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Old 09-04-2008, 01:51 PM   #1
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Bigger tires on a diesel truck

Well, I've tried downsizing tires on the Blue Beast, from 265-75R16 to 235-85R16. The new tires are a hairsbreadth taller than stock but narrower. I haven't seen much improvement in mileage.

I have been thinking of going in the opposite direction. BFG makes huge all-terrain 305-75R16 tires. Now I know that the tread on these tires is not aerodynamic and they are heavier. But it will effectively gear the truck about 8% lower for a given speed. The effective axle ratio will go from 3.73 to 3.47.

What's the group's opinion? Will big mud tires make my mileage go up or down?
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Old 09-04-2008, 01:55 PM   #2
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Yes it will effectively make your gears taller, but the RR of big, knobby mud tires will eat up all of your savings and probably some extra.

-Jay
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Old 09-04-2008, 02:41 PM   #3
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I agree with Jay.

I wonder about engine efficiency in diesels...is RPM as important? Based on the little I know, I'd guess that it's more important, but still not enough to offset the RR of big mud tires.

Tall, high-pressure highway tires, OTOH, might do the job.
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Old 09-04-2008, 03:32 PM   #4
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Yep. I agree. Some "E" rated highway Michelins would be nice.

-Jay
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Old 09-05-2008, 06:49 AM   #5
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Quote:
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Yep. I agree. Some "E" rated highway Michelins would be nice.

-Jay
The existing 235-85R16 tires are Michelins. They're rated for 80 psi, I keep them at 90. I wish they had helped mileage more, but nothing seems to help the Blue Beast's mileage significantly........
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Old 09-05-2008, 07:10 AM   #6
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18 mpg average is pretty decent for a "one ton" truck, even with a diesel engine.

Don't forget that if you change your tire diameter, it will throw off your speedometer and odometer, so you'll have to calculate that in when figuring MPG (or adjust your ScanGauge).
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Old 09-05-2008, 07:26 AM   #7
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Did you adjust your odometer readings for the new taller tire? If you went 100 miles on the old tires...the new ones might be traveling 104 miles but the odometer will still read 100. Taller-skinnier tires at higher pressures would be you best bet.
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Old 09-05-2008, 08:35 AM   #8
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The difference in tire height in miniscule. An odometer correction of just 0.2%
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Old 09-05-2008, 09:48 AM   #9
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If they're rated at 80 PSI then you've already got E rated tires. If you have E rated Michelins aired up to 90 PSI then I think that's all you can do as far as tires goes... Look elsewhere for mods. You've already hit the point of diminishing returns on your tires. I don't see any signifigant FE gains from different tires.

-Jay
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Old 09-05-2008, 05:23 PM   #10
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I always figured diesels would be good candidate for larger wheels. They make insane amounts of torque. If you just going down the highway unloaded, or even a light load you may only be using 300 ft-lbs. of that 480 lb-fts. of torque your producing. That leaves you 180 ft-lbs. to play with, without increasing the amount of fuel its burning. I'd look into American Force Wheels. They make some mighty big wheels with basically street tires, instead of those insanely wide mudding tires.
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