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Discussion Starter · #1 ·
I have a 2009 3500hd lmm with EFI live. It’s been tuned by Corey Willis and has also been built to some extent, it has 37s along with a fully loaded tool box “just a bolt in tool box that goes in the bed ” I have tried everything but I can only manage to get 12.5-13.0mpg. Any ideas on how I can improve this number?
 

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We have the same truck, I don’t have the tune though. Biggest killer for your fuel mileage is the 37s. Also, you say built. Depending on what you mean by that, it could also effect mileage.

You didn’t say if the 12.5 - 13 was in town or highway but am assuming town.

Mine gets about 13.5 - 14 in town, 17 - 18 highway unloaded with the cruise set at 78. When towing the 17k camper it drops to 10 towing at 65 MPH. My truck is bone stock in the drivetrain.
 
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If you're set on keeping the 37's, upgrading the axle's gear ratio to 4:10 would definitely help...
 

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Sounds like your mileage is pretty good. If you need to go off road all the time and you’re climbing over rocks and branches, or running deep muddy trails, your mileage is going to suffer.
 

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You might try driving in tow-haul all the time and stay out of 6th gear, just to see if more RPMs helps the mileage. This might be somewhat similar to a higher geared diff. The good thing about this is that the cost to try is $$zero.
 
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Discussion Starter · #6 ·
We have the same truck, I don’t have the tune though. Biggest killer for your fuel mileage is the 37s. Also, you say built. Depending on what you mean by that, it could also effect mileage.

You didn’t say if the 12.5 - 13 was in town or highway but am assuming town.

Mine gets about 13.5 - 14 in town, 17 - 18 highway unloaded with the cruise set at 78. When towing the 17k camper it drops to 10 towing at 65 MPH. My truck is bone stock in the drivetrain.
Its got head studs, aftermarket turbo, 160hp injectors, stage 5 sun coast trans rebuild kit and a fass 165. There’s more but these are the major things
 

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Discussion Starter · #7 ·
We have the same truck, I don’t have the tune though. Biggest killer for your fuel mileage is the 37s. Also, you say built. Depending on what you mean by that, it could also effect mileage.

You didn’t say if the 12.5 - 13 was in town or highway but am assuming town.

Mine gets about 13.5 - 14 in town, 17 - 18 highway unloaded with the cruise set at 78. When towing the 17k camper it drops to 10 towing at 65 MPH. My truck is bone stock in the drivetrain.
I get about 13.5 on the hwy
 

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The tires are killing your mpg - possibly in more than 1 way. Is your speedo calibrated for those tires? If not and you're using the odometer to calculate mpg, you're gonna be off...
 

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Ok, so it's just the weight of the huge tires. Rotational inertia is probably the biggest source of your problem. As other said, you can minimize that in city driving by changing your differential ratio. Might actually help highway mileage too if those tall tires are "lugging" the motor in the higher gears too.
 
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