17mph an average of 9 - 12 is more like it.
But on a flat with a small decline i have got 22mph so it is conceivable to think he is stating his top speed whilst out and about. as for the range if Sinclair says he can get 20miles + with his small battery i can again assume he is just as hopeful as Clive.
On a more realistic note - the person who buys it will probably not care they never get 17mph on a flat and he probably will never even consider attempting to go more than a few miles or even go out of his/her village.
And remember the c5 is a serious vehicle and could be used for shopping, commuting to work / school. If only Clive had told the truth - A c5 is one of the most crazy, fun, un-reliable vehicles, with endless possibilities for hobbyists to mod, he would still be selling them today and they probably would do 17mph an 50miles per charge!