Top tip - give them the cheapest truck, as it makes no odds whether they have 300bhp or 750bhp
Actually, this changed in a recent update of ETS2 the quality of the truck now somewhat affects the efficiency of the driver. I'll presume this also made it into ATS. Let me see if I can find the details of it.
Edit: It's actually the latest update (1.22.2). See update logs:
"Hired drivers are more efficient when equipped by good truck"
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