If you take the muffler off of a car is it more fuel efficient?

The answer is likely, the muffler on you vehicle has two main intentions. Being the limiting of noise and usually act as some form of catalytic converter to limit pollution. However this action limits the flow of exhaust from the engine and thus increases back pressure decreasing the efficiency of the engine. The removal of it increases the power of the engine and could in fact thus also increase fuel efficiency.

most car designeers and engineers design vehicles to use the muffler to perform well. no, or little backpressure can cause engine failure. if it makes you feel better, remove it, but 99 out of 100 times there will be NO improvement, and if there is, its not going to be anything more than .1 mpg. so you wont save any money. and in most states it is very illegal.

This cant be answered as a simple yes or no. Its possible that the result can go either way.