Silver-I may not be an engineer, but I did stay at a Holiday Inn Express once!
Alright..............let's look at it this way:
If we take the Vipers figures (3400 lbs w/ 650hp), divide the hp into the weight, we get the 5.23 figure. If this figure equals HP, then (as pointed out above by Anthony) the Viper would have 18,000hp - which it doesn't.
Let's say that the Viper only has 550hp instead of 650hp. Take 3400 and divide it by 550 and we now get 6.18 (a higher number than the 5.23 when dividing by 650).
Using Tim's method/logic (that the figure equals HP), and that the higher figure equals the faster car, that would mean that the 550hp Viper would be faster than the 650hp Viper.
Using MY method/logic (that the figure equals weight), it would mean that the lower HP car (550hp) would be SLOWER because it now has to push more weight = 6.18lbs for ever HP it produces instead of the 5.23lbs that the stronger (650hp) car has to push.