I have a 1979 Chevrolet Big Ten pick up that came stock with a 350 with the granny four speed. When we had the motor machined we punched it .040 over, added RV cam, and flat top pistons. we also made the mistake of putting a Borg/Warner 5 speed behind it (which did not last very long). When I shelled it I replaced it with another exactly alike.then i shelled that one maybe 3 months later. So I replaced with a saganaw 4 speed that lasted maybe a year before finally removing some teeth from first gear. after that I decided to go with TH-400 which has been good since. Now my problem is it is just sluggish. this motor should burn the tires off with no problem. I am thinking maybe the rear end gears are like 2.73s. which i believe would cause extra stress on transmissions and keeping me form exhibiting my acceleration abilities. i really cant afford replacing the gear lube at the moment ( new Born) so i dont want to pull cover. does anyone agree with my theory. i did notice that the TH-400 first gear is 2.48:1 and the granny second is 3.58:1 which leads me to believe the the rear end is my problem

 

i don't think trucks came with 2.73's. maybe 3.08, 3.42, 3.73, 0r 4.10's. the th400 is a power eater. great trans, but it takes alot of power to turn the turbine inside. it could be that someone swapped in a 2.73 rear end from a camaro or malibu or somethin. maybe swap the th400 for a th350 or 700r4, and find a rearend with 3.42's or 3.73's.

 

^^^^^
3.08 id say, with the 400 trans, sapping power, dead on with the grenaded trans too, any local junkyard will carry a replacement rear for under $200 when u want to change it (or afford it my blazer has been waiting for a new rear for 2 years, kids huh?)

 

Without removing the cover, you can check your ratio really easy - without even jacking the truck up. Park the vehicle and put a paint or crayon mark on the pinion yoke of the diff (or the driveshaft) at the exact bottom. Then put a crayon mark on one rear tire, right at the bottom where it meets the street. Now slowly push the truck forward so that the rear tire makes exactly one revolution and stops where the tire mark is facing straight down, while you count the number of turns the driveshaft makes.

Remember that every 90 degrees of pinion rotation is equal to .25 turns. So if the driveshaft turns 3 full turns, then ends with the mark facing horizontal towards the driver's side of the truck, that would be about 3.75 turns, or a 3.73:1 ratio. You'd be pretty amazed how easy it is to eyeball the exact ratio this way.

 

Hi briced08,

Welcome to the Chevy Forum.

If you have your Service Parts Identification sticker in your glovebox, just find your RPO code for your differential.

GU5=323 ratio

GU6=342 ratio

GT4=373 ratio

GT5=410 ratio

GT8=410 ratio

G80=POSITRACTION,Limited slip differential

I'm going to go out on a limb here and say that more than likely, it's a 342, which is great on the highway, but real doggy on takeoff. Don't take my word for it, though.

I know I don't even have to ask did you change the transmission filter...