Larger diameter tire for stock 16 inch wheels?

[farnorthdriver]

Are there any tires that will fit my current wheels and have a diameter of at least 34 inches and not more than 35 inches? --And be at least E rated?
I've got the stock "alloy" 16 inch wheels and 235/85/16 tires on my '99, F350 DRW 4x4.
I'd like to go to a larger diameter tire and lower my highway speed RPM; I have the 4.11 rear end and at 65mph, see about 2250-2300rpm.
The way I figure it, my current tires have a diameter of 31.7 inches.
A 35 inch tire would lower my effective rear end ratio to 3.72 (31.7/35 x 4.11 = 3.72) which actually might be a bit too low. A 34" tire would lower it to 3:83.

[huj]

try toyo open country at 315\75\r16 tire diameter 34.6 or same brand 305\70\r16 32.7 inch

[SmokeyWren]

Originally Posted by farnorthdriver

Are there any tires that will fit my current wheels and have a diameter of at least 34 inches and not more than 35 inches? --And be at least E rated?

No. Your dually wheels are only 6" wide. The biggest tire you can mount (according to the tire manufacturers) is LT235/85R16. You can go up one size to LT255/85R16 with no problems other than the tire manufacturers all say you need at least 6.5" wide rims for that tire. But don't even think about LT265/75R16 tires that come on SRWs. Those puppies require 7" wide rims, and yours are only 6". You can cheat by a half inch, but don't try for a whole inch.

Hui apparently didn't catch the fact that you have 6" wide rims, or he wouldn't have suggested tires that require 8" wide rims.

Insisting on a load range E tire for your dually is nonsense. Worry about the weight capacity, not the load range. LT255/85R16D tires have 3,000 pounds weight capacity per tire. That's 12,000 pounds weight capacity on the rear axle and 6,000 pounds weight capacity on the front axle. I hope you aren't stupid enough to ever overload your dually that much. So those tires are fine for your truck. They are 33.3" diameter, while your stock-size tires are 32". 625 revs/mile vs. 655, or 4.6 percent difference in your speedo, tripmeter, odometer, and effective rear axle ratio. So your 4.10 would "feel like" a 3.91. You need to calibrate the speedo to 625 revs/mile to make the odometer and tripmeter accurate, and the speedo would be very close to accurate.

Most so-called 34" tires are about 33.5" diameter. So that LT255/85R16 is very close to the diameter of a 34" tire. But use the revs/mile, not the diameter, in all calculations related to speed, distance, or rear axle ratio. The percent difference in revs/mile is the percent difference in speed, distance and rear axle ratio.

[farnorthdriver]

I didn't know load range wasn't equivalent to load capacity; it was an assumption I'd always made. Thanks for pointing that out. I'll just look at load capacity from now on.
As for stupidity and overloading, some amount of stupidity is required when you put a too large camper on even a DRW 350 :-) My level of stupidity is in exceeding the GVWR by almost 2000 lbs. As for tire loading, I'm at ~75% of the rear tire load capacity and 83% of the front tire load capacity. On axle loading, I'm at almost 100% of the rear axle and 75% of the front.
I have airbags, sway stops on the overload springs and heavy duty shocks and ride and handling isn't too bad.
I am considering having two leafs added to each of the rear spring packs to improve the slight amount of sway I still have. I've been told that will help out a lot but some say the unloaded ride will really become poor.
Another approach I've seen on rv.net is to replace the overload spring with some kind of three leaf spring pack that engages at a higher height. The couple folks that have done that seem to like it but am having trouble finding someone in my area who's even familiar with doing something like this.
Of course the other approach is to get a 450 or 550 but that's not an option.

[scoutguy66]

Originally Posted by farnorthdriver

Another approach I've seen on rv.net is to replace the overload spring with some kind of three leaf spring pack that engages at a higher height. The couple folks that have done that seem to like it but am having trouble finding someone in my area who's even familiar with doing something like this.

That setup is entirely possible, you just need a couple leaf packs and longer U-bolts. Sometimes its hard when your dealing with out of the box companies because if they can't order it, they've never heard of it and you can't get it on your truck. Where I live I have a blacksmithing/suspension shop and if they don't have it hanging on the wall they will make it for you. look for one of these places, they might hook you up pretty good.

[SmokeyWren]

I'll just look at load capacity from now on.

You won't have a choice much longer. "Load Range" is now obsolete, replaced by "Service Description". Service description has two parts = tire load index (i.e., weight capacity) and speed rating. No mention of load range.

For your F-350 DRW, you want a tire load index at least 119 (2970 lb) and a speed rating of at least Q (99 MPH). Your 235s are probably rated 120S (3042 lbs and 112 MPH).

[farnorthdriver]

All this leads to another question, tire pressure vs. load tables and whether I should be using them. Here's one of the many I found, http://www.goodyear.com/rv/pdf/rv_inflation.pdf
I have General Ameritrac TR tires, 235/85/16, E rated (for what that's worth) with a max single tire load of 3042 lbs and max dual tire load of 2778 lbs both at 80 psi.
The load on my rear tires is 2040 lbs/tire. On my front tires the load is 2310 lbs.
So from the tire pressure vs. load chart, I should be running ~50psi in the back and ~55psi up front.
That seems pretty low. --Until I got the the truck/camper weighed and found out how much weight was on each axle, I was running all the tires at around 72-75psi 'cause I thought they were close to their max load limit; as it stands the backs are at 75% and the front are at 83% of their max rated load.
Should I be using the pressure vs. load charts and the low pressures it comes up with??
I don't know the origin of load vs. pressure tables and what they're designed to optimize --safety? ride? wear? --a compromise between all three?

[SmokeyWren]

Originally Posted by farnorthdriver

Should I be using the pressure vs. load charts and the low pressures it comes up with??

Yes, with a caveat. If you read all the fine print in the Goodyear load/inflation document, you'll see that you should increase your PSI by 5 to 10 PSI if most of your miles are at highway speeds over 65 or 70 MPH. Since I usually tow at 62 MPH, I go by the table without the increased PSI.

I don't know the origin of load vs. pressure tables ...

The Tire and Rim Assn (TRA), which is an industry-supported group of engineers that test tires and wheels and come up with those load/inflation tables. The "industry" is composed of all the tire manufacturers that market tires in North America, plus the Rubber Manufacturers Association (RMA). The load/inflation tables and lots of other tire and rim info is published in the TRA Annual Report to Members. Various tire manufacturers pull info from those "official" tables and include it in flyers and docs under their own name. But Goodyear, for example, includes only the tire sizes they sell. So if you want a load/inflation table for some huge mudders that Goodyear doesn't make, then you probably won't find that size in the Goodyear tables.

Only TRA members receive the annual report. And the annual dues are a small fortune, so I don't belong.

The RMA included some of the TRA load/inflation tables in an RMA online report a few years ago:
https://www.rma.org/getfile.cfm?ID=606&type=publication

...what they're designed to optimize --safety? ride? wear? --a compromise between all three?

Yes to all three. Tires running the correct PSI will wear longer than tires that are overinflated or underinflated for the load. Tires that are overinflated won't have as good a ride as properly inflated tires. And underinflated tires are downright unsafe - remember the Ford Explorer/Firestone tire hubdub a few years ago?

Note that most "P" = passenger car tires are not included in load/inflation tables published by the marketers of car tires. Car tires should be inflated to the PSI on the Federal Certification Label on a doorframe. But LT and "real truck" tires should be inflated for the load they carry. The PSI on the doorframe label is only for when you are loaded to the GVWR of the truck. But when unloaded, you don't need that much PSI for long wearing, comfortable and safe tires.

[njneer1]

Very good info to have! Thank you
I heard a rumor Ford recalled some excursions and replaced load range D with E's. Anyone heard of this ?
I could not understand this since there is only on average 110 lbs capacity difference between the two. The D's I now have are rated @ 3305lbs and the E's I had were 3415 lbs. Can't see me loading the excursion heavy enough to touch what the tires will handle.

[jasonmt]

The 255/85/16 BFG KM2's come in LRE only anyhow, as do quite a bit of the choices in 255/85/16 so talking about a LRD tyre that is not available any longer is a bit of a moot point.

After having putting literately hundreds of thousands of kilometers on various 255/85/16 tyres mounted on stock wheels on Ford DRW trucks my advice would be to put a 1/4" spacer in between the duals and run in the higher range of pressures to keep your dual spacing wide enough.

Lots of choices still available as well:
Toyo M-55: LRE
Toyo Open Country M/T: LRE
BFG MT KM2: LRE
Cooper Discover S/T: LRD
Maxxis MA-761 Bravo Series: LRD
Maxxis MT-762 Bighorn: LRD
Dean Mud Terrain Radial SXT: LRD
SS Trxus Mud Terrain: LRD

Etc. and so on...

[Charlie]

Michelin 8.25R16 XZL. 34.0" diameter, 9.1" wide, will fit nicely on stock 6-7" rim. Hard to find and expensive but available in US/Canada. About 3130 lb @ 65psi, single.
there are also 14 ply rated 8.25R16s which are good for ~4500 lb each but usually with rib treads.
Charlie

[SmokeyWren]

Originally Posted by jasonmt

The 255/85/16 BFG KM2's come in LRE only anyhow, ...

You're right. I was going by an old specs sheet from 2005, and it showed the MudTerrain LT255/85R16D. But the current specs at
BFGoodrich Tires | Mud-Terrain T/A® KM2 | Tire Sizes & Specs
show it as LT255/85R16E.