Here's how you get around some of the advertising hype. First figure out the watt-hr capability of the battery you are using. Then estimate how many watt-hr/mile you will use. Divide the first number by the second, and knock off 10%. Sorry, electric vehicles are based on math.
1) You get the watt hours by multiplying the amp-hour spec of a battery by its voltage. So a 48V 10.5 AH battery is 502 watt-hours.
2)You take a typical number for watt-hr/mile. 20 watt-hr/mile is common, although my 250W hub motor bike only uses 8 watt-hr/mile at 12-14 mph with me pedaling a lot. However, my mid drive bike does use 20.5 watt-hr/mile at 20 mph.
3) You knock off 10% because the amp-hour rating on a battery figures you run the cells all the way down to minimum. In practice, battery makers shut them off with at least 10% margin. Also knock off another 10% if the battery is a year old.
I actually have that 48V 10.5 AH battery. Figure 400 watt-hr. Yeah, I might get 40 miles out of my little motor. I have done 28 miles and still looked like it was half used up. But I'm only going 12-14 mph.
I get a little under the advertised range on my RadRover 4" fat tire bike of +20 miles at 20 mph for a max 180 lbs rider (60 lbs bike, 48 volts/15 amps/22 amps peak, 11.6 Ah, 750 watt rear hub motor).
I use it mostly to commute to work around 13 miles round trip, 4900-5400 ft elevation range, I'm about 270 lbs, and the bike+gear around 75-80 lbs. I have it on the highest PAS (level 5, 20 mph max) and use most of my wattage in the afternoon since it is 70% uphill. I try to pedal as much/hard/fast as I can to keep the power around 150-300 watts. The combo of the extra weight and steady 4 mile incline zap about 10-15% of my range at the highest PAS level. I might only be at a 1-5% loss if I drop the PAS to 2-3; but, I don't want to spend more time in traffic at a slower pace just to save 1-2 cents of power.
I did take the RadRover on a endurance run last weekend (PAS level 3, 12-14 mph, power levels 50-150 watts of assisted power, flat terrain). I was able to go 36 miles in 3 hours before my legs just got too tired. I still had one solid bar left (blinking bar means almost depleted). I think I could have broken the 40-45 mile mark on level ground (last 4 miles was the 500 ft elevation climb uphill to my house).
I am getting up to 90 miles on my base Specialized Turbo (200W motor) with 691 Wh Turbo SC battery. At full assist (TURBO mode) I can average 20-22 mph and go 40 miles. At 40% assist (ECO40 mode) I can go over 80 miles at 15-17 mph. On the original 468 Wh battery, I got 25 - 50 mile range under similar conditions. I am 65 yrs old, weigh 235 lbs so I am definitely making use of the assist. My pedaling tends to contribute about 100-120 watts to whatever the motor puts out.