What qualifies as a "good truck"?

According to everything I could find on this, the more you upgrade your truck, the more efficient they will be. A "good truck" is just a more upgraded or better truck than the one they have currently. Some sources that seem to back this up.

so if you cheap out and give them a lower end truck then they can not work as hard!

source

I had a driver in a 310hp Stralis, and I upgraded it to the 560, and they've had an increase in revenue.

source

After doing some testing with various trucks of different upgrades, it seems any upgrade to the truck increases driver revenue, I'll have to do more testing later but it doesn't seem like the type of upgrade matters, just how it affects the trucks stats/cost.