What Kind of Vehicle information do I put in for paying a toll online?

I went through a toll and didn't pay it and I drive a 1996 Ford Explorer, it asks for my vehicle type and the options are
Auto - 2 axles; auto, motorcycle (which i think might be right?)
Small Truck (which also seemed right but it said something about 6 wheels? Nope)
medium truck
large truck (i don't think its either of the last two)

Auto 2 axles for that vehicle. (same as a car) Truck will give you the option to add an extra axle for a trailer.