before i put in the imjectors, dv, turbos, timing, etc. i was only getting 15mpg. only mods were afc gutted and no plate. intake, exhaust, and gsk.
now im getting 14-15 mpg modded. why is the mileage so bad? is it cuz the afc isnt tuned. wouldnt think it would make that big of a difference just crusing down the highway.
I think that I would start with the basics, tire pressure, tire rating E, are the brakes dragging, wheel bearings (temperature for balance to start), toe-in, wind drag... you started at 15, it should have been 20-21.
Six years ago I was driving a ton truck - the co had 2 the same - I made 16 mpg and the other made 9, we traded trucks and had the same results I made 16 with the truck that had been logging 9 and my truck made 9 with the other driver. It even made 16 when I put the Isuzu on a towbar to relocate, I took care to control windage with tarps and run easy. The other fellow drove comfortable - both feet on the floor when I rode with him.
The flatbed is hurting your mileage. I lost 3-4 mpg putting one on my '97 dually.
Even with 4.10 gears, I got 16mpg stock with it. And nearly 18 after I modded it. Flatbed knocked me back to 12-15 range. Mostly 13-14, highway or not. No matter how I drove.
Just for hahas check fuel pressure. I had a customer with a 95 12v burn through a tank in 100 miles. I found absolutely nothing wrong except low inlet fuel pressure. I put a new overflow valve in and it was fixed. I seriously thought someone was stealing his fuel, but he claimed he didnt do anything different, and the next tank was 500+. Probably not your issue, but worth a shot.
I had mine full forward, til i went to Colorado. Air is too thin up there to spool the 64 worth a crap. Put it back towards the middle helped and I kept it there. Worked pretty good.
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