You don't lose any MPG with propane. The city of Atlanta has converted all of their cars to be dual-fuel. They run on gasoline OR propane, but not both at the same time. Their cars get the same MPG no matter what fuel is being used.
The demo car I saw was a Crown Vic, with a propane tank in the back, and the plug and play system installed. The car starts on the manufactures gasoline system, then when it'* most beneficial it switches to propane. Car runs and sounds the same, just smells different since less emissions. Car got 20ish MPG on gas and the same on propane.
Now, like I said, this kit retails for $7,500. The dude told us though that if we were interested to call him directly to get some sort of student discount. I'm thinking about it on my truck to effectively double my highway mileage, if not better, using a 30 gal tank mounted in the bed.
2004 Chevy Silverado Z71 - 15% tint, FTDRL, All-on-high, 5000k lows/fogs, tinted tails, color matched grille, Sound by Kenwood.