Tire mounting... with but a few letters

RAY: I had to re-obfuscate this. I thought Sue Olster did a pretty good job, but I think I did a better job. We'll see.

She wrote the following:

I'm going to have winter tires put on my car. I bought four of them. I'm moving soon, and since I'll be taking the car to a different shop to have them changed back next spring, the tires have to be marked so they can be put back where they belong, i.e., where they came from. So the left front has to go the left front, the right front has to go there, and so on.

When I take them off, I'm going to ask the people at the gas station to mark them with letters.

The question is, what's the smallest number of letters needed to mark my tires to guarrantee that all four of them can get correctly installed with no chance of error or ambiguity in the spring?

Think you know? Drop Ray a note!

[ Car Talk Puzzler ]