amschind
Well-known member
I went through this with 265/70R20 BFG T/A KO2s. I tried to offset the increased weight with a set of flow-formed Weld wheels, which were only 29# per wheel vs 37#/wheel stock. Oddly, the forged wheels were all HEAVIER than this particular flow formed wheel. Anyway, overall increase was ~8#/tire, with the weight biased towards the outside, so the effect upon rotational inertia was magnified beyond the net weight change.
I find that adding tow mirrors hurts my mileage more than the tires, largely because I drive on the highway A LOT for work. To that end, I focus a lot on "drafting" (1 bar ACC behind a big truck, not hugging bumpers) and keeping it under 65 MPH. I can still average 26 MPG on the highway under good circumstances, but I can also hit 17 MPG going 75 MPH into a West Texas headwind. These trucks have the drag coefficient of a house, and mitigating that shortcoming is the chief determinant for highway mileage.
For city driving, the tires KILLED the previous sweet spot for mileage in this truck. Previously, it would cycle the ICE on and off in 10th gear at 47-50 MPH. The transmission CANNOT be cycled, even manually, into 10th below 50 MPH. With the greater rotational mass on the bigger tires and increased air resistance (I have a massive Fab Fours combo bumper/grillguard), the ICE won't cut off above 47 MPH. Previously, I had averaged 30 MPG in steady 30-50 MPH traffic on I-10 North of San Antonio over about 45 minutes. I tried to recreate that on a deserted road in the middle of a still night in SE Texas, and even with the transmission in manual mode, could not find a speed where the ICE could cycle off while the truck stayed in 10th. The mileage penalty for being in 9th overwhelmed any benefit of keeping the truck slow enough to permit the ICE to cycle off.
Bottom line, my next maneuver is to address this when I finally get the truck tuned, ideally moving the shift point for 10th down to 45 MPH. Globally, the better solution would be wider gearing for the hybrids, but I have no idea if that's even possible. We don't benefit nearly as much from the close gearing that the stock 10R80 was built to provide because we have a giant, 47 HP "synchronizer" that powers the whole system through acceleration in a way that a pure ICE/transmission combo can only dream of. Sadly, the close gears really rob us of torque on the low end and even better fuel economy on the high end. Changing the rear to a 3.55 or 3.31 wouldn't help because the truck needs the torque multiplier from the 3.73 to keep the ICE off when traveling at low speeds, but that ratio really hampers mileage. A 0.5:1 10th gear OD would be amazing, while spacing the lower gears out to push 1st all the way down to 5:1 would make this thing even more of a beast.
I find that adding tow mirrors hurts my mileage more than the tires, largely because I drive on the highway A LOT for work. To that end, I focus a lot on "drafting" (1 bar ACC behind a big truck, not hugging bumpers) and keeping it under 65 MPH. I can still average 26 MPG on the highway under good circumstances, but I can also hit 17 MPG going 75 MPH into a West Texas headwind. These trucks have the drag coefficient of a house, and mitigating that shortcoming is the chief determinant for highway mileage.
For city driving, the tires KILLED the previous sweet spot for mileage in this truck. Previously, it would cycle the ICE on and off in 10th gear at 47-50 MPH. The transmission CANNOT be cycled, even manually, into 10th below 50 MPH. With the greater rotational mass on the bigger tires and increased air resistance (I have a massive Fab Fours combo bumper/grillguard), the ICE won't cut off above 47 MPH. Previously, I had averaged 30 MPG in steady 30-50 MPH traffic on I-10 North of San Antonio over about 45 minutes. I tried to recreate that on a deserted road in the middle of a still night in SE Texas, and even with the transmission in manual mode, could not find a speed where the ICE could cycle off while the truck stayed in 10th. The mileage penalty for being in 9th overwhelmed any benefit of keeping the truck slow enough to permit the ICE to cycle off.
Bottom line, my next maneuver is to address this when I finally get the truck tuned, ideally moving the shift point for 10th down to 45 MPH. Globally, the better solution would be wider gearing for the hybrids, but I have no idea if that's even possible. We don't benefit nearly as much from the close gearing that the stock 10R80 was built to provide because we have a giant, 47 HP "synchronizer" that powers the whole system through acceleration in a way that a pure ICE/transmission combo can only dream of. Sadly, the close gears really rob us of torque on the low end and even better fuel economy on the high end. Changing the rear to a 3.55 or 3.31 wouldn't help because the truck needs the torque multiplier from the 3.73 to keep the ICE off when traveling at low speeds, but that ratio really hampers mileage. A 0.5:1 10th gear OD would be amazing, while spacing the lower gears out to push 1st all the way down to 5:1 would make this thing even more of a beast.
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