Yeah, I've done a bit of experimenting with "unusual" tire choices . . .
Yes, Aspect Ratio counts here, as does rim width.
There are some nice tire size calculators on the 'net. I find
this one is easy to use.
A 190/55-17 is pretty close to a 205/50-17 (a
very common car tire size), with a diameter difference of 0.6%. But that extra 15mm (0.59") gives a width increase of 7.9%.
Note that all of these are based on "standard" tire sizes, and individual tire brands and models can vary a small amount.
For example, I measured the actual width difference between the stock 190/55 Z8 rear and a 205/50 car tire mounted on the stock GTL rim as just under 3/4".
In terms of the K16, the single-sided Paralever swingarm has some room to go taller, but not much room to go wider. There are also some concerns where the swingarm curves in at the front corner of the tire.
In order to fit the Euro-profile car tire to the GTL, I needed to make a 3/8" spacer disk, and use longer lug bolts with special tapered adapters (those were a pain, as BMW uses the more unusual Fine metric thread).
It works, but the bike does handle a bit different. I can still push hard and never have had any traction concerns (in some cases traction is actually better, especially compared to the factory Z8), but it is an "acquired" taste. I have had a few friends think the car tire might slow me down some, but a local canyon ride usually takes care of that notion rather quickly.
I also played with a 130/80-17 "rear" tire mounted on the front. My thoughts were that the thicker tread would considerably increase mileage, although there are some concerns around the different compounds and profiles used, and the general weight differences between front and rear tires.
I chose to use a 130/80-17 Tourance tire, as it had good reviews and I wanted a more aggressive tread pattern for some dirt and gravel road work (yes, on the beast of the GTL).
What I found was that the tire fit between the Duolever forks, but the taller diameter interfered with the fender. I looked at making brackets to lift the fender 1/2"-3/4", but decided to just run my test with no front fender.
The bike rode fine, although it did steer a bit heavier. Traction didn't seem to be an issue, although I never pushed it to the absolute limit. It did work very well in gravel and soft dirt, as the K16's massive torque meant the 205 rear car tire just dug in and pulled hard, allowing the front's larger contact patch to float over the soft stuff. It was still a 700+ lb, limited-clearance enduro bike though, which meant it was a handful . . .
The biggest downside of that oversized front tire was that the 6.7% larger diameter threw off the wheel speed sensors. The larger tire rotated 6.3% slower for the same distance, which the bike interpreted as the rear tire rotating slightly faster than the front, so it constantly initiated the Dynamic Traction Control system. I was able to disable that (had to redo it on every startup), but even then, the ABS system flashed errors, and my cruise control wouldn't set.
That might be solved by using a Yellow Box (device that intercepts and modifies speed sensor readings to correct for Odo errors) on the front only, but I removed the larger tire before playing with that.
So yes, you can do some playing around with different tire sizes, but the disadvantages usually outweigh the advantages pretty quickly, so for most riders, it just isn't worth it.