How often to NFL favourites win but not cover?

How often to NFL favourites win but not cover? Does the size of a spread have a significant impact on whether they cover or not?

I tried a few google searches and got a virus. Figured I better ask here...

Can anyone recommend a website that would have these types of statistics readily available?

Im not exactly sure about faves covering. However  I think it's pretty accepted that the spread does not come into play 90% of the time in the NFL. Of course, if the dog wins straight up then the spread doesnt come into play 100%of those instances. The dog doesnt win straight up most of the time so that would lead me to believe that if the fave wins they would cover between 70- 80% of the time. Just pick who you think will win the game.

Take week 3 for example. 13 Faves won straight up. 3 dogs straight up. Of the 13 faves who won only 3 didnt cover.

'Cause I was thinking...Bet the dog to win straight up AND bet the fav to cover the spread. The only time you lose is when the fav wins but doesn't cover. I've played with the odds and it nets a 7-9% return if you place both bets when the fav is -3 and moneyline for the dog is around +150. Higher return if the spread is greater of course. For example San Diego is favoured by -9 over Oakland. Betting both the moneyline and the spread gives a 40% return. But without having the hard stats determining the likelyhood to win but not cover, I don't want to take an unquantifiable risk.