Hello,

I thought a few wiser heads may be able to answer a couple of questions for me.

1) What is the speed which gives optimal fuel efficiency on the 1.9 diesel?

2) How fast does it tail off after this point?

3) Does having climate control on econ use a significant amount of fuel? Does this vary on whether it is heating or cooling?

Thanks. (Feel free to poing me at an FAQ if this is all old ground).

I wouldnt go for a set speed in any car for best mpg as conditions will make a massive difference the less you have to press down the go pedal to maintain any speed is the best mpg so say 60mph might be fantastic mpg but with a slight gradient the torque available might be better at 55mph etc. I would add to the point that being in top gear 5/6th depending on model is a common mistake for fuel economy. I often will see better mpg in 5th over 6th on single carriageways where the national speed limits apply, as hand in hand with the above I am not having to press the accelerator down as far to maintain the speed.

Air con hot/cold I can see no reason why hot/cold would make a difference but tbh especially in a diesel it makes little difference due to the amount of torque and I would not choose a tiny tiny increase in mpg over comfort.

Some relevant stuff from the new Haynes manual for the 1.9 & 2.0 Octavia diesels about driving economically:

"Air conditioning absorbs quite a bit of energy from the engine - typically 3 kW (4 hp) or so. The effect on fuel consumption is at its worst in slow traffic. Switch it off when not required."

and

"The engine is at its most efficient when running at a steady speed and load at the RPM where it develops maximum torque. (You can find this in the car's handbook.) For most cars this corresponds to between 55 and 65 mph in top gear. Above the optimum cruising speed, fuel consumption starts to rise quite sharply. A car travelling at 80 mph will be typically be using 30% more fuel than at 60 mph."

2000 rpm / 60 mph and I average 52 mpg. Anytime I charge up and down the motorway it drops below 50.

After I changed to Michelin Energy Saver tyres fuel consumption bumped up to 57 mph.

However, very quickly thereafter I found a gradual fall off over few months. A replacement MAF cured this and now I am back to 57 mpg. :sun:

Edited 13 March, 201214 yr by FriendlyFire

MAF?

my 1.4tsi Max torque - Nm/rpm 200/1500-4000

meaning anywhere between 1500-400 rpm?

Max torque on the 1.9 PD comes in around 1900 - 2000 rpm, max power around 3300 rpm.

MAF?

Mass Air Flow sensor, or Air Flow Meter