This is for what it’s worth: Just listened to a “classic” podcast, #1139, 9/17/11. Mark, from Richmond, VA bought a Chevy Envoy and was suffering horrible mileage, 15-16 highway, empty, when he should have been getting 20+ and was asking about additives, which of course don’t work. However, there is a software program that some shops have, that will do a custom re-flash on many GM computers. I have a Chevy 3/4 ton crew cab pickup with a 454 big block. Stock, it was getting 6-7 mpg towing a loaded horse trailer and 13-14 empty, with so-so power. The shop I use, Richard’s Diesel and Automotive, a large shop in Idaho Falls, plugged in a laptop, asked if I wanted power, mileage, or combination, was I going to keep the same size tires, etc. Result was 10-11 mpg with the horse trailer, 17-18 empty at 75-80 mph and much more power, up one gear on known hills. You’re welcome to call Richard’s to verify. I’m just a customer, not a rep for Richard, but he’s sharp. A car guy, he races stock cars. A young, hot rodding shop teacher at a high school in Idaho Falls was the first I knew to own the program. It is legitimate and many other shops around the country would have it, I expect. It isn’t available for Ford or Dodge, unfortunately.
We used to own a 460 Ford that pulled a Class C motorhome. The best mileage we could get was about 9 mpg and that was going 55-60. Of course a motorhome will have a larger frontal area but you get 17-18 mpg at 75-80 mph? That, sir, is a miracle.
A 455 in a Firebird never saw 18 mpg…I don’t see how an Envoy could do it…
I ddn’t know that Chevrolet even built an Envoy… 