The big questions
Three things come up in every darkside thread: will it pass inspection, will my insurance still pay, and can the bike still corner. Here's what I found on each.
Will it pass inspection?
Depends on your state. Most states don't inspect motorcycles at all. The ones that do each have their own rules, and some inspectors are failing bikes with a car tire on the back.
Virginia is a good example of how murky it gets. A Venture rider posted in 2025 that a car tire is an automatic fail at Virginia's annual inspection. But when another rider pulled up the actual rule, 19VAC30-70-400, it doesn't mention car tires at all. It does reject a tire marked for a use other than the highway. It also rejects a directional tire mounted against its rotation arrow, and that one matters for double darksiders who reverse the front.
Check your state's inspection rules and ask your inspection station before you mount a car tire, not after. The written rule and what the person holding the clipboard does aren't always the same thing.
Will my insurance still pay?
This is the scariest-sounding one, and the one with the least behind it. The story goes that if you crash on a car tire, the insurance company will find out and deny the claim. I've looked, and I haven't found a single documented case of that actually happening. Neither has anybody on the forums I've read, as far as I can tell. Right now it looks like an assumption that got repeated until it sounded like a fact.
That said, I'm not going to tell you it can't happen. Policies differ, and Rider Magazine raised a fair point: if you hurt someone else, their lawyer could make an issue of a tire that wasn't built for a motorcycle. Safety writer David Hough put it plainly, saying the owner who installs a non-motorcycle tire takes full responsibility for the results.
Call your agent and ask how your policy treats non-standard tires. Get the answer in writing if you can. Then you know instead of guessing.
Can it still corner?
Yes, but it feels different, and you should know why before your first set of curves. A motorcycle tire is round across the tread, so the bike rolls onto a fresh edge of rubber as it leans. A car tire is flat with a squared-off shoulder. Lean it over and you're riding up onto that shoulder, and the bike takes more push on the bars to stay leaned. It also wants to stand back up over crowned roads and ruts.
Tire makers don't sugarcoat it. Rider Magazine quotes Kumho saying its car tires aren't made for the smaller contact patch at high lean angles. Riders who run them say you learn to countersteer a little harder and you stop noticing after a while.
The best answer I've found to "can it really lean?" is a video. One rider mounted a camera on the belly of his bike, pointed right at the car tire on the rear, and rode the Tail of the Dragon. That's 318 curves in 11 miles on the Tennessee and North Carolina line. You get to watch what the tire actually does in every one of them.
Watch what the tire does, then take your own first twisty roads slow and build up.