<DIV>They should reduce your Rpm's but I've also heard that some guys are loosing on their fuel mileage due to engine not running in it's power band so to speak.</DIV>
<DIV>I still have the 245s on mine, I've have also heard of warranty issues going to A bigger tire on the Duramax/Allison setup.</DIV>
From what I've heard (for what it's worth - nada), Slow driving, around town, starting, etc.., the mpg is worse due to the force (moment) it takes to turn the bigger radius tire. When you're up to speed, mpg is better, due to the fact that the rpms are lower & are not sticking near the 2000 range, where mpg drops off significantly.
It seems that the best mpg rpm band is about 1800 rpm. When I go mph with my (bigger) 255's, it's about that (1800), where it used to be closer to 1900 rpm with my 245's.
I'm trying to rember off the top of my head here, so my rpms might be a little off...
on edit: and we are talking very small differences - probably near 1 mpg.Edited by: conradv
i am running 315s and the rpms are lower. on the highway I run about 70 - 75 and with the 245 used to be about 2100 rpms. now, i run around 1850 - 1900 and noticed that my mileage is about the same, maybe a touch better. isntallign the tires coincided with the pre OEM cat filter so whatever I lost with the larger rubber I made up for with cleaner fuel.
1Gunner, I now have the 285's like you want. I have noticed a slight decrease around town, but i did increase on the highway. For the little it changed, I'll take the better look and wider tire any day. After getting stuck in my yard down in W.V. because the stockers were too skinny, I went home and bought the 285's. And now she looks awsome. And for the bfg's they have been great tires for me.
I went from oem to H2-315'2. What beasts these are. They really bogged the truck down. So I added a hot juice. Now its faster then before. My in town heavy foot mpg is 12. My freeway 70mph cruise is 19. My rpm's dropped 3-400 at freeway speeds. I can go 9mph faster now.
After you upgrade to larger tires you have to calculate your "new" gas mileage by accounting for the difference your speedometer will be off (which, for the 245-285 jump will be about 8%-9%). So, add about 9% to the miles on your odometer and you'll be a lot more accurate when you calculate your MPG's.
The change in the rear gear will cause more of a performance problem then the weight of the tires will. You can estimate what your effective final drive ratio will be using this simple math formula:
A little different spin on things okay? The actuall wheel you purchase and the actual tire you purchase will have a big impact on the overall effect on gas milage and performance. The wheel weight is a factor. Tire factors include weight, roundness and tread pattern. These are all factors that go hand in had with the overall final outcome of going with a larger tire. I wish I had some real hard data as to what tires are more forgiving but I don't. Logic tells me that the lighter rims will be better. Logic also tells me that the roundest tire is the best. Logic also tells me that more aggresive tread paterns are more likely to produce the most rolling resistance. Just a little different spin on things.
Going with what JimWilson said about 8%-9% gain in speed with the over all tire diameter, that's how much your RPM will drop at cruise CONSIDERING you compensate on your speedo for the extra speed. So now instead of traveling at 60mph according to your speedo you should travel at about 55mph and your actual speed will still be 60mph with the 285's.
I changed out to 285's from the stock 245's on my truck and saw a substancial improvement in fuel economy on the highway, but then I decided to put more lift in and went to 315's and saw my mileage go in the toilet. But that's ok, I can afford the fuel and I love the look!!
i agree with bronco lets think of the extreme if you were to put a tractor grip on a truck tire every grip would hit and bounce hard witch would slow you down so guys that say thier milage went down with bigger tires you should also consider youer tread of the tire more agresive tread will give you poorer mpg my 2cents woeth
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