Restrictor plates are necessary and required by NASCAR to keep the spectators in the stands safe from accidents. You need to understand that crashes often happen in NASCAR whenever cars are crammed together, and whenever at slower speeds cars stay in a pack. To give you an example of a NASCAR accident that occurred before the implementation of the Restrictor Plates, in 1987, Bobby Allison crashed at the Talladega Superspeedway, going over 200 mph. Because of the high speed, his car lifted up and was thrown into the air. Fortunately, it was caught on the screen protecting the sports spectators. However, the tremendous speed of the crash and the amount of debris that flew into the spectators made NASCAR re-evaluate its safety rules and policy. Thus restrictor plates were made a prerequisite to racing in NASCAR. On the other hand, Restrictor plates are not required or needed in IndyCar, which makes their race speeds faster than that of NASCAR races. So, yes, IndyCars are definitely faster than NASCAR’s because of this restriction. Currently, The top speed in NASCAR is only 200 mph, while the top race speed in IndyCar is 236mph. However, some experts estimate that the heavier NASCAR car can run at speeds of 225 mph, which is still slower than the IndyCar record but only a few mph. And considering that NASCAR cars are twice as heavy as an IndyCar, the V8 engine in NASCAR makes a considerable difference compared to the V6 run by IndyCars.