UK-based motorists who find themselves crawling the streets of the capital on a frequent basis may be tempted to swap their BMW or 4x4 for something a little more economical. Specifically, they may wish to dodge El Presidente Ken Livingstone’s punitive £8 congestion charge by purchasing something that causes less congestion.

However, Livingstone is a politician and his rules are therefore not really on nodding terms with logic. Buy a small, economical, inoffensive city car like a Fiat Panda and you’ll currently pay the same congestion levy as the driver of a Hummer. Which, in short, sucks. A lot.

Plans have been broached to rebalance matters - but not by making Pandas welcome. No, the proposal is to charge as much as £25 per day for drivers of the most fuel-thirsty eyesores (those with CO2 emissions above 225g/km) and to exempt the most miserly cars (below 120g/km - the lowliest Panda 1.1 chucks out 135g/km). But these changes are just plans at present, and unlikely to be enacted before 2009. Although you never know with Ken – he is not the kind of politician to be hampered by any contradictory statements he might previously have made – so it might happen tomorrow.

Anyway, back in the grubby present buyers are left to ponder the currently exempt vehicles, which range from expensive and ugly city-only electric cars, such as the G-Wiz, hybrids such as the frankly quite large Toyota Prius and the even more ecologically iffy hybrid SUV, the Lexus RX400h. To qualify, hybrids have to be classed in Band 4 of the PowerShift Register [PDF] maintained by quango the Energy Saving Trust which, judging from the RX’s presence in the band (it emits 192g/km) must be a bit of a doddle if you shove some batteries in the boot. (There has been talk of removing the RX from the exempt band, but the official info suggests it is still enjoying a free ride in the C-zone: Auto-IT would welcome clarification from owners.)

So clearly there is not much point in thinking clearly about preventing pollution or taking up less roadspace. Better to check the exemptions and... prop up Toyota’s market share.

Or, maybe, choose one of the other exemptions. You could go large. Vehicles with more than 9 seats don’t have to pay. It doesn’t specify how big the seats have to be, and you could probably bolt seven of the rear seats from an Audi TT coupe into the back of an ordinary hatchback, if you were so inclined. Not very pretty, but it might work.

Or you could go very small. There is a motor tricycle exemption. You don’t have to pay if your car has three wheels and is less than a metre wide and less than two metres long. Unfortunately this is somewhat narrower and shorter than a Piaggio Ape (which is basically a Vespa scooter with a roof). It’s hard to think of anything that fits this footrprint except the Piaggio MP3 (which is really a motorbike not a car and so exempt anyway).

Oh well, a hybrid it is then.