[joggerfogger]

I seriously doubted that a $625 programmer (I paid $375 used) could actually make a difference.

The problem is that the programmer screws with the fueling tables in the ECM and in the past massively screwed with the DIC (driver's information center on GMs). So the Dashboard computer would say 23mpg, and when hand calculated, the number was more like 17mpg. Throw into the mix that almost every diesel truck driver puts on bigger tires (including me) you've got a semi-complex math problem for the average diesel dunderhead.

So I hook up the programmer and immediately I see +3 mpg averages on the DIC. I immediately didn't trust it as it had a +/- 0-5% error every tank anyway vs. hand calculating.

So I finally fill up again and the DIC says an optimistic 15.8 and hand calc was 16.1. For the same driving before the programmer was around 13mpg. Now one tank is not a good study, but it's amazing considering the only thing I did was hook up the programmer.

I use this truck for many different tasks, but the only time it sees the highway is with a 6,000lb RV trailer behind it. The rest of the time it's 6mi to daycare, then another 6mi. to work, haul a load of firewood, move a friend, etc. etc. repeat.

Yes, I get really bad economy, but I drive in the worst possible conditions for short trips, in bad traffic, over two 900ft. hill two times every day. I'd love to run a whole tank on the highway, but if I was going to do that without the RV, I'd just take my 32mpg Golf.

An improvement of 3mpg is substantial to me.