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1.
- refers to a culture of people with rear wheel drive vehicles (usually ae86 corollas and S-bodied nissans) meeting up and doing donuts at crossroads, shopping centres and industrial parks, especially late at night. It`s seen by some as a starting point towards drifting where people can learn how to control a slide without serious risk of injury, but is widely sneered at by mature drivers.
People interested in diffing(differs) tend to be very young drivers who cannot afford to attend drift events and see diffing as a way of being able to be involved in the modified car scene.
Are you coming out diffing with us tonight?

We were at a (insert cruising forum name) meet last night and Dave was diffing his brothers twincam.
by tarmac surfer September 07, 2009
 
2.
Term used in Ireland for spinning a RWD car in circles, what Americans would call a donut. Very popular in Mk2 escorts, 80s/90s BMWs and Twin Cams
Sir, you shoulda seen the lads diffing the twin cams last night outside Dungannon
by Doire Abú September 22, 2009
 
3.
When a group of knob heads gather to wreck their cars by spinning around in circles, usually done by joy riders from west belfast and little boys who think twin (tin) cams and old beemers as old as the hills are cool
diffing - does she go round mister..
by diffin is for losers July 23, 2008