So as the question says... any body have a clue? Trying to figure this out getting ready to go pick up our trailer and were going to need a winch to pull cars up... Any equations any body would know?

The rule of thumb is 1.5x the weight of the vehicle, but there are other factors, like how far down on the drum you are. Also, the use of ****** blocks gives a much greater capacity to any winch. Seen some BIG TIME weight moved with some relatively light duty winches using ****** blocks...

The rule is 1.5x for offroad trucks that get stuck. You can use far less for winching rolling vehicles up a trailer. I use a 3000lb harbor freight winch that I got new for $50. It will tow a 6000lb truck with a flat tire up on my trailer (18' flat deck) without a problem (using included ****** block) but its slow. I would suggest maybe a 6000lb winch if you can afford it. That should pull any light duty pickup with 4 (or six) flat tires onto your trailer if you use a ****** block.

We pulled an old S10 Blazer up onto a trailer a couple of months ago with my buddies little WalMart winch that I bet is about the same thing as your $50 one and while I agree with you that a lower capacity winch will do it and often just fine, I took the OPs word "cars" as in plural that he'd be doing this on somewhat of a regular basis. If this is going to be done with any regularity, I'd still be purchasing a winch in the 8-9k range.
Some fairly decent Chinese import winches out there for $299 in the 8k range and $399 in a 10k flavor...

I bought a 10,000 lb winch at Harbor Freight for like $300.

It's better to have more and not need it than need more and not have it!!

Not the fastest out there but it has been dependable and pulled me out some bad situations in my offroad truck.

He's pulling a car onto a trailer, not gnarfroading.
OP, you should be able to get away with a 2500/3000lb winch no problem. If you could even just use a heavy duty come along if you wanted to save money. I've done it before and will do it many times again I'm sure.

If your looking for something small and cheap, I would think a 3k winch with a block will be plenty, if your looking to have the right tool for the job and not just cheaping out get a decent 5-6k winch and dont worry.

A buddy of mine uses a 6k for his buggy and has never had a problem has pulled it on the trailer a couple times with a flat and the winch never slowed down, buggy probrably weighs right around 6k.

A 6000lbs buggy..? That thing better have 20 seats and a nuclear reactor. Class 1's barely weigh over half that, an those are fully built race vehicles.

Hunting buggy has tractor tires on it, no racing buggy.

i agree with the people that say go with 6,000#-10,000# winch. a smaller winch would have to work harder to do the work of a heavier winch so it may fail sooner. plus with a heavier winch if you do ever decide to start off-roading you have a heavy duty winch.