When I was a lad, there was nothing about the BMW 5-Series that remotely interested or excited me, and I've always found the idea of fitting a small engine to one of these large saloons rather depressing. The new 520d shows how wrong I've been on both counts. I really must grow up one of these days.Its engine produces a maximum of 163bhp, and although this isn't the most that anyone has squeezed out of a two-litre turbo diesel (there are several units on the market offering more than 170hp) it's enough to give the 520d an impressive turn of speed - very similar, on paper, to what you can expect from the more expensive and considerably less economical 523i.Actually, there is no other 5-Series that's as economical as this one, even though it's alone in the range in not having been given BMW's full EfficientDynamics treatment, which provides greater performance along with better fuel economy and lower CO2 emissions. Mechanically, it's the same as before, whereas the 525d has been improved substantially, and frankly the 525d has all but caught up.The larger-engined car is quite a bit quicker, but it comes uncomfortably close to the 520d in terms of combined economy (45.6mpg compared with 47.9mpg) and CO2 (165g/km verus 158g/km). The saving grace for the 520d is that it's nearly £4000 cheaper.Although this is the slowest 5-Series, it has enough low-rev grunt to let you make a decent progress even if you impose a 2000rpm limit on yourself when changing up through the six gears in the manual transmission. And this is by no means the only thing that makes the 520d a particularly relaxing car to drive.For example, it's reasonably quiet. You can never entirely forget that there's a four-cylinder diesel under the bonnet, and there are certainly other 5-series BMWs that offer a more refined engine note, but the noise is never intrusive.Another plus point is the ride, which is the best I've experienced in any BMW for years. That's despite the fact that it uses run-flat tyres with a relatively low profile suggesting that the recent problems with unforgiving rubber (quietly acknowledged by a German BMW spokesman I had lunch with shortly after my stint in the 520d) have been almost entirely solved.This is the least expensive car in the range, but unless you drive a more lavishly-equipped 5-Series on the same day it hardly feels like a cheap option. The test car's seats were very comfortable, there's enough adjustability in the driver's seat and the steering wheel to allow most human beings to find a position that suits them, and there's a lot of space in both front and rear.Plently of room for luggage, too. The boot has a very long floor and, unlike those found in other large saloons, it has a wide enough opening to let you load large items without too much heaving and cursing. Maximum capacity is 520 litres, which is actually 20 litres more than the Touring estate version provides with the rear seats in position, though if you fold them down you open up a volume of more than 1650 litres. The saloon can't begin to compare with that, of course, but the flipside is that the Touring costs £2010 more.Another way of spending extra money is to ignore the SE specification of the test car and go for the M Sport instead. For the £30,100 that that car costs, you get fancier trim, larger wheels (18" instead of 17"), lower profile tyres, upgraded trim, an aerodynamic body pack and sports suspension. The appeal of all these things is obvious - well, to varying degress, anyway - but you know what? If it were my money, I think I'd be satisfied with the subtler charms of the SE. Engine 1995 cc, 4 cylinders Power 163 bhp @4000 rpm Torque 251 ib/ft @2000 rpm Transmission 6 speed manual Fuel/CO2 47.9 mpg / 158 g/km Acceleration 0-62mph: 8.6sec Top speed 139 mph Price From £26216.00 approx Release date 24/03/2007