In response to "Ryan'sTc" question..
Torque is NOT really the clear answer, although it is a large part of the answer. At a given rpm, the engine can produce only so much torque (and HP). At both 25 and 65 mph, if you're at the same rpm, the WOT (wide open throttle) torque/HP available from the engine is the same (but not the HP you are using to cruise at a steady speed - see below).
What's obvioulsy different is the gearing (5th vs. 2nd I'm guessing). Lower gearing makes for acceleration, higher gearing makes for speed. In a sense, the gears act as a torque multiplier - remember, HP = torque x rpm. Aside from drive train and parasitic losses, engine HP = wheel HP; and if wheel rpm's are lower than engine rpm's, the wheel torque has to be higher. The lower the gear, the greater the multiplier. Also, in low gear, when you "punch it", small incremental changes in speed equate to relatively large changes in engine rpm (thereby very quickly providing even more torque/HP up to the engine limits). In high gear, small changes in speed produce smaller changes in engine rpm (and therefore slower/smaller increases in torque/HP).
What's not so obvious is the fact that at 65 mph, you're already using more HP/Torque just to maintain that speed vs 25 mph....tire rolling resistance, body wind resistance, bearing losses, and [final] drive train losses are all greater as speed increases. Therefore, you also have less HP/Torque available for accelerating. (Remember, you're crusing, so not at WOT, and therefore not at max torque fro the given engine RPM)
That's why you might be able to outrun a higher HP car in the 0-60 or even quarter mile (gearing, weight & skill), but almost never be able to reach as high a top speed, even if you had gearing that would allow such speeds... You run out of HP before they do.