PROTIP: Always try to buy ramp height, fuel etc. in BULK. I.e save up, and select two or three stages ahead of where you currently are. WHY? Each upgrade has a fixed price. To get from 0>2 you need e.g. $25 AND $50=$75. To get straight to 2, it still costs just 50$. THUMBS UP SO PEOPLE CAN SEE :D
There are some nice gameplay details in this. For instance, armor values exist, and your average German infantry will just ping off the front of an M8, whereas the sides and rear are a different story... 2D world of tanks on the go :)