Given what a tour costs and the fact that you have to follow their schedule, I've never really seen the point to them unless you're unable to get around otherwise. If, as glc suggested, you book through a European consolidator, you can usually avoid the 'underage driver' fees that tend to push up the car hire rates. Besides carhire3000, some others are AutoEurope and Europcar. If you manage that, then you have a great advantage over being on a tour. Not only will your expenses be lower - you can share the cost of travel and lodging rather than paying for everything on a per-person basis - but you can go where you want on your schedule.

Speaking of schedules, you apparently are planning on about a month for this trip which would be enough to let you see a good chunk of America and parts of Canada if you're of a mind. September into October can be a great time to visit the US as children have returned to school and most holiday makers are off the road by then. One thing I would suggest, though, is that you start with 'elsewhere' and finish with New England. Northern New England in particular can be spectacular in the late fall, with peak foliage season coming around the second week in October.

For some suggestions on what to see, check out these discussions of New England, and these attractions in each state and province.

AZBuck