To swap or not to swap

[zackary027]

my 81 stepside is pushing about 3k rpm at 65mph i feel like its turning a little too hard. i currently have a set of limited slip 3.73 in the truck but my parts truck has a set of 3.08's. would the gas mileage increase be worth swapping them or should i keep the 3.73. i do some in town driving with about 40% highway at 65 mph. im running a 350 bored .40, flattops, set of fairly decent heads. tranny is a turbo 350. tire size is 275 60 15

[71swb4x4]

A 700r4 would give you the best of both worlds, is that an option?

[zackary027]

it could be an option later but i just built this tranny it has maby 300 miles on it

[BigRed76]

3.08's will definitly lower rpm's and help fuel mileage. It will hurt your acceleration a little bit, but with your tire size it wont be that bad.

[gchemist]

With that much highway, yea swap in the 3.08. I run that gear with a 700R4. If you were closer, I would take the 3.73 axel off your hands.

[zackary027]

You would be surprised how many people want a 3.73 limited slip

[71swb4x4]

You have an engine with what seems a few performance goodies in it, but then you want to raise your rearend gearing to get better mileage. Will you be happy with the swap?

[JaggedEdge]

According to the variables you have given (Turbo 350 {final drive ratio 1.0}, RPM, and tire size) this calculator indicates 63 mph at 3000 RPM is only achieved with 4.11 rear end gears. Perhaps your tach or speedometer is in error.
You can calculate the rpm you will need to turn with the same combination of variables with a 3.08 rear gear by trial and error in the RPM box, or using the slider tab. Just keep testing rpm numbers until you get the speed you are looking for.http://www.markwilliams.com/calculators.aspx

edit to add: you can change variables like transmission type, wheel size ect. to look at different options. This is a fantastic planning tool to use!!! A 700r4 AND 3.08's will cut your rpm in half!!!

[cheepin]

Originally Posted by JaggedEdge

According to the variables you have given (Turbo 350 {final drive ratio 1.0}, RPM, and tire size) this calculator indicates 63 mph at 3000 RPM is only achieved with 4.11 rear end gears. Perhaps your tach or speedometer is in error.
You can calculate the rpm you will need to turn with the same combination of variables with a 3.08 rear gear by trial and error in the RPM box, or using the slider tab. Just keep testing rpm numbers until you get the speed you are looking for.http://www.markwilliams.com/calculators.aspx

edit to add: you can change variables like transmission type, wheel size ect. to look at different options. This is a fantastic planning tool to use!!! A 700r4 AND 3.08's will cut your rpm in half!!!

You have to factor in the torque convertor adding about 200rpms.Them calcs. don't do this.