Having just now learned that there are some riders who use car tires on their motorcycles I have a couple of questions. Please some honest answers no rhetoric.
Since you are on an R3T, my comments may not directly apply to you since I'm on an R3 Standard. I'll answer based on my experiences. Since I am primarily a commuter/touring rider, our riding style may be similar since you went for the R3T.
1. What exactly is the advantage of using a car tire?
As with any modification, there are compromises. The good for me has been stopping power, longevity, slightly improved gas mileage, and a slight correction to the speedo error. My tire is slightly larger in circumference than the stock tire. And you can't dismiss that it looks cool, too.
2. Is it improving the handling?
This is a totally subjective answer. Some may say that the handling is the other side of the compromise. However, my experience has been very positive in regards to handling. I "feel" more secure using my c/t than using the standard Metzler. The countersteering effort is increased slightly but I didn't notice it after the first 100 miles on it. Yes, at slow speeds there is some leveraging when crossing an irregular surface but, again, I barely notice it. I'm fairly large so my weight may help in taming these issues.
3. Is the stopping distance improved because of a bigger contact patch?
This is the primary reason why I went darkside. On 2 occasions I had to make a sudden stop due to traffic and both time the rear brake locked up the stock Metzler and the rear end started to slide out. Fortunately, I was able to recover before highsiding. Unfortunately, several other Captains on this board did not and ended up with bent and broken parts. I personally, and it is my own opinion only, is that the stock Metzler is a safety hazard. I will never mount another on my Rocket. If forced to, I will sell my bike. With my c/t, I have never had it lock up and sudden stops are controlled and immediate.
4. What are the down sides (if any) of using a car tire.
The main downside is finding someone to mount it.
5. Do you use them front and back
No. Just the back. Yeah, Batman ran darkside front and back but I wouldn't go there.
6. Compared to a motorcycle tire is the price much different
Metzler : $220 or more + $50-$75 mounting good for 6-9 thousand miles.
Riken Raptor : $85 + $10 mounting good for 20-30 thousand miles or more.
Economics are compelling. And yes, for me, the economics are part of the reason I bought a motorcycle. It's a $15k motorcycle and I'm concerned about the expenses. That's part of the reason I didn't buy a $23k motorcycle.
7. Are they limited to say drag racing or do they make a good all around touring tire.
I just completed an Iron Butt Saddlesore 1000 and 4 days later did a Bun Burner 1500 on my c/t. Yeah, they're great for touring.
8. Are they safer then using a motorcycle tire?
This is another completely subjective answer. For my application and my riding style, yes, I consider it safer than the stock Metzler. But that is for my scenario. Others with more aggressive riding styles I wouldn't necessarily recommend it.
9. Why isn't everyone doing it?
It doesn't apply to everyone's riding style or needs. Some have tried and couldn't make the adjustment or their mind couldn't let go of the idea of running a c/t on a motorcycle. I really can't speak for them. I was sceptical at first but am thankful that I did it.
10 Lastly why is anyone doing it?
For my reasons listed above. The Metzler was scaring me so much that I was considering selling the bike. Also, the downtime and the economics of spending over $800 per year changing out rear tires didn't appeal to me either.
If you're down Chesterfield way, come on up to Fredericksburg and we'll go out and you can give it try on mine. I'd love to spend a few miles on an R3T as well.