Even people who are sick from doing Atkins wrong don't become flammable.
Besides, zombies on fire isn't always an improvement over zombines not on fire. What you want to do is set up a burn pit that's too deep for them to climb out of, with a funnel leading them in, and then set up appropriate bait on the far side. Even fast zombies are mighty stupid.
Some malls and other big shopping centers have semi-truck loading docks that are below ground level, with huge concrete ramps that start off extra wide but narrow toward the bottom, where there's a concrete surround and big heavy metal doors. Try that, with gasoline or kerosene, if you don't have time to get a backhoe, but follow fire-safety guidelines. The ramp will have drainage grates at the bottom, so you may need to plug those up ahead of time.
A lot of those ramps have gates at the top, but it's faster, easier, and safer to just park something across the entrance once you have a bunch of zombies in there. Add accelerant, add spark, retreat, repeat.
I still think driving around and mowing them down in a parking lot is even easier, but you do have to have the right kind of vehicle to make it easy.
