MPG - a piece of string

[watdaluck]

According to studies backed by the department of energy, the average car will be at its advertised MPG at 55 mph. But as the speed increases:

* * * - 3% less efficient at 60 mph
* * * - 8% less efficient at 65 mph
* * * - 17% less efficient at 70 mph
* * * - 23% less efficient at 75 mph
* * * - 28% less efficient at 80 mph

Just tuck in behind the car in front = less aero drag.
pump your tyres up = less road drag
clean your car out = less unnecessary weight and less road drag

TaDa

 

[chriswales6]

The combined MPG figure is usually quoted and 20 years ago you had a reasonable chance of getting that. I can normally get close to it with my 110 TDI, but that has a 1990’s fuel control computer. As improvements were made to computers in cars it increased the power produced and manufactures also programmed the fuel systems to perform well in the fuel consumption tests. Resulting in diesel cars with more power than mine advertised with the same MPG figures and cars with same power as mine supposedly getting 20MPG more. In reality the more powerful cars like the PD150 sometimes getting under 50MPG and the new 1.6 110 TDI getting mid 50s the same as my old 1.9 110 TDI.

The official tests are done at an ambient air temperature between 20c and 30c, which we only match in the UK for a few weeks a year – if we’re lucky. They’re on a rolling road that doesn’t take account of drag from wind or having to drive up hills. The acceleration sections of the test would seem painfully slow in today’s traffic. The Extra Urban part of the test does take the car up 75mph, but the test only lasts 4.3miles and the average speed is 39mph.

Having spent a few years commuting on motorways, when I try to travel at 75MPH often I’m not. Because I have to slow down, then give it a good boot full of fuel to get back up to my cruising speed. Which will end up using more fuel than travelling at a constant speed, also it’s rather hilly around my way so I’m doing fairly well to match and occasionally better my official combined MPG.

 

[Maria]

I had a 20 year old Granada that was still returning the 21 mpg it was officially launched with.
For the past 12 years I've always had diesel and those have always been pretty close to their official mpg regardless of age which has been more than 10 years old in all cases.
Petrol however a different story. Two hire cars that should have done better than my big diesel lumps have both failed miserably on 65-70 motorway runs. Diesel I'm sure returns much better fuel economy at speed. Some (Rover springs to mind) lets you down badly around town. Peugeot non-turbo and HDi truly awesome regardless of speed. My Leon does the best overall but above 80 it can't compete with a large Pug on fuel. 800 motorway miles on a 70 litre tank at motorway speed ... that takes some beating.