So after that 180 mile ride, my bike took 13 1/2 litres as an absolute max I could fit in there. On google it says the tank holds 16 litres, which is 3.5 imperial gallons.
Since mine used 13 1/2 in 180 miles (I fully brimmed it before the long ride), that makes 2.969 gallons, just call it 3 gallons used.
180 divided by 3 is 60. Colour me massively impressed at that!

Note I am in the UK, so I assume in the US you use US gallons which would change the result.
Either way thats almost double what my car can do.

Edit: Also note I recently did the following:

Brand new tyres
Coolant change
Braided brake line and fluid change
Cleaned out the brake drum and correctly set the lever
Spark plug change
Oil and filter change
Fork oil change
New air filter

Riding 1 up, fairly large screen and lowers.

You used 3.603 us gallons, for an average fuel economy of 49.95mpg. About average for a C50 ridden at moderate speeds. You had .485 gallons remaining in reserve, which would give you a maximum theoretical range of 200 miles. Of course, fuel economy will vary, so do not take one tank to mean you will always see such performance. Ride it a little faster or fill it just slightly less and you'll be pushing the bike.

My old girl still gets about 180 miles before I fill up with 3.2 US gallons (mostly doing 65mph on superslab).

My C50 got 52 mpg and my Vl800 gets 46 mpg. I always get gas around 125 Miles. I get gas for the Goldwing at 225 miles.

Just rolled 97,000 miles on the odo; filled up after 201 miles and the bike took 3.7 US gallons for 54.3...mpg....not bad for a 12 year old bike with 97,000 miles and most of this tank was superslab running 70-80mph....love it...approx same as my Prius C on the superslab...

I usually get 200 miles per tank. Now, to clarify my commute is usually 60 mph and that seems ideal. Single rider with no extra weight.

I fill up at 135 miles if I ride single, but I have noticed when I have a nervous female clawing me in the midriff if I go over 40 I make it to 150 and one would think that with the extra weight I would get less milage

I recently came back from a 10 day trip on my C50. Mostly highway. I was stopping every 100 to 130 miles to fuel up to give myself a little break , so I wasn't "testing" the range on this ride.

During the trip I had one stretch where I went as far as 149 miles before fueling and I still had one bar left and the second flickering. This was with all my camping gear etc., loaded on the bike, so I can well believe I could get over 175 miles+ before the fuel light turns on.

I'm happy with that.