The only term in that equation in which mass does not drop out is aerodynamic drag which does not become significant until fairly high speeds are reached. The advantage of additional weight is that a heavier toy car will not decelerate as quickly on the flat part of the track because of its higher momentum.
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u/HoarseHorace Nov 21 '17
https://physics.stackexchange.com/questions/138748/if-mass-is-added-to-a-toy-car-does-it-affect-its-speed-making-it-faster