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Yeah, it seems to be primarily 3.31's for mileage and 3.55's for towing performance. I have the 3.55's with the 18" wheels (stock Lariat FX4 Michelin ~33" tires) and can't keep it up over 18 with the DPF causing regens that waste too much fuel.
How often are your regens? I have 3:55 with 20's and see 20-21 on the highway regen every 100-200 miles and regens drop my average mpg for a few miles then work there way back up pretty quickly
I swear my truck almost never regens. I wish the message was longer though. They have no problem harassing me about the oil change that's due, but apparently don't want to call any attention to the regen process. I watch the mpg numbers like a hawk though, and catch the regens that way ~50% of the time, and on the message the other 50%. 100-200 miles would be about every other day for me, there's no way it's that frequent. I'll have to start paying closer attention and writing it down and see what the actual frequency is. Maybe I'm just missing them.
Most trucks with 3:55's have 20's and 3:31's have 18's and because of tire height differences the gear difference cancels out.
Your truck must have been ordered with that combo
Not literally, but just as well. I actually wanted the 3.31's with the 18's but was talked into the 3.55's by the dealer that was willing to work with me. My truck came from the factory with the nice Lariat 20's but they traded those off for another truck that came with the Lariat 18's, which is what I wanted, and put the 20's on that other truck instead, and knocked the price down accordingly. I have no real regrets. I may put larger tires on these 18's one day (to match the factory ~34" tires) which may or may not help with MPG, but I have no complaints about lack of power.