Hey guys, I am having some issues with the mileage in my 03 Duramax. First I will tell you about my truck. 2003 Crew Cab SWB 4x4 auto k&n air filter has right around 49,000 miles, tightened torsion bars, put on 16x8 wheel with 285's. I was told the truck has the 3.73 rear end, but it doesn't feel that way. How do I check to see which rear end the truck has? Any way back to my real question. I bought the truck a little over a month ago and drove it to TN and back home, South GA and I maxed out mileage at 17.75 mpg, this was with the truck completely stock. Since then my mileage has decreased. I was expecting a little mileage drop when I change the stock tires to 285's but I wasn't expecting my mileage to drop to 12 mpg. I was more expecting my mileage to stay around 15 mpg. Anyone have any ideas on whats going on?

I believe the only axle ratio available with a duramax is 3.73:1. At least that is all that shows on order sheets. Did you correct for the increased tire diameter when you checked mileage?

yeah I have accounted for the increased tire height in my mileage calculations.

does the factory have a flash available to help correct the speedo for truck with larger tires? I know that dodge's have this option.

yeah I have accounted for the increased tire height in my mileage calculations.

does the factory have a flash available to help correct the speedo for truck with larger tires? I know that dodge's have this option.

All San Antonio dealers I've talked to say GM hasn't given them a program fort speedo recalibration but I've seen a nuimber of posts one here where guys say thier dealer recalibrated for them. You lose some power at the rearf wheels with the larger tires and your mileage decreases but I don't know how much. Maybe someone with a Predator who has changed to the big tires cantell you what thier ex[erience has been. I have a 03 and my mileage has fallen off, from16.5 to 14.4. This is mostly urban driving and change of tires to 265s.

I have seen many posts indicating people get 20+ MPG with their D/A and I just want to cry because I drive 150 miles per day minimum 5 days a week on I-55 at 70mph.
With 68,000 miles the best I have ever gotten even with the mods and using several fuel addititives including Primrose 405C and 402 has been 18.9mpg in the summer. In winter I average 18.3mpg. Like some I have monitored every single drop of fuel that has gone into the fuel tank and changed filters and lubes like clockwork. I have one of the slowest off-the-line takeoffs that even a grandmother could imagine and NEVER tromp it. Pulling my Crossroads Cruiser 28CF 5er I get 11.2 mpg at 65mph. I have yet to run the Edge above 1 hoping for better fuel mileage, but continue to be disappointed. I've been a GM guy for life (long one so far), but I am disappointed in the D/A from an economics standpoint. Possibly expectations were too high.
Thanks to all for the vast knowledge exchanges and special thanks to all the supporting vendors on this site.

Kaiserrw....acutually, I have talked to several people who have had better fuel mileage improvement with the Edge set on a higher level. They saw nothing at level 1 but did see close to 1/2 at level 2.

My brother has a 2003 with the same tires as you and gets about 23 highway mpg (hand calculated, JJ approved). You should be doing better, I assume you have had a couple of dealers check it out?

Bob ... I believe the dealer can set the speed for up to 265 tires since I think they are available on a 3500. Unfortunately, that does not work for your 285s.

It may not work for my 285's but it will be a little bit closer than what it is set now.

I have seen the best fuel mileage results at level 3 and the low boost fueling at 1. Do you compensate your mileage with the larger tires?

yeah I do compensate for the larger tires, but at most the tires would only make me 1 mpg less. From stock to now is only a 7.8% difference in tire revolutions per mile, which only makes my odometer off by 23 miles every 305 miles, which isn't even 1 mpg.