Kind of long but it needs to be said as I'm a retired automotive engineer and know and want to share the contributing facts.
I've noticed that many posts mention they had a large drop in fuel economy when the 285's were put on their trucks. Let's analyze what's really happening and why!
The OD of the 285's is 32.8" so it rotates 633 times to travel 1 mile compared to the OEM 245's OD at 30.5" that rotate 682 times to travel 1 mile distance. These are the accepted numbers derived from years of testing. This means your odometer and speedometer will read approx 7.3% slow and the actual distance traveled will be recorded as -7.3% with the 285's. The result is that a 100 mile distance traveled with the 245's on will only be recorded as 92.7 miles with the 285's on the same truck even thought you actually traveled the 100 mile distance. You did not really lose as much fuel economy as you think by far. It will become apparent as you read on. Actual fuel mileage is quite comparable to the 245's fuel mileage in reality. 285's will use silightly more fuel than 245's but very slightly.
Why??? The 285's have a little more friction/contact with the road surface due to their wider width but most of this is offset by it's larger diameter and thus less fore/aft footprint length coupled with less flattened tire surface making contact with the road from weight carrying.
You can prove it to yourself by traveling the same exact distance from one point to another twice with having the 245's and then the 285's on your truck and traveling at the same speed, load, and wind conditions/etc. Like, from one place in one city to a place in another city place, say, maybe 100+ miles away. Note the odometer reading of each trip and it will become very obvious as the the discrepancy. Now, since you use the odometer reading of distance traveled divided by the gallons of fuel used, your calculations must include the +7.3% for travel with the 285's on. Generally, the 285's will use approximately 2% more fuel when the truck only has 1 or 2 passengers and no cargo to approximately 3+% less fuel when the truck is at GVWR. Larger tires carry larger loads more efficiently.
Another varying factor to consider is that to be accurate and using apples to apples, both 245's and 285's must be new as the OD lessens with miles traveled because of tire wear. The 245's have 16/32 tread depth and the 285's have 17/32 depth. 16/32 is 1/2 inch and it's the same 1/2" all the way around. For instance: The 245's have a OD of 30.5" when new but only 29.5 inches when bald. That makes a difference of about 3.2" of distance gained with each revolution. The 682 revolutions when new increases by approx 22 when bald to cover the same mile of distance.
There's more to it also. The true distance traveled by a tire is not the rotational circumference distance of the tire when rolled off the truck. The true distance is calculated by measuring the actual distance from the center of the axle shaft to the ground surface and multiplying by 3.1416 to find the distance traveled with one complete revolution of the tire with the load it's carrying. Why the difference? Look at your tire and where it contacts the road's surface and notice the tire compression or (squat) represented by the bulge. The distance from the center of the axle to the ground has been lessened do to this squat and this lessens the distance traveled by one rotation compared to the tire jacked up off the ground. It's then back to full distance from the axle's center to the tire's OD. More load, more squat and less distance from the axle's center to the ground! A smaller circle diameter is the result. Larger tires can disperse more weight at the same PSI due to more air volume. It's this volume of inflation pressure that determines squat amount on like tires with size being the only difference.
Never under inflate your tires as that what quickly causes blowouts and/or tire failure and short tread life. Also makes for dangerous vehicle handling conditions. Under inflation causes the tires to get hot and this breaks down the carcass structure due to excessive flexing. Heat kills tires!!!!!
Hey guys, I'm new the the forum and just purchase my 2nd duramax. I now have a 09 crew cab short box. I love the truck and the power is amazing compared to my 05. I'm doing something wrong though. I put 265-75-16 on the truck right after I brought it home and I'm having an issue with the front drivers side rubbing while I'm backing up.. Probbably while I'm driving forward also I can just hear it while I'm in reverse and turning to the left.. Any ideas? I wouldnt mind raising my front end up but when I did that with my 1500 no one could get it aligned for me and would this hurt anything if I did this?
So, it sounds like I can do 285's on my stock 16's with no changes (leveling kit or lift), but I might have to trim a little.
My next question is what size offset should I get on the aftermarket rims I want. I plan on going with THESE XD wheels. So it looks like I have to go with 18's. What size tire will I be able to run on those w/o making significant changes to my suspension? I know this is a little broad, but please give some input. Pictures are awesome if anyone has any!
You might want to keep the same offset that you have now depending on the actual specs of the exact tire you are going to purchase. Tires do vary in spec dimensions from model to model of tire and also among the various brands. You have to use the tire manufacturer's specs to make a final determination for offset tempered by inside side of tire clearances. You must clear brakes, calipers, etc on the inside while doing the best you can for the outside. Inside items are not moveable while some trimming can be done outside.
Something to be very aware of is that the 285's have a higher sidewall than the 265's and quite a bit higher than the OEM 245's for the 2500HD. This greater sidewall dimension means more flex and WILL contribute to additional wandering or wallowing especially when carrying heavy loads such as a slide in camper or a heavy pin weight 5th wheel. Basically, you rob Peter to pay Paul in your quest to have a higher capacity tire/rim. There's exact no silver bullett! Needs, setups, cases, and manner of driving varies greatly between actual trucks. One "size" does NOT fit all!
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