As a teenager I worked a garage forecourt ... in the days before "self service" ... from my point of view the best customers were those who wanted 4 gallons of "best" (5 star) .. simple reason ... it was 4s 11d a gallon, they would pay a pound and allow me to keep the change if I had done the tyres, oil and windscreen .. so I made 4d ... doesn't sound a lot but my wages were 2s 6d an hour.
Putting that into modern amounts .... 4s 11d ... a fraction under 25p a gallon ... 2s 6d an hour ... 12.5 p an hour... a 4d just under 2p .... but .. looked at another way ...
2s 6d (30d) an hour was .5d per minute so a 4d tip was the equivalent of 8 minutes work ....
So if someone today is on £12 an hour it would be a £1 60p tip .. and I don't see many folks giving £1 60p just to have their tyres and oil checked and the windscreen cleaned .. !!!!
Everything is relative ....