I am pretty new here. I have an '08 stock 2-DR X with an auto tranny and 3.73 gears. I just installed some new 255/75/17 BFG M/T's on 17x8 black rock wheels. Do I need to have my computer reflashed for speedo and shift points? Also I orginally had 255/75/17 Goodyear Wrangler SRA's. Seems like my MPG went from around 19 to 16.5 now. Any reason why since the tires are the same size, would the weight of the new tires affect it that much? Thanks for any input.
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if you didn't change the size of the tires, then your speedo and trans shouldn't know the difference. As for mileage variation, check your tire pressure. Maybe too low?

 

I am running them @ 35psi right now, mostly on the road during the week in some mud and sand on weekends. Should I up the psi for during the week on roads?