Thought I'd post my experience upgrading(?) tires and wheels. I must say that studying all the pertinent posts in this forum gave me some very good insight, and some pause, into choosing appropriate wheels and tires. Until this process I was under the naive assumption that getting new wheels was simply a matter of looking at a catalog or the web and picking out something that looked cool. Not so. Other than max tire diameter I never realized all the details involved: backspacing, offset, positive, negative, weight limits, etc. It's been quite an education! I worked with my local tire store, Big O, and I'm confident they wouldn't have installed a wheel that didn't fit, but having read all the information and done the research [SMBforum, calls to SMB West, Quigley (both very helpful), web searches, YouTube wheel videos, etc.] I was more confident about picking an appropriate wheel.
Here are the details of my process:
2002 Ford E-350 EB 7.3l diesel Quigley 4x4 Sportsmobile
Original upgraded tires and wheels from SMB West in 2002:
American Racing Python aluminum alloy one piece wheels; lug circle 8 on 6.5; 9/16th lug bolts; 4.5" backspacing, 0 offset
Load Range D BFG All Terrain
radials: width 11.4", diameter 32.8", max load 3305#
As commented by many users on the forum, and echoed by both SMB and Quigley, it's recommended to keep the backspacing and offset similar to original, to maintain normal loads on vehicle and suspension. Quigley, mainly for liability reasons I believe, parroted that only
the original equipment should be used, emphasizing I should stick with the stock size of LT245/75R17 as delivered. When mentioned I had been delivered and running 16" wheels (since it was a 2002 before the move to larger brakes and wheels) it was suggested unconvincingly that I then stick with 16" rims. Later, as Peter of SMB recommended, I was able to speak with Tiger at Quigley, and he was extremely knowledgeable about the vehicles and the changes over the years. With a wealth of information regarding my rig (axle types, configurations, etc.) he too suggested that I keep the backspacing number close to the original wheel specification, but that 17" rims would be fine. One side benefit of getting in touch with Quigley, and to their credit for keeping good records, was that, with the last 8 of my VIN they were able to email me a PDF of the complete list of parts installed on my vehicle.
Armed with my newfound knowledge and wisdom I then set about to measure the backspace and offset on my wheels. First thing I noticed was that my 16x8 wheels were not actually 16x8"! Or at least they weren't 8" wide: they were more like 9" wide. It appears that the wheel dimensions given are measured from inside both outer wheel flanges, making most wheels in reality about an 1" wider than the stated #. However, the important number, backspacing, is still measured from the wheel's inner rim (closest point of wheel to vehicle) to the mounting surface, and I measured mine at 4.5". Offset looked to be about 0, as the mounting surface appeared to be midline. And this is the interesting part, if I measure my backspace at 4.5", and the wheel is known as a 16x8 at 8" wide, then the offset would appear at first not to be 0, but 1/2" or 12mm positive. However, if you consider that my wheel in reality, inner face to outer face measures ~9", then with a backspace of 4.5" the offset would be 0, or centerline, as it indeed is. As it turns out, a far easier way to determine backspace and offset, as it occurred to me later, was just to look up the numbers on the internet. Even though the wheels are 11 years old, their specs are still posted and my official backspace was 4.5" (exactly as measured) with a 0 offset. [Incidentally I found the website http://www.performance4trucks.com
to be very helpful in locating backspace, offset, and max. load #s for various wheels.]
Next I found a new set of 17x9" wheels that I liked with a reasonable backspace of 5" and 0 offset. Here again you can see that intuitively one would think a 9" wheel with 0 offset (mounting surface at centerline) would automatically have a backspace of 4.5", but the backspace is 5", taking into account the actual width of the wheel to be at 10", or 1" more than designated. With this set up I was still mounting my wheels at centerline (0 offset) but the wider wheels would extend the wheel width 1/2" in each direction.
New tires and wheels:
17x9" KMC XD808
aluminum alloy one-piece wheels; 8 on 6.5; 5" backspacing, 0 offset; max load 3600#
load range E 10 ply Nitto CrossTek HD
tires: width11.34", dia. 32.7", max load 3195#
I went to 17" rims partially for my personal aesthetics but plus to be able to accommodate the newer bigger brake systems if I ever decide to go that route in the future. As for tires, I had only an 1" clearance between the back of the tire and fender with a 30° turn so I didn't want to go any bigger in diameter. Size makes a difference in overall diameter but so does tread pattern. I went with a 285/70R17 to keep the max. diameter and fit for the new CrossTeks exactly the same.
As for the CrossTek, l was looking for a quieter and more comfortable ride. Realizing that I'm not doing any hardcore offloading, in reality probably 80% highway driving, I moved from the BFG A/Ts (which I think are a great tire, never had any issues, only 1 flat) to a more All Purpose design in the CrossTek. I considered Michelin's LTX M/S2, but liked the styling of the Nittos better, and some Coopers, but couldn't find them in 10 ply E rating that I deemed important.
Back to installation: fortunately, all the information proved useful, as unlike the newer SMBs which are able to accept deeper backspaced wheels, my Quigley van would accept a maximum estimated backspace of 5.25" with 17" rims to have the proper clearance. As it was, the new wheels had just shy of a 1/2" of clearance between the rim and the outer tie rod end. Incidentally, a 16" rim with 5" of backspace would not have fit, and an 18" rim could probably have a little more backspace if desired. As it worked out, Big O has as their usual practice, and this is probably true of most tire shops, to simply mount a single tireless rim to first check the fit before going ahead with full installation. Fit looked good with adequate clearance and full installation commenced.
I will say that upon leaving the shop in my new digs I noticed an immediate difference in the smoothness of ride, that wasn't just psychological. With the BFG's more open tread (and perhaps contributed to by the tires being older and stiffer) there was a minimal but perceptible constant vibration, unrelated to suspension. With the CrossTeks (and I'm sure any more highway favorable tire) this went away, and the ride felt noticeably more comfortable. Now come winter when I'm side slipping down a snow covered fire road in Colorado towards a cliff (based on a true story) I may wish I still had the BFGs under me, but for now I'll enjoy the plush!