If you fully upgrade a truck you’ll be given the option to unlock a 'Custom Import' version of that standard truck. Choosing this will unlock a custom color variant of that vehicle where its upgrades are set to 1.

The first two ‘Custom Imports' include a performance boost that increases the upgrade limits of that new vehicle and will be represented as a GT variant for the first upgraded import and as SX for the second; for example, Speed 1/10 might become Speed 1/12.

Any subsequent new imports of that truck will only change the color but will remain SX level imports.