When configuring a truck for NG+ races, or checking diagnostics for New Game races, the important things to consider are primarily engine power (bhp/kw) and gear ratio.

In the current version of the game (as of 14/10/2020) the best trucks are the Peterbilt 389, or the Kenworth W900, as they have the most powerful engine available: Caterpillar C15 625 (625 bhp / 466 kw) They also have access to the transmission with the best top gear ratio: Allison TC10 (Ratios 7.40-0.86; Differential 2.64x) Combined, these allow for a top speed around 107-108 mph / 172-173 kph. For NG+ profiles, these both unlock at level 18.

For the sake of handling, a Short chassis is likely best, as the 200 gallon capacity is sufficient for the current longest race routes without refuelling, with 'realistic fuel consumption' disabled.