SLA, eh? Even high-current NiMh is a very dramatic improvement - isn't it nice NOT to deal with the extreme weight, gradually sagging voltage and oversold AH capacity of lead chemistry batteries? Only downside seems to be the cost but even that works out over time if you take care of 'em.
In my experience, no peddling, 1AH per mile is about average for my 48V 20-30 amp BLDC hub setup. Hard acceleration eats into range and of course hills and headwinds can create large differences over the same route but it seems like you're getting excellent range.
I'm happy with a 1AH per mile - easy to calculate that way. Need 15 miles - remember it's not an exact science. Widely variable resistance experienced during each ride complicates estimating so get 20Ah battery and go easy into winds and from stops until you can see the charging destination!
Oh, and if into really understanding a nice battery pack and how it performs - one of these kicks major booty:
http://www.hobbycity.com/hobbycity/s...dProduct=10080
nwmtbiker (sp?) posted a link before and I bit on one. Best thing I ever did for best understanding battery performance. For folks like us it's extremely useful to know how much electricity you take out and put back in...
edit: I see your other thread - nicely done. haven't been around much. too busy building and riding!