JImbo1

My truck calls for 215x85x16, but in all the posts I've seen nobody uses them. They all seem to have 235 or 255x85x16. What's up with that. Should I get the 235 or 255. Will they rub? Will I get better fuel milage? Will it put more strain on the truck when haulling my 5th wheel? Does the # 235 mean a taller tire.

Sams club does not have an alternate tire size for my truck, only the 215 Should I let well enough alone and stay with the reccomended tire size?

magnum

the first numper ie 245 means the width of the tread across the top of the tire in Millimeters (mm) The second number ie 75 means the percent of the first number relating to the height. and 16 is the inch size of your rim/wheel.

So.... in mm, a 285x75x16 tire would be 285 mm accross the top, the sidewall would be 75% of 285, or the height from rim to top would be 213.75mm. Hope that answers your question.

magnum

Oops--sorry, yes the taller the tire, the less revolutions per mile, and yes it would change your gear ratio. But with all the torgue you have with the CTD, I really don't know if you would notice much difference.

I'm running 285x75x16 Bridgestone duelers "Revo" on my 2001 6 speed. I'm getting better mileage, just have to re-figure everything cuz the speedo is now off about 2 1/2 MPH

JImbo1

Thanks MAgnum..In that case to get better fuel milage wouldn't I want a 215 vs 235 because the 215 is narrower which would mean less rubber touching the pavement thus less friction and better MPG?

magnum

JImbo--you are getting into Physics, a topic at which I flunked in college long time ago because I felt the need to argue with my professor on a daily basis. (didn't really flunk, just can't remember s___)

Just remember, the taller the tire, less revolutions per mile, less revolutions should transform into greater fuel economy, unless your freight needs the RPM's to tug it along.

MnTom

Sometimes going to a bigger tire will hurt the fuel mileage. When you go way oversize the effective gear ratio is so far out of the power range that it compromises economy. Your truck came with 215's on it? Most all that I have seen have had either 245 or 265 tires on them. The differance between the 215 and 235 as far as friction is probably negligible. I would tend to believe that the differance would be more in the inflation than any other thing since a tire that is under inflated takes more power to turn.
Tom

magnum

MnTom--absolutely---thanks for saying it better.

HOHN

Your mileage will go up with a larger tire IF and ONLY IF the effective gear ratio increases MORE than the increase in rolling resistance.

It takes a certain amount of HP to turn the tires. How much is determined by inflation and design, as well as size. If you take two identical tires (in terms of inflaion pressure and design), but one is a size larger, it will ALWAYS take more energy to turn that larger tire. That's because the contact patch is larger, so there's more rubber to deflect, which means more rolling resistance.

Higher gear ratios often mean better MPG, but not always. This is because diesels vary the fuel ratio with load, so while a taller gear gives you lower rpm, it makes the engine run at a richer ratio, so it uses more fuel. The two tend to cancel each other out, though not completely. So don't expect better mileage from larger tires.

If you want better mileage from increased gearing, add an aux OD tranny like a gear vendor and keep the same tire size.

If it were MY choice, I would stick with the 215s......

Justin

gunracer1

well if you look at the later duallys they are running 235s. you cant beat the 235s for price and they are available every where. i blew out more than a few 215s on my motor home, not so easy to find running across new mexico. i have 235s and 3.54 gears and i would still like a higher gear.

JImbo1

Thanks all for the valuable info. The tires I ordered Michelin XPS 215x85x16E just came in at Sams club. I think I'm going to go with them. They are $146.00 each.