HP= Torque/5252 x RPM
You can treat Torque as a constant of a car or imagine the Muscle of the car. While RPM is an engine variable.
HorsePower is to describe how fast you can move a thing from one point to another point. or how much work can be done in a specified time.
Petrol car , lower Torque , higher RPM 8000rpm
Diesel car , high Torque, lower RPM 5000rpm
Both also can achieve a 200hp like this
309.73lb-ft (420NM) Diesel
200hp ~= 309.73lb-ft x 3400rpm /5252 , At a gear ratio
184lb-ft (250NM) Petrol
200hp ~= 184lb-ft x 6500rpm /5252 , At a gear ratio
You can see that the Diesel generates 200hp at 3400rpm whilst Petrol at 6500rpm. So how is the performance? Shall petrol goes faster? you shall see how the transmission work for you. For Petrol car because of lower torque, requires higher final drive ratio to compensate, on the other hand, Diesel car lower rpm range shall compensate with lower final drive ratio, but lucky it has huge torque.
check out this link, you can see the diesel final drive ratio is lower and therefore engine don't need to work as fast as petrol car. (let me know if I am wrong)
http://www.e90post.com/forums/showthread.php?t=101507