I took Aidyn out many time in my sisters 3-door car.
When the carseat was still in the reversible position, my knees were rather close to the dashboard, but it was still ok.
We had the compact model Fisher Price carseat, so it was smaller than the average carseat... if we'd had one any bigger I dont think you could have fit anyone in the front passenger seat!
It was a bit of a pain manouvering him into the carseat, but would be easier if there was someone to help.
Once they are in the forward facing position it becomes a bit easier.