I recently purchased an 07 2500 mega cab. the dealership I bought ir from had put new tires on it prior to my purchase. 315/75r17 vs he factory 265/75r17. as a result the speedometer was off 8.739%. mpg taking the tire size adjustment was about 18mpg in city. they recalibtated the speedo to within 2% and now he mpg is 13. is is possibly they accidental changed othrr parameters to efrect the change. truck seems to have less power now as well. suggesstion are very welcome.

I this I am going to ask them to undo what they did and just deal with the speedo being off


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Something doesn't sound right. 18 in the city with that size tire? I had a 1500 re-geared with 4.56 and 285/70/17. My best in the city 14. Have them undo it, you can buy a tool that just recalibartes the speedo. You will feel less power those are big tires. Alot of weight. You can get it back with re-gearing.

 

it is a 2500 with the cummins 5.9 and a 3.73 gear. I have with an 03 similar tires with the same gear that averages 24mpg but that is with a banks programmer. another friend with an 04 with a 4.10 gear averages 16-17mpg and the 3.73 should get better than that.


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also the truck had the power with those tires before they calibrated the speedo


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Are you hand calculating the fuel economy, or just relying on the overhead display?

 

I have done both


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Having the speedo off doesn't magically gain 5mpg. 13 in town sounds about right. What are you getting highway?

Absolutely no reason to regear for those tires.