Driving a £21 million Gullwing in the world's wildest classic race

All up, it weighs less than 900kg. The first examples had even higher door sills than this car, but the door was made bigger, and the sill lowered, to aid entry in the Le Mans-style sprint starts. That nod to practicality extends to luggage space too—a decent amount behind the occupants, plus a boot that holds a spare wheel and would have housed other tools at the time.

The interior is nicely finished for a nearly 75-year-old racing car. These were endurance racers, and driver centricity was, as it is today, at the heart of those. There's a tartan pattern on the seats, rough durable carpets, a chrome strip along the dash and even suede on the dashboard top. An overlap of materials on the transmission tunnel creates an envelope for storing timecards or race paperwork, just beneath the extended gearlever, which snakes backwards in an S from under the dashboard to where it more easily meets the driver's outstretched hand.

There's a large, wood-rimmed, removable steering wheel, which if you mount with spokes diagonal makes it easier to read the top two instruments — speed and revs — and the bottom four, most importantly today, given the amount of traffic about and the fact it's a 30deg C day in June, the water temperature gauge.

Water temperature gauges are, I suspect, one of the most important things in what is an unusual event today. I doubt any other dials are watched as much in Italy over the five days that the modern Mille Miglia takes place. Old cars, particularly old racing cars, were not meant to sit stationary in modern traffic, and modern Italy has an awful lot of stationary traffic.

As a result this road rally, which ostensibly takes place on open Italian roads still subject to normal Italian road laws, becomes a little more anarchic. That's exacerbated again by the local plod, who accompany the event and will run traffic control at roundabouts and junctions, holding up regular traffic so that competitors can pass unimpeded as often as possible. They also act as motorcycle or car outriders for small convoys, giving these classics an easier run through roadworks or into town centres, where they'd otherwise risk expiring in clouds of steam.

This sets — let's be euphemistic — an enthusiastic precedent. Say two enviably stylish motorcycle coppers turn on the blues and those rather charming Italian twos and head down the outside of a queue of traffic to a set of temporary traffic lights, initially Simple tailed by a couple of Alfas that they've beckoned in behind them. By the time they reach those lights, they'll be being followed by about 20 classic cars, a few support cars, and an Audi TT who's trying it on.

This goes down fine with some road users — and all spectators, for the Mille Miglia is an impossibly popular event, with tens of thousands of people lining the route to cheer on these old supercars. Nowhere else beyond video games can you drive through historic towns and piazzas that would normally be closed to vehicles.

But it is not universally popular behaviour, which is understandable. If one is pootling home in a Fiat Punto and, as one rounds a corner, is greeted by a Porsche 356 driven by a Swiss dentist and his companion — that surely must be his daughter, right? Right? - careering towards said Punto on the wrong side of the road at 60mph, and who thinks that it's the Punto's duty to dive onto the verge to avoid a collision, I can imagine that it would cheese one off mightily. On video since the event, I've seen a Porsche turn sideways into a passing police motorcyclist, a car upside down in a ditch (saw that one live too) and an old Alfa understeer off a mountain hairpin into the side of an oncoming saloon car.

Today, then, the 1000 Miglia is not universally loved. When you see an oncoming driver hooting and gesticulating wildly out of the window, it can be a sign of great enthusiasm, or very much not so. It's surprising how you come to know which at a glance.

Into this melee, then, Mercedes-Benz, which is running three of its own museum cars and seven cars on behalf of customers, has felt secure enough to hand me the keys to a priceless 300 SL. Frankly, bonkers. Plus it has trusted me not to crash the head of Mercedes-Benz Classic, my companion Marcus Breitschwerdt, formerly a chief of Mercedes-Benz UK, who was headed for a quiet retirement until CEO Ola Källenius convinced him that running MB Classic might be a good way to postpone his dotage.

It isn't chaotic everywhere. In some hills and on good country roads, Breitschwerdt notes that regularity sections have been inserted just at the place things might get out of hand to slow competitors down and spread them out a bit. A typical trial will consist of, say, half a dozen short sections that have to be driven at a precise speed — something like 22.9kph for 20 seconds. It's meant to calm the field. And usually it works. Whether slow or more enthusiastically, this car is magnificent.

How does a 74-year-old Mercedes feel so good? When they put these things together, they were not messing about. With a thumb on the start button and a couple of foot stabs on the throttle, the straight six coughs into life. The clutch is heavy, positive. The gearshift long throw, a little notchy, but surely impossible to mis-shift. There's an umbrella-handle fly-off handbrake on the dash — drilled for lightness of course — and with the clutch pedal eased out and the weighty throttle pressed, away it eases.

The gears are so widely spaced and the torque band so broad that you can hook second as soon as you're rolling and only need to shift to first at a standstill or crawling uphill. The noise is gruff, basic, loud and exhaust focused — which pokes out of the right-hand bodywork, and there's no window there, so as the car passes mountain hedgerows and village walls, the cacophony rebounds into your delicate eardrums.

You can rev it towards 6000rpm — though I feel Breitschwerdt would rather I didn't too hard — but it doesn't necessarily sound like a big six. Those are legendarily smooth but this sounds a bit less so. It's not blisteringly fast outright, but in making so much torque, and with such little weight, it's plenty quick enough for modern traffic. Hot hatchback performative.

The steering, a bit vague around straight ahead and with a bit of flex in the wheel rim too, weights up incredibly quickly and very heavily in corners. The weight will be on the front more than the back and the Dunlop SP Sport 185 VR15 Radial tyres on their knock-off 15in rims feed back to the driver ample messages.

Handling balance? I'm not that inclined to find out, but the front feels heavier and the weakest point is probably the swing arm rear suspension that even in mildly spirited road driving suggests it could become lively. Everything is unassisted and deliberate and slow, so I wouldn't want to have to try and catch it. There's a modern GT vibe to the SL, and on loose surfaces — much more common in the '50s — one can imagine it skidding about gaily, steered on the throttle by a hero as it soaks up lumps and shocks with the kind of solidity and durability that it still exudes today.

And it did rather well, the W194. Between those few production cars, it won at Le Mans, it won the Carrera Panamericana even while collecting a vulture through the windscreen and it finished in the top four positions at a Nürburgring race. But it was the attention it garnered in the US that really put Mercedes back on the sports car map.

Max Hoffman, a Jewish Austrian, had fled to America in 1941 and by 1952 was the American importer of Mercedes-Benz. He told the factory they should make a road-going variant of the 300 SL for sale in America, where the jet age was arriving and prosperity was high. Mercedes told him they would, but only if he promised to order 1000 of them. So he did, and they did. The Mercedes-Benz 300 SL Gullwing (W198) you see below is the result.

It has all that was good about the competition W194 but glammed up, styled, and arriving as one of the world's first super-sports cars. Even lined up right next to a W194, it looks bigger, and it is a bit but that's deceptive — most of it is down to the chromed bumpers. It retains a 3.0-litre six and H-pattern box, plus the rest of the layout, although there is fuel injection rather than carbs and the road car made rather more power than the race car — around 240bhp at 6000rpm.

Entry to it is still a little challenging, beneath the overhanging door, and the steering wheel hinges out of the way rather than being completely removable. (The squared-off wheels of today were, thankfully, not even a twinkle in the eye.) The gearlever comes out of the transmission tunnel where it ought to, and the inside is finished more pleasingly and luxuriously than the race car—albeit you still know, really, the origin of what you're in.