Jay Leno turned the position down because he knew it wouldn't work like the British show. In Britain, Top Gear is paid for by public funding, much like PBS here in America. Because of this, they can be honest and don't have to worry about keeping advertisers (the stuff that they are reviewing) happy. When you have people paying money to have their product featured on the show, everyone suddenly gets a wonderful review.
Between that and the piss-poor cast, I think that the BBC America ratings for Top Gear will be higher than the History Channel ratings for Top Gear America.