1. Reserve is about .5gal. At your economy of about 38mpg, that's about 19mi range.
2. I've noticed a 3-5% error in the speedo compared to my wife's GPS when she's pacing me. It has been hard to get a consistent error, so I normally assume less than the typical 5% European delta and go with the 3%. So at 70mph, that would be ~67.9mph.