I am currently running a 1985 BMWK100Rs with a 1980 Vetter terraplane sidecar. I constructed this rig about two years ago after restoring the K bike and the sidecar, mating them and then learning how to pilot the rig. I have michelins on the bike and the sidecar and will be installing a leading link front end that was fabricated for me by Wasp sidecars in the UK. Since restoration I have driven the rig approximately 3000 miles and have noticed accelerated wear on the rear and front tires of the bike, replacing the front and rear tires long before I would have in normal two wheel use. The sidecar tire has exhibited very little wear in comparison. My question is , are there any car type tires that are available to replace the tires on the standard K100 rims? I have looked into most of the manufacturers for auto tires and the sizes are limited to 14 and 15 inch rims, the K100 being 17 and 18 inch diameter. Any information in this regard would be helpful, Thank you, Craig

You will find an 18" sidecar square profile tyre in both Heidenau and Metzeler brands.   While there are 17" car tyres available these days, there is nothing in the 130-140 range.  Years ago I could buy a retreaded tyre with a car tread bonded to a used 130/90-17 bike tyre casing.  It was cost effective and I never had one fail. Sadly no longer available.

A rear car tyre on a K100 means a custom rim of some sort; this can be a full custom wheel or a narrow 15" car wheel (VW, Volvo, Renault etc) on a machined adapter.  

My sidecar is fitted with a custom built 15 x 4 aluminium  rear rim that I run a 165/80-15 tyre on, it's a good combo as it's the same overall diameter and circumference as the original 130/90-17 so no gearing changes or height changes.  An RS may need some gearing reduction for better rideability but the RT final drive I use is fine.

I'm sorry but I have absolutely no idea where you would find any of these (wheels, adapters) in the US.

Al

Smart car wheels apparently do the job for the rear wheel using an adaptor plate.

If you are in contact with Wasp, they do or did make 15" wheels to fit the K series BMW's. I have them on my K1100LT rig. They are fitted with 145x15 Nankang tyres.

It's an easy job to fit a car rim on the rear arm. Just need to machine an adaptator plate that fits both the drive and the rim holes. You don't even need to think about centering, there's plenty of room on the left side.
Just check before if you easily can purchase tyres matching the rim.
For the front wheel, Metzeler Block C or Heindenau K28 can do the job also. Not the top, but they are available and long lasting.  Some prefer round profile tyres on the front, arguing that an automobile tyre is only parallel to the asphalt in straight line, not when cornering. Geometrically correct, but in reality the reasonable section of an automobile tyre makes no difference.
The real difference is about the life expectancy: I changed the tyres on my side car when I bought it in 1995. At this period I was student, had no money and bought second hand tyres Citroen 2cv tyres, 125x15 dimensions. They lasted until 2021...and I had to change them because they were so dry that knobs were tearing, but they still were high enough for regulation. And I used this sidecar daily for at least 10 years.

So even if it may be expensive to convert the bike to auto rims, you'll rapidly save money on tyres wear.
Besides that, a smaller diameter will increase road handling, braking capacities (same strenght on discs for a smaller wheel development).
In Europe it's easy to find 14 or 15 inches narrow rims because it was a standard on cars. In USA it may be more difficult, but, apart Smart rims, think about old beetles rims. And many electric cars use narrow rims today to preserve efficiency.