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73-87 Chevy _ GMC Trucks => 73-87 Chevy & GMC Trucks => Topic started by: mjolinar on September 05, 2006, 12:13:00 am

Title: Poor Fuel Economy
Post by: mjolinar on September 05, 2006, 12:13:00 am

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?

Title: Re: Poor Fuel Economy
Post by: Blazin on September 05, 2006, 05:31:00 am

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

Title: > Re: Poor Fuel Economy
Post by: Lt.Del on September 05, 2006, 12:11:00 pm

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

Title: Re: > Re: Poor Fuel Economy
Post by: VileZambonie on September 05, 2006, 09:44:00 pm

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.

Title: Re: Poor Fuel Economy
Post by: mjolinar on September 18, 2006, 11:38:00 pm

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