gas mileage with 383

[jklonowski]

i have a 383 with 700r4 setup and a 12 bolt with a 3.08 gear in it. i have read a few threads over the last month or two and i was wondering why people would say they would get better gas mileage with a lower gear such as a 3.43 or 3.73. that didn't make too much sence to me that's why i was asking. thanks for the replies you guys are a lot of help.

[N2TRUX]

There are a lot of things to factor in, but lets start with running gear. What cam the motor has, coupled to what transmission is sigfinicant. Combine that with tire size and height and you can see there are a lot of variables.

As an example, if a truck is running a TH350 and a 3.08 gear with 29" tall tires. The TH350 has a moderate first gear thats rather sluggish when combined with 3.08s and taller tires so it might get better MPG with a 3.42 gear.

In your case with the 700r4 having a lower first gear, the 3.08 is probably more efficient. Unfortuntaly it's a trade off for torque that you will feel at midrange. With the OD transmission, a 3.42 would be a great gear but you will loose 1-2 mpg. You will loose another 1 mpg buy going with a 3.73

[fencepost]

Couldn't figure out how to post the actual chart but here is the link to it. It shows ideal RPM's at 65mph to get good mileage and power pretty neat.

http://www.4lo.com/calc/geartable.htm

[piecesparts]

Originally Posted by jklonowski

i have a 383 with 700r4 setup and a 12 bolt with a 3.08 gear in it. i have read a few threads over the last month or two and i was wondering why people would say they would get better gas mileage with a lower gear such as a 3.43 or 3.73. that didn't make too much sence to me that's why i was asking. thanks for the replies you guys are a lot of help.

I am one of those that got better mileage with a lower set of gears. My Son's 86 Chevy truck had 3.08 gears and we put in a 700 R4 tranny and LOST MPG, it dropped to 10-11 MPG. We replaced the rear end gears with a set of 3.73 gears and the mileage increased back to the 14 MPG. The engine was basically stock 350 replacement engine with a upgraded cam to an RV style set of lobes, an Edelbrock 600 CFM carb and an Edelbrock Performer intake.

The truck, with the 3.08 gears was running down the highway at 70 MPH and turning around 1800 RPM. That allowed for a lot of the engine trying to recover speed after a hill or when you tried to pass someone. With the lugging of the engine below the power band, it was working to hard. The 3.73 gears put the engine into it's power band at around 2300 to 2500 RPM(at about 70 MPH) and it doesn't have to work as hard to get rolling or maintain speed. I also have a 700R4 tranny in my 84 GMC truck and it has a 385 stroker engine with the 3.73 gears. I used to run the 3.43 gears and the installation of a lower gear set allows for better acceleration and easier maintenance of speed on the highway. The average gas mileage on my truck is around 14.5 to 15 MPG. That is why the lower gears helps out in some cases.

[nico]

I have a 383, with a Comp 280H (.480 lift, 230 duration at .050) plus 1.6 rockers, headers, MSD ignition, Edelbrock Performer RPM intake and a 750 Demon vacuum secondaries. It runs through a 700R4 with a 2500 stall lockup converter and 3.73 gears on a Detroit Locker... With my 275/50/17's, I get around 12-13 mpg cruising at 65-70 mph.

I have always had the 3.73 gears, but nearly doubled my mileage by switching from a manual-valvebody TH400 to the 700R4...

[jklonowski]

thanks guys i appreciate the help and all the replies