85' C20 rear end ratio question

[fightthewake]

my daily driver is a 1985 C20. not sure what rear end gear it has but can i swap it out for better MPG ? not really doing any towing..

[zieg85]

First of all since this is your first post I'd like to welcome you from NW Indiana.  We need additional info to help you out.  What is the current mpg, engine, trans combo. Is the engine stock with the original carb?  How do you use your truck, highway, in town, lots of hills, how fast do you generally go?  If you still have the sticker on your glove box it should say the ratio or at least give the RPO code for what came in the truck originally.  I have a 85 C20 myself.

[fightthewake]

thanks for the welcome !! current mpg around 9 or 10 i think with stock 350 with th400 tranny.rochester carb.mostly highway back and forth to work, but sometimes around town.and usually running 70, or 65 mph on trips to work. the glove box i replaced a couple years ago cause it was cracked.

[zieg85]

Well the speed is what is sucking the go go juice.  You should have a 14 bolt semi float axle.  Highest gear set for that one is a 3.42 I believe.  You need to determine what you have by taking off the rear diff cover and see what is stamped on the ring gear.  You may be able to squeeze 15-16 mpg out of it if everything is perfectly set up and you keep it under 65.  My $.02

[blazing816]

I have a k10 with a 305 and a TH350 with a Holly 600cfm, when I am not towing and keep it right around 65 on the highway, I can pull about 12-13mpg. That is with a 3.08 gear in the rear and front and mine goes through the transfer case (being 4x4)...which supposedly reduces MPGs. However whenever towing or passing people on highway and go above 65 the gas can drop down to like 9-10mpg (especially without overdrive like you do not have). So it can be done you just have to fine your perfect setup (like zieg said), mine is not even ideal for me because I tow a 23 foot travel trailer and let me tell you that 305 and 3.08 gear does not let me get up and go at all with the trailer....it is bad on the highway.