SKORPION G DOES NOT HAVE 650 HORSEPOWER. It actually has 850 horsepower. This also means it actually has 22 HP/TON In-game xml code displays this for the tank: <smplEnginePower>850 (int)</smplEnginePower>
I performed several tests with the Skorpion G. Here were the results:
Skorpion G (17 hp/ton, 1.15 resistance on hard, 1.44 on medium) vs. 113 (16.67 hp/ton, 1.05 on hard, 1.15 on medium):
On an uphill, hard terrain 195m track, Skorpion G took 30.67 seconds to complete. The 113 took 36 seconds.
On a flat, medium terrain, 200m track, Skorpion G took 1.5 fewer seconds to complete the course. 113 capped at 40 km/h, Skorpion G capped out at 42 km/h
Skorpion G vs. StuG III G (20 HP/ton, 1.15 on hard, 1.44 on medium):
On an uphill, hard terrain, 195m track, Skorpion G took 30.67 seconds to complete. StuG III G took 31 seconds (Neither exceeded 35 km/h).
On a flat, medium terrain, 200m track, Skorpion G took 30m fewer to reach 38 km/h (StuG's top speed).
Now, if the Skorp G actually had the advertised 650 horsepower, 113 would have would have tied with it on the uphill run, and 113 would have won on the medium terrain run. As for the StuG, it had both a HP/ton advantage and a ground resistance advantage for uphill - It still lost. On medium, StuG should have reached 38km/h first - It didn't.
Testing for other tanks confirms a similar story. There is a different value used for straight-line acceleration, than is used for traverse speeds.
I will make a full post on this matter later, with full testing results for all tanks. I'm also communicating with kittens, creator of tanks.gg, about this matter. He is currently working on updating values for tanks to display effective horsepower, rather than the garage-advertised matter.
And yes, that means even horsepower and hp/ton have "hidden stats" now.