Five different tire brands on one car
 

I picked up a compact car from Avis for the weekend and have just checked the tires - all five are different brands (including the spare!). The car has done 13000 miles. Is this normal? Shouldn't they replace the tires as a set and of the same brand?

Far from normal. Tires should last far more then 13,000 miles with normal driving
Different tires can effect handling & braking. More so with different front tires left and right. Tires as front & rear pairs (L & R the same) are OK.
18 months ago I had to replace 1 tire on one of my cars. Purchased a pair (left & right) so handling would be the same

I'm curious where this was. It wouldn't surprise me at all in Caribbean or Central American locations but would be unusual in the US, Canada, or Europe.

It was in the UK.

I didn't mention anything about it when I returned the car, just in case they accused me of swapping them.
While the tires on the car did seem to be in good condition, the spare looked a lot more worn.

Do regular renters usually check this sort of thing?

It certainly is a bit of a red flag if you saw it when picking up

How do you define the term "regular renter?"

I don't own a car and I rent one about 6-8 times annually. I've been doing that for 5+ years. I've never once checked to see if the tire brands match. Until I read this thread, I never even considered it. Basically as long as they aren't flat/under-inflated, I don't care if they don't match.

5 or more a year

That seems reasonable, I don't want to have to check this every time either.

I'd estimate that 75% of my rentals are 1 day. The remainder are 2 day rentals. Add in the fact that I have no affinity for car brands equates to me not caring much about what I'm given. As long as there's no smoke odor, I'm good.

I'd probably be worry about it a little more if I were keeping the car longer. Although, I probably still will never look at the tire brands unless I were driving in the snow.

Driving in the wet (= heavy braking) is where odd unmatched tires can make a difference (for the worse)