A: John wants to debate with his friend about which of the two cars has better gas milage.

B: The argument would be like beating a dead horse. He has signed the contract with the dealer and paid the front money.

Note: The idiom means to “bring up an issue that has already been concluded, something that’s considered to be pointless” or “pursue a futile goal or belabor a point to no end.” This phrase may originate with horse racing, where horses are sometimes beaten by their riders to get them moving faster. Depending on the rules, a jockey usually has access to a riding crop, which is sort of like a miniature whip, and this is used to slap the horse on the thigh. The horse responds either by running faster, or not at all if it’s too tired. If the horses were dead, then it’s no use beating them.