Well, I'm stumped! At 11,001 miles:

Had GM's Performance after market 4" cat back SS exhaust system installed by the dealer.

Then went to the America's Tire store and had a set of BF Goodrich Commercial Traction LT245/75R/16 tires installed on new rims. Same size as OEM.

Then had California Automotive Concepts install Autometer Pyro and Boost gauges in a pillar mount.

Now, I find that my combined city-highway mileage for the last tank was 28.75 miles per gallon! (391 miles /13.6 gallons) :eek: Fuel use was calculated from receipt mileage, which matched the trip odometer. I thought maybe I forgot to record one fill up, but no, the math showed 391 miles on 13.6 gallons. The fuel gauge showed 1/2 tank used and I filled the tank where diesel was visible in the filler neck. Previously, my best mileage was 17.4 mpg. I have used Primrose Power Master 405 additive since day one.

Gotta be a difference in tire size - right? Well the Bridgestone Duravis M77311 OEM's show a diameter of 30.6", while the BF Goodrich replacements show 30.5". The tread width and Section width are identical. Both inflated to 60 psi. The Bridgestone spec lists 680 RPM and the BFG 683 RPM. That's a match of 99.6% for RPM and 99.7% for diameter, or less than .5% between the two.

Just how sensitive is the odometer program on these things? Is a .1" diameter and 3 RPM difference in tire specs going to make that big a difference? Seems a crock to have to reprogram for every tire change in the same size category (I'm thinking - needlessly burning up warranty mileage with incorrect programming). Unless the dealer tweaked the tune when they installed the exhaust, I'm stumped. There's gotta be a trick somewhere here.

I don't think you can get an accurate economy check on one tank of fuel-- If you run it for several tanks that will give you a better average and it will be valid. Sounds like the old story of the guys that kept adding fuel to someones tank of his new car! LOL:D If you are getting that mileage-- GM would like to buy it back!:cool:

Measure or calculate the tire circumferences, that is the true distance a vehicle will travel per tire revolution. Don't calculate your percentages from the diameter, diameter is used to determine clearance in the wheel well. Also, tire measurements shrink as they wear out.

I think you didn't get a complete fill up, your odometer reset at some point during that tank or you filled up without resetting. Few, if any, alterations will increase fuel mileage without a deliberate effort by the driver to operate the vehicle economically.

JC1834 - That thought entered my mind maybe somebody added fuel to the tank! Maybe something happened at GM when they did the work and had to repair the fuel tank and refill (joke's on me in that case).

Lurker Carl: I measured the tire circumference as you recommended, which proved the OEM Bridgestone’s are 95.75” or 7.98 feet in circumference, while the BFG’s are 96.5” or 8.04 feet. That means the tires are within 99.2% of each other or .992. Taking the 391 miles and multiplying it by .992 = 387.8 miles, divided by 13.6 gallons = 28.51 mpg. However, I doubt the GMC’s electronics are capable of registering a difference of .008 between tire circumferences, we’re talking about a difference of 1,838 revolutions over 391 miles or about 2.7 miles.

My fuel gauge indicated ½ tank, which coincides with 13.6 gallons. I filled up at ½ tank because it seemed to be taking too long to burn the tank up. I thought they may have messed up the fuel gauge when installing the pillar gauges.

I'll watch the next tank - this is just too good to be true. Of course I do drive like an old guy - but then I am an old guy. Since I got the boost gauge I’ve been driving to keep boost as low as possible around town for economy: more boost = more fuel. Even then, a mpg increase of 10 mpg is a bit astounding, I was expecting maybe 1 or 2 with the exhaust upgrade. If the next tank is suspicious I’ll have the dealer check the odometer. Thanks guys - more brains = better results.

Sounds like somebody added fuel to the tank...


EDIT: whoops somebody already said that.

I got the same exhaust as you and I went from 17.3 mpg in summer before exhaust to 15mpg on winterized fuel. go figure.

Isn't a half tank used more like 18.6 gallons and not 13.6 gallons? More like 21 mpg. I get 17.6 with a 2006 3500HD so your 21mpg may be more probable.

Wayne z.

Ok a CC short bed has a around a 27 gal fuel tank if its a long bed I believe its a 34 so around 13 gal is yes a half a tank.;) As far as you getting 28 mpg, no freeking way:rolleyes: on an 06 lbz maybe 18! I get 330-340 per tank wether I beat on it or baby it!:D

I wish someone would play this prank on me :)

I wish someone would play this prank on me :)

And kept doin it every single tank, lol

Driving down hill?

Ok a CC short bed has a around a 27 gal fuel tank if its a long bed I believe its a 34 so around 13 gal is yes a half a tank.;) As far as you getting 28 mpg, no freeking way:rolleyes: on an 06 lbz maybe 18! I get 330-340 per tank wether I beat on it or baby it!:D

Somthing has got to be wrong with my truck. The best I can get right now is 12mpg. I drive 8 miles one way each day to work mostly highway. I tool around town a bit during the week and still end up with about 250 per tank adding about 20-21 gallons a fill.

I've been in Florida for a week or so now, my mileage has gone way up from being in Ontario Canada where it was cold and had winter fuel.

I'm averaging 19.5 on short runs and 20.5 on a trip to Ft. Myers and back which was 200 mile return give or take.

Checked my mileage using calculator and fuel receipts on the trip from Ontario down to Sebring Florida and averaged 12.5/gal US towing a 29ft Cougar 5 wheel trailer.

Pretty good considering the average speed was 70 mph towing the trailer down through all the mtn's etc....

The best I've ever got was 22.5mpg in constent flow traffic going 80.
Smooth roads+ Steady heavy traffic(less wind resistance)+ Cruise Control= BADASS FUEL MILEAGE. :anitoof: