‘A thousand miles, on one tank?,’ the man at the petrol station counter smirked, handing me a folded VAT receipt. ‘Hope you’ve got breakdown cover ready – you’ve got no chance.’

I had a little more faith in the task ahead than he did, but not much. If fuel economy claims sound too good to be true, there’s frequently a reason for it, and on paper at least Vauxhall’s refreshed Insignia is nothing if not ambitious.

Its subtle design update last year tackled a number of D-sector criticisms head on. List prices came down, discounts were removed, new equipment made an appearance and, most importantly, the two most popular engines for fleets dipped under 100g/km for the first time.

At 76.3mpg, the pair of 2.0-litre diesel engines are frugal enough to give the Insignia the longest range in its class without resorting to downsizing, hybrid technology, wacky wind tunnel design, or stripping equipment. You even get a full-size spare wheel in the boot.

In theory, that means you can fill the Insignia’s 70-litre tank and travel 1,175 miles before grinding to a fuel-starved halt. Enough for us to travel from Vauxhall’s head office in Luton, to sister brand Opel’s headquarters in Rüsselsheim, just outside Frankfurt, and arrive back at the start point with range to spare. Big news for a car which will do most of its work on the motorway. In theory.

In practice, though, it has to be able to do this without needing to hypermile. Time is a precious commodity for business drivers and a 56mph motorway trip is a good way to add unnecessary hours to the working day. So in the spirit of real world driving, we’d set the car the additional challenge of covering the return journey without modifications, without coasting downhill, slipstreaming or driving at abnormally low speeds.

Our car had come straight from Vauxhall’s business demonstrator fleet, and had spent the last 2,500 miles being run in at the hands of potential corporate customers up and down the UK. However, concern number one: the indicated range when brimmed was 690 miles, which if true would mean rolling gently and incongruously to a halt on an Autobahn verge somewhere on the way back. My only hope was that the computer was used to test drives being carried out at more pace than we were planning.

We filled the tank thoroughly in Luton and my lack of confidence was short-lived. Recent improvements to the Insignia’s aerodynamics were showing up in near non-existent wind noise, and the new 16-inch alloy wheels were smoothing the rougher bits of the British motorway network. With the Insignia refusing to break the 70mph limit I’d set, we’d reached the Channel Tunnel showing an indicated 72mpg.

Coquelles brought the day’s second coffee, and five minutes to set the navigation for Rüsselsheim. The first section of the route through Belgium was to be vital. Contrasting the undulating motorways of southern England, the Belgian A10 highway casts a roughly surfaced but flat, straight route towards Brussels and the needle continued to climb, reaching 75mpg around Brugge and barely dipping as we crept through roadwork-induced traffic.

Sixth gear was the perfect tool for the job, and at 70mph just outside the peak torque, but with the still large capacity engine allowing it to waft along with barely any input from the turbocharger and no need to downshift. We reached a rest point at Dutch-German border with the display reading 74mpg, feeling relaxed but in need of caffeine and fresh air. Surely this was going to be easy, we laughed. We were to be proved wrong.

The Autobahns showed up just how slow our UK speed limit is compared to Europe. Crossing the border into the steep inclines of the Eifel mountains, our limited 70mph meant thorough checks of the rear-view mirror to avoid being wiped out by the high speed convoy of German executive saloons in the outside lane, while the sun set behind us.

But it’s a safe and relaxing way to travel. Highlightyed by our third traffic jam of the day, caused by a BMW 3 Series spinning off the carriageway under heavy braking in the wet, barrelling onto its roof at the start of a section of roadworks.

The Insignia rolled into Rüsselsheim at 10:30pm, with ache-free occupants keen for a rest after the hours of concentration. The Insignia tinked gently in the underground car park, showing just over a third of a tank was missing and an average of 73mpg over the first 484 miles. Enough to make the return journey look simple, even with a detour into Opel headquarters the following day.

A car park lined with Insignias awaited us at Adam Opel Haus the following morning. The company’s glass-and-metal clad headquarters form a small part of a site over half the area of Monaco, where its founder began building sewing machines in 1862, and almost 110,000 Insignias leave the factory here each year. A small hint of the importance Opel places on its flagship car.

I had a brief three hours to take in the facility and meet Opel fleet boss Ian Hucker, before beginning a time-conscious trip back to our scheduled Eurotunnel crossing that evening. Driving steadily on the way over had made the journey a relaxed one, with no concerns about straying over unfamiliar speed limits or collecting fines.

The return journey had a deadline, but with a little restraint we left feeling confident that we could get back to our start point.

Belgium had other ideas. With the clock ticking down to our train crossing, the Brussels ring road had choked on a combination of pre-rush hour traffic and an accident, and the Insignia’s navigation diverted us into the city centre. This was a gamble, and it didn’t pay off. We rolled into 90 minutes of gridlocked traffic, roadworks and every-driver-for-themselves aggressiveness that we couldn’t contend with while maintaining a steady right foot.

Frustrating, but we were powerless. No amount of aerodynamic upgrades, clever gearing or engine tuning can mask the damage done getting the Insignia’s bulk moving in bumper-to-bumper stop-start traffic. Short of pushing the car we were short of options other than simply watching hours of hard work ebb away.

By the time we reached a service station just outside the city, the all-important average economy figure had dropped below 70mpg for the first time and the weather was starting to turn. That valuable economy-boosting section through West Belgium and Northern France did us no favours either, the trees craning backwards in a strong headwind and the wipers roaring up and down the windscreen with the additional drag.

Unsurprisingly, our economy continued to slump all the way back to Coquelles. By the time we parked on the train, the gauge was showing 67mpg – a disappointing place to be after such a promising start, but Luton was still within reach.

On the positive side, Brussels had pushed our repatriation back beyond the rush hour and the motorways were clear, even through the dreaded Dartford crossing. Without the headwind, our economy figure was beginning to climb again, and as we passed the M11 junction of the M25, the dashboard finally told us that we should be looking for a fuel station soon. We’d covered 933 miles.

I could sympathise. With eight hours of travel under my belt, refuelling was on my agenda too. The tiredness had started to materialise in clammy oxygen-starved palms, uncontrollable slow yawns and dry eyes that no amount of mineral water could cure. But, with over 100 miles of range left, the detour around the two closed M1 junctions into Luton didn’t become much of a concern.

A different, but similarly perplexed, fuel station attendant greeted us in Luton, where the Insignia took just over 64 litres of fuel on board, equating to around 69mpg. I paid up, we checked again and with a gurgle the tank belched out one last gasp of air. Another 1.7 litres disappeared down the filler neck, bringing it back to an unshakable brim.

It had breezed it. With two adults, 30kg of photography gear, a headwind and Brussels traffic against us, the Insignia had not only got us back to Luton with range to spare, but it had made it look easy. We’d averaged 67.8mpg over 986.2 miles, enough to cover an incredible 1,050 miles on one tank of diesel, and all without relying on a breakdown truck.

Insignia significance: 

James Taylor, fleet sales director at Vauxhall, explains the importance of the Insignia in a diversifying market, and how the new 98g/km engine will make a significant difference. 

Overall Insignia’s about 10% of our sales, and of increasing importance, it’s 18% of our fleet sales, and 35% of our true fleet sales. It’s our most important car within the user-chooser segment.

The upper medium segment has dropped, but Insignia shares have either stayed the same or increased. We predict the segment will plateau out because of new model introductions and, with Insignia at 98g/km, there’s less need to downsize, so that should stabilise the segment as well.

It had a very good reputation for durability, reliability and real-world fuel economy. The feedback we took on board was a radical change in the P11d pricing, so from a tax point of view coupled with the 98g/km engine that made a significant difference.

Critically you’ve now got a mainstream model with lower CO2 than some of the premium cars. Our true fleet order take is up about 50% old to new car, so undoubtedly I think we’ve got it right.

Getting the most out of an Insignia: 

• Small wheels – the new 16-inch alloy wheels introduced last year not only improve real-world economy, but they’re good for ride quality too. 
• Tyre pressures – Vauxhall has two recommendations for tyre pressures. We used the Eco setting, adding 0.5 bar to the front and 0.6 bar to the rear compared to Comfort. 
• Speed limiter – most core fleet cars include a speed limiter as part of the cruise control system. We set ours to 70mph, the UK motorway limit, and maintained it. 
• Air conditioning – with the ventilation system on, we were able to keep the air conditioning switched off and stay comfortable.