I know this is going to sound dumb, but here it goes. I own a vintage Porsche, circa 1953. The original wheels on the car are 16" x 3.25". The only tires that I can find to fit those narrow wheels are vintage tires from Coker (5.00x16, 5.50x16, etc). I really don't want to use those tires because they are too tall for how I want the car to look.

Here come the dumb question. Looking at some motorcycle tires, I found a bunch with the right diameter and look that I'm after. The weight load is the same as the tires from Coker, but the motorcycle tires are H speed rated, which is far better. Does anyone know of any verifiable reason why I cannot use a rear motorcycle tires on my car?

Any input would be greatly appreciated!

Well here's what I think. If you take a look at a motorbike tyre you'll see that it's not flat like a car tyre, but rounded so when the bike leans when turning it still has good contact with the road, now your car won't be leaning so I would guess that you might only have 50% or less of the tyre touching the road on the car compared to a proper car tyre. Hope it makes sense but I don't think it would work, take a close look at a bike tyre to compare.

HTH, G :)

Don't try it. Motorcycle tires are not designed for complicated slip angles found in automobile suspensions. Tire failure is almost 100% likely.