@1olbull and
@barbagris I got an answer from Deget on converting a 16x8.5 rocket wheel to a 17x8.75 wheel.
After close examination I figured out their method, its quite ingenious. The cut off the lip of the stock rim, then take a new hoop and slide it around the outside. I can only assume they weld them together along the inside of the rim, because there is no weld visible on the wheels they've done and it would make balancing easy, just balance, grind, balance grind etc until its weighted evenly. It's not likely to ever fail, or leak, and doesn't require precision machining that would be involved in cutting off the whole hoop and mating another one.
Price came out to - 780 euros for the work, using a forged aluminum hoop to increase the diameter for strength.
Shipping to me was quoted around 140 euros if a tire is installed, so a bit less without.
In the US we can pass on the TUV certification, meaning you save 60 euros.
Roughly translates to $980 shipped for a 17x8.75. As a 17 you can now fit the Pilot Power 3, a tire literally unmatched as a street tire if you consider both wet and dry traction. If you go 18x8.75, you get the Dunlop Q3 as an option, wears much faster than PP3, has less rain grip, but has significantly increased dry grip.
An option would be to buy a rim on ebay.de, have it shippped to them, and then do the work and forward to you. It would save you the downtime while your rim was being worked.
IN EITHER CASE, the PP3 and Q3, are both so vastly superior to the 16x8.5 options, it's irrelevant which you pick unless your planning on racing your R3 on the track, then go with the Q3 lol.
