How much can you gross with a single axle tractor and a double axle trailer?

Well, you're limited to 20,000 on the single axle. If you have at least a 35 ft. bridge to the trailer tandems, you can gross 34,000 on the trailer tandems (or 40,000 if you have a 10 ft. spread between the axles). As a rule of thumb, 12,000 is going to be your limit on the steers.