Ok here's the deal I have the p1850 issue with my Tranny U know the hard 1 2 upshift , so I have always driven the truck in 3rd (i was told this was ok just harder on gas), So I usually only got about 450 KM per tank (I think the normal is 500 km) so no biggie. Now this past year I seem to be only getting about 350 km a tank of gas. I have changed my plugs and wires, air filter, gas filter, everything else seems ok. So why the poor mileage?

 

I'm sure you had the problem fixed by now, but I had a similar problem..took it to shop, mechanic found i was leaking fuel from spider connection under the hood. I'm not too sure what that is, but im hoping that my 6mpg goes back up to 20mpg.