I came upon the phrase ‘the mechanic’s gallon’ a while ago. Teams in the 60’s were trying to save weight and make the car as light as possible, inc. putting as much fuel in the car at the start of the race so that it lasted JUST to the end. This generally worked but a few times ran out of fuel with a few laps to go. So unbeknownst to Colin Chapman, the mechanics when fuelling the car would add an extra gallon, just to make sure it was enough, hence the ‘mechanic’s gallon’!