Given that my Audi A2 was returning 75mpg in normal use, I had idly wondered what I could get out of this 1.4-litre diesel hatch if I tried.

It was a bank holiday Monday, I was in no rush and I was driving from Bicester to Petersfield and back to visit family. So I had a go. Traffic was light, I found the odd van to slipstream and nobody had binned a BMW M3 into a hedge on the A272. The answer, without trying that hard, because I don't like being a 40mph menace, was 94mpg.

I figured I could do better than that. In fact, I thought I could get 100mpg out of it. Which is why I'm on the A34, again, with nearly a full tank of fuel, again, and slipstreaming a truck, again – not so close as to be a nuisance but not so far back that there's no benefit. This time, though, I have a goal, and a Morgan Plus Four with a cameraman hanging out of it for company.

I'm driving from a filling station near my house to the seafront in Lee-on-the-Solent. Snapper Jack Harrison wonders if it's because that's where the hovercraft museum is and hopes we will have time for a visit.

Alas, no (and it's shut on weekdays): it's just precisely 100 miles from where I started, according to a map. I've calibrated the odometer (against motorway kilometre markers) and the trip computer (via brim-to-brim fills) so I know what level of fibs they're telling.

Not very big ones, as it goes, and they're pessimistic ones. If the trip says I've done 100mpg, I'll have done closer to 103mpg. If the trip says it's right, it's right: I'll have used less than a gallon of fuel to cover 100 miles in a 22-year-old hatchback.

I love this old hatchback. To recap, early last year I paid only £500 for it, because it didn't have an MOT. I thought I'd keep it for 1000 miles and then sell so I could write about how you can be paid to drive, but it has become a car I use all the time and can't imagine selling.

I was going to check the wheel alignment before today. But with the kind of disorganisation that comes with being an idiot and working on a weekly car magazine, all I've done is remove the back seats, pump up the tyres to 40psi and give it a clean.

I have a few concerns about my mission. One is HS2-induced roadworks near the start of the journey, the other a congested ring road. But here I shouldn't have worried, because it turns out that 40mph-ish touring on a little more than idle is actually about perfect. By the time I reach the A34, only a few miles into the journey, the average is around 104mpg.

It turns out that the A2 prefers motorways or dual carriageways less. When one Luton box van I'm behind departs at a junction near Oxford, I'm sitting at 55, 54, 53, 52mph... watching my average mpg drop from the high-90s to the mid-90s. And I'm waiting, hoping, desperate for another to arrive from an adjoining slip road.

What to do? Drop back and become a mobile road block until I can latch onto a passing truck? Or put my toe in to catch a lorry half a mile in the distance? In the end, a couple of times, I sort of do neither, flounder, and by the time the A34 meets the M3, I'm not at all sure I'll manage it. The economy readout sits deliciously in the balance: high-90s. If I get a break, the A2 might just do what I need.

My break comes in the form of 40/50mph speed limits on the M27 through roadworks. The A2 is in its element and my consumption meter is heading closer and closer to 100mpg. Joining the slower roads to the coast seals it: the odometer thinks my journey isn't quite 100 miles, so after a brief tour of Lee-on-the-Solent I pull into a seafront car park, shamelessly park nose first and switch off the engine.

The car says I've done it: 100.9mpg. I'm very pleased. Beyond very pleased. Yes, I know if it were electric I'd have burnt no fuel whatsoever and we do live in an age when there should be abundant clean energy. But that wasn't the point of this exercise.

This car might have expired after a couple of months and I couldn't have complained about it. But several thousand miles after buying it, I've totted up the sums and, if I sold it for what I think is its market value, it would still have cost me approximately zero pence per mile to run. But I think it's time I spent a few quid getting it looked after. It deserves it.