Aww, heck, I'll go for the straight answer.
V-8s get better mileage.
As far as the engine choices for Chevy trucks, comparing SB V8s and stovebolts, the V8s will give better mileage. Why?
Better 'specific fuel consumption;, i.e. how much fuel is burned to make how much HP per hour. This is typically measured on a dyno. The V8s give better SFC than the sixes.
Why? Better breathing and higher compression ratio.
It takes a certain amount of HP to push your truck down the road at 55 MPH. Whether it's a 6 or an 8, the amount of work being done is the same. (Actually, given than the SB is about 100 pounds LIGHTER than the 6, it's doing *slightly* less word, but the difference is very small.)
The SBC will turn in better mileage than the 6. Now, if you take advantage of all the extra V8 power and decide to drive at 70 instead of 55, you will get worse mileage, because it takes a lot more HP to go 70 than 55.
Usual disclaimers: we're talking properly tuned street engines here, etc.
Cheers!
George
