Basically, you hold on to a vehicle's back and a indicator would show up depicting how much items you can fit in before becoming full.
Or, like the seatbelts mod, you input a command at the vehicle you are looking at and it shows you how much stuff you can put in, and you use your binded throw item key to store things.
Going 30 blocks per second and crashing would result in 2 or 3 of your items being lost if compatible with the seatbelts mod.