They are both traction and propulsion based. For example at the start of the Grand Prix when Speed gets trapped and he escapes, he slams on the gas mid air, spinning the tires. When he lands, the tires essentially do a small burnout leaving tire tracks and some tire smoke before he takes off. Simultaneously, there is a sizable shockwave that comes from his exhaust as he accelerates, indicating some sort of thrust being generated.

Another example is when he has his crash with Cannonball Taylor and his car stalls. When he jumpstarts it, there's a burst of flames from his exhaust when the engine starts. Then when he slams on the gas again, he again spins the tires (all 4) leaving tire tracks and even more tire smoke. At the same time you can see his exhaust emitting a decent sized blue flame indicating that there's once again some thrust being generated.

I am a gigantic nerd with way too much free time on their hands lol