A few years ago I did a project on a tank which basically involved turning it into a big remote controlled toy.I was very surprised to find out it ran a jag 4.2L petrol engine.The fuel consumption is scary ( 2 mpg on road, 1 mpg off road ).
The german tanks of WWII ran on petrol, which made supply difficult in the latter stages of the war. Nowadays, the choice of fuel will be diesel for normal engines or power units. The americans did experiment with gas turbine engines on the Abrams M1A1, but switched to deisel engines. Most tanks will have an engine with 20 or more litres cubic capacity, and around 15 times more horsepower than a family car.