RPM's and MPH?

[L31MaxExpress]

My 97 Express managed 18-19 mpg with the 350. Stock, tuned, cam, headers, 10 bolt 3.42 to 14 bolt 9.5 with 3.73 from 60E to 85E it never changed much. 5.13s did knock it down about 1 mpg, turning 3,000 rpm @ 75. I put the 3.73 10.5 14 bolt in it with the 383 and when it had the Rhoads V-Max lifters, made a ~500 mile road trip that managed 20.6 mpg. Best I have ever seen out of it. Removing the rhoads lifters and installing the marine intake killed mpg by about 3 mpg. Best I have seen since is 17-18 mpg. Wind even a crosswind and rolling grades can kill mileage off a bit. Alignment, brake drag and tire pressure can also drastically effect mpg. About 5 years ago, I had a brake hose collapse internally, slightly hanging the RH front caliper, warping the rotor that caused a slight front end vibration and cost 3-4 mpg on the highway. The rotor was not smoking hot either, but the wheel was about 50*F hotter than the LH side after about 1 hour on the road.

 

[L31MaxExpress]

My 97 Express managed 18-19 mpg with the 350. Stock, tuned, cam, headers, 10 bolt 3.42 to 14 bolt 9.5 with 3.73 from 60E to 85E it never changed much. 5.13s did knock it down about 1 mpg, turning 3,000 rpm @ 75. I put the 3.73 10.5 14 bolt in it with the 383 and when it had the Rhoads V-Max lifters, made a ~500 mile road trip that managed 20.6 mpg. Best I have ever seen out of it. Removing the rhoads lifters and installing the marine intake killed mpg by about 3 mpg. Best I have seen since is 17-18 mpg. Wind even a crosswind and rolling grades can kill mileage off a bit. Alignment, brake drag and tire pressure can also drastically effect mpg. About 5 years ago, I had a brake hose collapse internally, slightly hanging the RH front caliper, warping the rotor that caused a slight front end vibration and cost 3-4 mpg on the highway. The rotor was not smoking hot either, but the wheel was about 50*F hotter than the LH side after about 1 hour on the road.

The fuel economy math I setup in HP Tuners is pretty darn close. This was rolling 70 mph on flat road and almost no wind.

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[L31MaxExpress]

The fuel economy math I setup in HP Tuners is pretty darn close. This was rolling 70 mph on flat road and almost no wind.

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I had similar math setup in my old EBL setup on my 83 G20. It was a stock L05 350 with 1.6 rockers, performer rpm intake manifold, 454 TBI unit on a marine adapter and thorley tri-ys. 700r4 and 3.08 gears. I cruise near the speed limit with the flow of traffic, 70-80 mph most of the time.

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[Sean Buick 76]

Cool combo

 

[Sean Buick 76]

Having about 100,000 km on these trucks I will tell you. With the TBI 5.7 keep it at 1800 rpm on the highway and just go slow to make that happen. Doing this with a worn out TBI 350 I got 12 mpg and once I swapped in a brand new stock crate engine I saw 15 mpg. Although once I had the new engine I tended to drive faster and had more fun lol… and I dint think I checked the mileage after that.

 

[PlayingWithTBI]

I have the same Cooper Cobra tires (275/60R15 on all 4) as OP's with 4.11s. It runs ~2800 RPM @80 MPH and I don't care about mileage - just keeping up with traffic

 

[95OBSMan]

Thanks for all the reply's, excellent info and visuals!

So it sounds like, run it as is, and go with the flow until its time for a new engine! If it ain't broke don't fix it! Thanks again guys!!!