I've run a lot of post-apocalyptic games, and have a large collection of vehicles and whatnot.
In my experience, while 1/72 is technically accurate for 25mm, it doesn't look or work right on the gaming table. 25mm-28mm minis take up a lot more space than they technically should, especially when you take the base into consideration (the base is usually about 5 ft across at scale, and adds a foot to the minis' height).
I find that 1/48 tends to work a lot better. You can also find loads of cheap die-cast toy cars commonly at 1/43, and they work great as well.
The larger the vehicle, the closer you want it to "actual" size. A small car at 1/72 would look ridiculously small next to 25mm/28mm minis, but it's less pronounced for a semi or a helicopter.
Mongoose made a prepainted modern warfare minis game a few years ago that included a lot of vehicles; maybe you can find some on eBay or whatever. They're nice, but they're a good example of what I'm talking about--they look stupidly small on the gaming table, even though they're exactly to scale.