Hi all, after some advice on Fuel Consumption.. old e53 V6 Petrol with LPG fitted and the Computer shows 16.8 mpg on the motorway at 65 mph. I know it has the Aerodynamics of a London Bus but it does seem high! Plugs and Air Filter all been changed....it does use a small amount of water, about 400ml on a 150 mile run, no visible leaks, Head Gasket? any suggestions please? Thank you in advance

Morning Martin

Welcome to the Forum

First your engine should be a straight six in Petrol or Diesel flavour. Obviously Petrol in your case as you have LPG fitted (if it is a V6 then it isn't a BMW engine)

As a comparison my 2003 V8 4.6is with 157k on the clock shows an average 18.8 mpg mainly local over the last year I also tow a horse trailer, 300+ miles to a tank is average but when we visit our Daughter in Cannock if the Motorways behave with cruise on at 75 we have made the 360 mile round trip on a single "tank.

Don't want to "teach Granny" but many things have an effect on MPG 

Roof Rack. a roof carrier left fitted can knock mpg for six, one of our sons has a bike rack it knocks around 20% off his mpg (he runs a diesel Peugeot)

Tyres, are they in good condition correct for the car and properly inflated (no signs of odd wear)

Suspension, worn suspension bushes on a heavy car like the X5 will cause odd tyre wear 

Brakes are they all in good condition and working correctly (no binding)

Engine, is it starting straight away hot or cold? Does it tick over smoothly at around 5/600 rpm. No misfires or major leaks? A pressure test would help find your water leak 

Gearbox and the drive train (Transfer box and Diffs) when were they serviced no noises clunks or rattles.

All of the above will contribute to poor MPG put any 2 or 3 together and it will wreck your economy.

The 3 litre straight six is a very economical engine in the 5 series and 3 series cars, in the X3 and X5 AWD takes its toll as does the body weight so economy is similar to the V8's as the six has to work harder to achieve the same level of performance.

Good Luck Enjoy your X5

Dave