If it's snowing: 20%
"There are a lot more obstacles on the road when it's snowing. Gas mileage is also a lot worse when you have to cruise around in second gear. Snow and icy roads can be pretty dangerous, which is why when it's snowing, I'll have a really busy day. Some people tip really well in the snow, especially regular customers. However, we tend to get a bunch of new customers when the weather is terrible, and the general rationale seems to be, 'We won't order from them for a while so it's okay if we tip below average'."
If the delivery comes an hour after the food is ordered: Same as Average Service
"Usually it's not the driver's fault if the food is late. There are many other factors, especially when you order from a place with a small kitchen. If you order in the middle of the lunch rush, your order may not even be noticed until someone has time to check. Don't take it out on the driver. If they are really late, whoever took the order should have given a more accurate estimated delivery time. This is why when you order on the phone, they tell you the estimated delivery time last: They want to bring in business and hopefully exceed your expectations. No matter how late the food is, the driver still needs to cover gas, insurance, maintenance, meter parking, etc. I've even gotten towed once and had to give the guy $60 on the spot to get my truck unhooked. (The person I was delivering to didn't tip)."