LIVING the life of a sales rep on the road for a 1,100 mile week lets you quickly discover what's really important in a car - and what isn't.

Well, it was a rep's life at the wheel of a Vauxhall Insignia Grand Sport during the day but not quite a budget hotel in the evenings, with something smarter for a holiday break.

This particular big Vauxhall might have been designed for someone with appointments to keep in far flung places but who was still tackling the lower rungs of the company promotions ladder.

Sitting three from the bottom of a pile of Insignia models that stretches 40 cars high, the Design Nav trim has the essentials for a hassle-free working life - with satellite navigation as most needed bit of kit.

Equally necessary for a happy working relationship - until very recently - might have been a diesel engine under the Insignia's bonnet, complete with promises of Scrooge-like economy.

Well, you'll have clocked the bad feelings that encircle everything diesel right now (deserved or not) and a change of car in the Vauxhall press office meant the test Insignia was powered by petrol instead.

And an interesting comparison it proved with all the diesel versions I had driven before.

For starters, the petrol model saves £1,440 over the cost of an equivalent diesel (which will concern the company fleet manager more than the driver) but uses more fuel while about its business.

The petrol Insignia claims an official 47.9mpg to the diesel's much more frugal 70.6mpg but these statutory numbers don't translate into real life figures. They never do.

Over 1,100 miles of mixed motoring, from long stretches of the A1 to some challenging Scottish byways the test Insignia showed 42.6mpg on its trip computer. I'd expect the diesel to do better, with mid-50s a likely outcome.

There were other differences too - starting with the silence that reigns when kicking the petrol version into life first thing in the morning. So unlike the clatter a diesel kicks up until things warm through.

But the biggest difference was the amount of work needed from clutch foot and gearchange hand in the petrol model. A modest motorway incline had the dash's gear change indicator urging a downshift, even at 50mph.

A diesel version would have raised its eyebrows at the very suggestion, making for a more relaxed journey thanks to the pulling power that comes as standard with any decent diesel.

Given some stick, the petrol Insignia gets along at a fair old pace but the softish suspension of this lowly version really preferred a gentler life.

Very comfortable, though, and in a car that's so roomy front and rear (and in the vast boot) that you wonder why anyone ever needs anything bigger.

Which brings us to the spec on this Design Nav version. I challenge any long term owner not to put a dent or two in the bumpers without the help of front and rear parking sensors - a £460 extra. You might add £240 for a reversing camera, to keep you out of the bodyshop.