Please direct me to the appropriate section if this isn't it.
Situation: 2005 SRX N*V8, by previously owner converted to gasfuel, don't know if it was biogas or LPG. But the gas distibutor went out of business, so the car was convereted back to regular gasoline.
The mileage is way too poor. I have not ever had better than 19 mpg, mostly it's 18 and I only drive highway. Average speed when I fill up is typically 40-45mph.
The shifting is hysteric (not hysterical), the motor revs too easily. It shifts down and revs up to 3000rpm or more at any mild hill or mild acceleration, but runs at an even 2000rpm @ 70mph.
The car simply does not seem to take advantage of the torque inherent in the V8.
If I compare with my 1995 Eldorado that gets 25mpg easily, and 29mpg with conservative driving, my SRX is a lemon.

How can I tune the engine away from "race settings" to more consumption friendly low end torque and a more favorable shifting pattern? I guess it's a computer thing, but I'm a fish out of water with computers. So I will need help from somewhere to get it done. The problem may be universal, the practical side is me and my var is in Europe.