Volkswagen is keen to point out that the Jetta does not share a single exterior panel with the Golf, or indeed with any other car. This puts it in a slightly different category from its saloon ancestors - the first Jetta, the Vento, the Bora and then another Jetta - which could all reasonably be described as "a Golf with a boot".Under its elegant skin, though, that's still more or less what this latest Jetta is. And nothing wrong with that, I say. If you're going to base a medium-sized saloon on anything (which isn't a big issue in the UK but definitely is in the US, where saloons are much more popular than they are here), the Golf is a great car to have available.The current Golf is more closely related to its immediate forebear than previous Golfs were to theirs, to the point where it actually shared the same body structure. The situation with the Jetta isn't quite as simple. It's 90mm longer than its predecessor, and most of that extra length has been created between the front and rear axles, and most of that (we're at 67mm now, or about two and a half inches) is devoted to rear passenger legroom.Slightly disappointingly, there has been a decrease in luggage capacity, though in fairness it's quite a minor one, from 527 litres to 510, which is still a great deal more than the 350 litres available in the Golf. You can, of course, increase the Golf's volume to well beyond Jetta levels, but only by folding down all or part of the rear seat and reducing its ability to carry passengers.You might reasonably expect that, being based on the Golf floorpan and with running gear carried over from the Golf, the Jetta might also be built like a Golf.Well, there's a thing. The test car had covered barely more than 800 miles (yes, eight hundred, not thousand) before I climbed into it for the first time, and already parts of the trim were starting to rattle and creak. This probably hasn't happened to a Golf of similar mileage since the 1970s.Thing is, the Golf is built in Germany, where they do this sort of thing rather well, but the Jetta isn't. It's built at VW's Puebla factory in Mexico, which makes sense because Mexico a) has cheap labour and b) is right next door to the car's most important market.It would be all to easy at this point to start thinking unworthy thoughts about Mexican building practices, but hold on - the last Jetta I drove (a 2006 Sport which I liked very much) was built in Puebla too, and it was as tight as a drum. Either the factory needs a quality assurance process to be inserted where the sun don't shine, or this was a one-off problem specific to this car. I hope the latter is the case.At the time of writing, four engines are available for the Jetta, and the most sensible of them is the 104bhp 1.6 TDI turbo diesel which has combined economy of 67.3mpg and an impressively low 109g/km CO2 rating, assuming manual transmission rather than the optional DSG semi-automatic.It's not very quick, though, and I'm inclined to think that the 2.0 TDI tested here offers a better compromise between performance and greenness (it can get from 0-62mph in under ten seconds, which the 1.6 TDI could do only if shot from a cannon, and its economy and CO2 figures are 58.9mpg and 126g/km). If you want nothing to do with diesels, the 1.4 TSI turbo petrol engine is also in the range in 120bhp and 158bhp forms.The lowest of the three trim levels in the Jetta range is called S, but you can't have that with the 2.0 TDI engine. The SE is the cheaper of the two remaining ones; for an extra £880 you can have the Sport, which has stiffer suspension, larger wheels and lower-profile tyres (not a wise move, I reckon, since the SE rides and handles pretty well and these things might spoil it), sports seats, automatic wipers and driving lights, an auto-dimming interior mirror, front foglights with a cornering function, a flat tyre indicator and rear privacy glass. Engine 1968cc, 4 cylinders Power 138bhp Transmission 6-speed manual Fuel/CO2 58.9mpg / 126g/km Acceleration 0-62mph: 9.5 seconds Top speed 130mph Price £20,005 Details correct at publication date