When an e-bike is listed as having a range of, say 20 miles does that mean 20 miles with no pedal assistance? Would that mean that if setting it on 50% of pedal power you could go 40 miles?

Possibly. Here is how you can get a good judge of the distance that you will get without pedaling. First of all, make sure the battery is either a NIMH or a lithium battery because you don't want to be buying a ready-made e-bike that uses SLA (lead-acid) because it just sucks.

Ok, find out what the battery capacity is and take that and multiply it by 2 and that's the likely distance that you will get without pedaling if the controller is a 20 amp controller. If it's a 35 amp controller, and you use it at full throttle then you'll probably only get about 1 mile per 1 AH. The 20 amp controllers give you more range because they have less acceleration and thus you get more range. You could get the same range on a 35 amp controller but you'd have to only turn the throttle about 50 %. It's just like with a gasoline car, if you floor the pedal, you'll get less gas mileage. So if it's a 36v10AH battery then you can assume to get 10AH x 2 = 20 miles range without pedaling. You may get more if you weigh like 150 lbs or something. And you'll of course, get even more if you decide to pedal a little.

It may vary due to a variety of factors including a person's size, topology, power demands of the motor and controller, whether the rider is pedaling and battery storage capacity (the more AHs the better). I added a Currie, non-hub motor conversion kit to my 21-speed mountain bike last summer. It comes with 1 24V 10AH SLA battery and I easily biked over 25 miles, with pedaling and still had power in the battery pack (I got power assist to help my old runner's knees tackle the steep hills here in the mountains of the far northern US Rockies, I like pedaling and don't really ride without pedaling). This spring I built my first LIFEPO4 pack, a 24V 20AH pack. What a difference that extra 10AHs makes. I haven't taken this new battery pack down to the deepest depth of discharge that's safe yet. I calculate that with the new LIFEPO4 pack and my original SLA, which is still working but starting to loose capacity, I can easily travel around 40 miles or more (yes, with pedaling, but I like to pedal). I plan on building a second LIFEPO4 pack and it will be 20AH again.

nwmtnbkr , those Thunderskys are very safe too. Much safer than the ping battery packs. If you look at the top, they have vent holes in them for if they ever get too hot. That prevents a fire. If you go on endless sphere, you'll see that some people with those duck tape ping packs have had fires on their bikes.

First off I don't think any bike gets the range that the monufacture says it will. It's a lot like the MPG that the car manufacture says it will get.......NOT.... As you have already been told the range will very depending on riding on flats, hills, speed and how much you help (and lets not leave out how much you weigh). If the kit or bike you are looking at says 20 miles figure it for 10 on mostly flats and you peddling your heart out. I say this because they also likely rated the range if you are willing to run the battery very low. This is not a good long term idea. Doing so will kill any type of battery you are using.

Max range is almost always listed with a light rider, tailwind, pedal assistance, and probably downhill.

I agree that there are some companies don't really mean
what they are saying regarding on the range that a specified bike could go.
And you can assume that half of the range they say is what actually the bike can go in reality.
Well i think, the best thing to do before you choose to get one,
better to make a brief review first from people who have experienced that specified bike.

I too questioned the 20miles @ 20mph. I purchased a 500w 36v kit w/ lifepod 4 battery and performed a test in my area. Hilly New England. I went 20 miles and it was done. I only peddled to help the system up hills to keep the speed up. I am ~6 feet tall and 210lbs. I purchased a new, cheap, mountain bike and converted it. If I replace the knobby tires with Kevlar road flats I might get more range but I have those tires @ 60psi( 5 below max) and only the center 1 inch in touching the road.

I would recommend buying a good system from a reputable company that dose nothing but e-bikes. I was not looking for peddle assist but something that could replace a 50cc scooter. I peddle when I need or want to, less on the way to work and more on the way home.

Treks Ride+ series of ebikes claims (on the video) 15-45km (9-27 miles). But it actually gets at least 30 miles for me - but I pedal a lot and use the regenerative braking. I think I can get a max of around 40 miles, but when I did a 43 mile trip (the Five Boro Bike Tour in NYC) I didn't start using the power until mile 27 so you can go a lot further. The bike is really pretty light for an ebike, and uses a 6.4AH battery. Yes, only 6.4. Their motor appears to be very efficient.