FWIW, the longest stretch of interstate without services is I-70 in Utah, at 106 miles. I don't think I-80 has any >100 mile stretches without gas, but possibly 80 mile stretches.

Off interstates though, the west can get pretty empty. This is near the Nevada/Oregon border:

http://i675.photobucket.com/albums/vv114/cherokeeprogressive...