The vehicles become 1/1 creatures, then get +2/+0, making them 3/1. The effect of Splendor making them 1/1 applies over their printed power and toughness, even if those stats exist only once they are turned into creatures. And effects that apply to power and thoughness with +'s or -'s always apply over effects that set base p/t to a number.
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7/14/2017 Overwhelming Splendor overwrites all previous effects that set a creature’s base power and toughness to specific values. Any power- or toughness-setting effects that start to apply to a creature after Overwhelming Splendor becomes attached to its controller will overwrite this effect. For example, the enchanted player’s Riddleform will become a 3/3 creature if player casts a noncreature spell.
Start Your Engines does not have an effect "that set a creature’s base power and toughness to specific values."
Unlike other noncreatures that can become creatures, vehicles have printed P/T values. When they become creatures, these values work like they do with regular creatures. This is not a P/T-setting continuous effect.
Overwhelming Splendor's ruling relates to things like a manland's "this becomes a x/y color type creature that is still a land" ability, but not to a vehicle just becoming a creature and using its own regular printed P/T.
So, any P/T-setting effects such as Splendor's will always be applied "over" a vehicle creature's base values, and then any +x/+y effect such as Start Your Engines's +2/+0 will modify this new base P/T. Hence, the vehicles are 3/1.
What happens?
Unlike other noncreatures that can become creatures, vehicles have printed P/T values. When they become creatures, these values work like they do with regular creatures. This is not a P/T-setting continuous effect.
Overwhelming Splendor's ruling relates to things like a manland's "this becomes a x/y color type creature that is still a land" ability, but not to a vehicle just becoming a creature and using its own regular printed P/T.
So, any P/T-setting effects such as Splendor's will always be applied "over" a vehicle creature's base values, and then any +x/+y effect such as Start Your Engines's +2/+0 will modify this new base P/T. Hence, the vehicles are 3/1.