99% of vehicles might be on the high side of that estimate
I’d be more worried about getting the gas to run them. Fuel pumps run on electricity and are pretty much electronically controlled.
Fuel farms (tank farm) that distribute the fuel would have issues as well. Most all are electrically run.
Electronics, and the electric grid would take a hit. Definitely
Anything with electronic ignition would be toast. Unless you have a distributor, coil and points you would be dead in the water. I stand by my 99%. That is one classic car out of 100 cars on the road. Trucks would be even worse. How many semis do you know of that are running on the road today that were built in the late 60’s? I dare say there may be some show trucks but essentially zero. All rail would halt. Farm tractors would be in the 90% range since there are some oldies still in use.
Fuel would not be as big an issue since there would be zero demand. Brass and lead would be the issue of the day.
Cities would run out of food in a few days. Anything in walking/bike distance of a city would be looted. Cities would go cannibal. To say it would be Armageddon is not an over statement.
Have about 10-20 feet of Teflon or PVC tubing that’s 1/4-3/8 diameter with a one way check valve on the end. With owner knowledge/permission (and you hopefully capable of paying with something of post SHTF value) access the tank fill port.
If straight open, you’re good to go but if there is an anti backflow valve in it you’ll need to prop/hold open with something like a broom handle. Run the check valve end of the tubing down into tank and work it up and down vigorously. Gasoline will come up the tube for dispensing as needed...