Poor Fuel Economy

[mjolinar]

I have a 78' 4x4 SWB 1/2 ton with a Small block 350 and TH350/NP203 converted to part time, with 3.73 gears in the axles. Carburation is a Holly 670 truck avenger. I get about 8 mpg on the freeway and think it should get much better than that.

I have seen the exact same truck with a 4 speed and NP205 get 17-18 mpg on the freeway.

Any suggestions or ideas?

[Blazin]

Tune up, fuel / air filters, timming adjustments, carb adjustments?

[Lt.Del]

If all you do is freeway driving and no hauling/towing, then get rid of the 3.73 and throw in a 3.08 or higher rear.  
My 79 Big 10 with th350/383 stroker engine gets about 13 city, or higher, 16 mpg on the interstate.  I have a 3.08 and she will get up and fly on the interstate.  It has a new qjet and I don't open up all four often.  

SgtDel

[VileZambonie]

Meter down that carb first of all. 8MPG if properly tuned is pretty bad. Leave the 3.73:1 and swap in a 700R4/np208 drivetrain.

[mjolinar]

Changed the jets. Seems to have improved it quite a bit. Going to mess with timing adjustments next.