I'm not going to start this with the cliche "not to be racist/stereotypical" because the nature of this question is about generalizing people.

The elderly and Indian people come in with a million coupons and often ignore the fact that the coupon says you can only use one at a time. They will also demand to use expired coupons. The elderly will do this and they will also split the check 12 ways, plus their coupon, and they'll barely tip at all.

Black people don't tip as well as other groups of people. Most of the insulting tips I've ever gotten (like $.83 on a meal that cost $60) are from Black people.

Families with children usually tip well. Probably because they feel guilty that their child spilled shit everywhere and screamed during the entire dinner service.

Teenagers usually tip about 10% of what they order, but they usually only order a salad and split it so 10% of $9.95 is nothing.

Asian people are a grab bag. It's hard to tell whether they're americanized enough to know to tip 10-15% or if they're foreign in which case they almost always only leave $1-2 no matter how much their meal was. They also don't go out to eat frequently and I rarely see them (I worked at an Italian restaurant though so maybe that's why).

White people are most likely to order alcohol and tip appropriately (15% for good service, 13% for standard service). However, they are also the most likely to eat all of their food then say there was something wrong with it and demand that the food be on the house.

College students are really cheap and will often take advantage of whatever complimentary items or specials that are there. Like it's only college students who come in and have one person order never ending soup and go around splitting it between 4 people despite having the manager come over multiple times to tell them to knock it off.

If you see just two women come in, be prepared to have them sit there for 3 hours talking only ordering 2 drinks the entire time and not giving a shit that the restaurant is closed or that it's a busy dinner service and there's a long wait.