Gas mileage increase- six to BB OD
 

Just wanted to post this for who ever is interested, but I replaced my 250 six Powerglide trans with a .030" over 454 and a overdrive automatic and got over two miles per gallon better mileage. Always got around 9.8 to 10 with the old power train. Just got 12.3 mpg the last tank with the big block overdrive.

Re: Gas mileage increase- six to BB OD
 

What trans? I made the switch from 250 to 402 but I kept the TH400 and I have not had a chance to check the MPG because the TH400 gave up the ghost.

Re: Gas mileage increase- six to BB OD
 

Good that the milage is better, still seems low to me. 6 with powerglide I would have guessed 15-18 mpg....What gears are in the rear?

Re: Gas mileage increase- six to BB OD
 

The best I got with mine with the six was 14 with the stock 3.73 gears.

Re: Gas mileage increase- six to BB OD
 

Running 3.73 gears. Tires are 31.3 inches tall. Running 2474 rpm at 80 mph. The new transmission is a 4L80E with 2200 to 2400 stall factory converter. Running two Edelbrock 500 cfm AVS carbs on the 454.

Re: Gas mileage increase- six to BB OD
 

Certainly the real smile on your face comes from the most important gain of the swap:

HORSEPOWER!

Could there be a worse slug than a 250 'glide in one of these heavy barn shape trucks (??!!) Maybe a diesel Chevette...... :)

Steve weim55 Colorado

Re: Gas mileage increase- six to BB OD
 

I drove a diesel Chevette. It was slow!

Re: Gas mileage increase- six to BB OD
 

I ran my 250 six for a few years with the stock 3 spd and 3.73 rear and the best I ever got was 11-12. Really liked that motor though. Always lit right off and idled like butter. Kinda mad at myself for yanking it out for a 350 many years ago. Looking back I wish I had just added a T5 behind it.