Why?
In Europe there are minimum passing distances set in a number of countries, in France the rule is that motorists need to give cyclists a minimum of 1m in towns (just over 3ft) and 1.5m (just under 5ft) on other roads, Germany and Spain go for 1.5m.
Because the road space is there and if there's not enough left for oncoming motorists to get through, why not use all of the width? It's also easier to judge the space remaining on the driver's side than on the far side.
UK guidance suggests you should allow at least 1.0m gap when passing at 20mph and 1.5m at 30mph, which combines with the normal cyclist's 1.2m dynamic envelope and 0.5m envelope-kerb clearance (all from LTN 2/08 - sadly these expectations aren't in the Highway Code or law yet) to mean your left edge should be at least 3.2m from the left kerb - which in a car of typically 1.9m width (published spec of Ford Mondeo) means your right edge should be at least 5.1m from the left kerb.
The widest standard lanes in the Design Manual for Roads and Bridges are 5m (an S2W lane, typically a rural single-carriageway A road) and you need to be 5.1m out, so even then you require the next lane right to be clear and you may as well change lane to pass a cyclist in secondary position at 30mph or more. A standard traffic lane is 3.65m wide - so if more than your left wheels are in the lane or if the cyclist is not in secondary position, you'd always be passing too close, so please don't take chances with cyclist lives and change lane fully to pass.
If there's two abreast, add another 1.2m to the widths, which means you're still in the next lane right and not beyond. In other words, whether a group cycling is singled or not is usually irrelevant because we should use the other lane to overtake when in a motor vehicle. If anything, two-abreast means shorter means quicker to pass.
