Originally Posted by 95geo
because when you add a chip it changes the injection pulse width and timing from what the ecm/pcm "knows" and it no longer knows how much fuel is being put into the cylinder.
i have been towing with 500hp recently and i have been getting 14.5-15mpg on 35" tires. i have gotten a best of 17 towing and 19 unloaded. i have yet to run my mileage tune which should net me about 21-22 mpg if driven conservatively.
this is with an 04.5 duramax/allison and tuned with efi live. i wouldnt run more than a 60-70hp chip unless you plan on trans upgrades. exhaust should be done reguardless becasue it drops your egt's about 100* and helps the turbo spool quicker.
This is hand calculated and speedometer was 6% off according to GPS for the tires, so added 6% more miles to the tripometer and got 12 to 12.8 mpg's. The best mileage I got with the Duramax is 12.8 @ 70-75 mph towing my jeep, two 4x4 quads, gear and 4 people. I had the chip on safe tow when I calculated this. My best approximation is I was hauling ~7700 lbs. Me or my brother have never gotten better then 18 mpg empty.