighs a lot more than a Camaro........ An overdrive will pay for itself eventually depending on how much you drive, but you used a junk yard trans. My th400 has over drive

what yall bitchin bout...everybody knows a powerglide is where the mpg is at....I think my race car does about 3 gallons to the mile lol

Yeah I ahvent seen a carbed car get 25 and I've had plenty even with a crappy 700r4

Secondly where is ur source for 31 mpg? Epa suggests 25 mpg highway

Diesel swap!! 18~25 mpg

4L80E. That is what will soon be in mine. Baumann Electronic Controls has a stand alone controler called the Optishift. Shiftworks has the tail housing to hook up the speedo cable. All gearing in the 80E is the same as the "stupid" th400( dont understand that comment) AND has OD

Maybe you read me wrong, I'm saying that MPG isn't the only factor in determining the ROI on an overdrive swap or gear change. Reduced wear on the engine is another point to consider...which directly contributes to ROI just hard to quantify it.

Richard