So is there a stat that Mafia 2 gives you that governs acceleration? It only gives you weight, basic max power, and horse power.
No... there could be, but it's just not in the demo, but then again you'd think that something like that would be in the demo to show off some more of the car stuff. So I don't think it's in the final game as of yet. All they have is what you said, and those don't tell you anything about the cars acceleration.
But you could, if you really, really, really, wanted to know a car's acceleration. Get a stopwatch (yes and actual stopwatch), find a nice straight road (like the one that's in front of your house in the demo), park the car at one end of the road, and see how fast it takes your car to reach 0-60 in such and such time. But I'm not sure you want to go that far into it? You can also then estimate the length in time it takes to reach 0-100, etc., etc.