Fuel Economy Reporting Thread

hewhoisnotintheknow

i drove from southampton to dover m27 m3 m25 m20 (?) at around 55-60mph and achieved 74.4mpg average. This is in a 2.0 hdi. I then got to france and flew down the a28 and the average shot down !

tomstickland

That's a very interesting article and is finally based on some data. Aerodynamic drag forces are approximately proportional to speed squared and this is the bulk of the work that's done at high speeds, so you'd expect lower speeds to return better mpg.

tomstickland

I've got the results from my 1700 mile trip to France and back.
This is based on driving at a near steady speed on motorways, though I did slow on large hills to keep the indicated instantaneous mpg above 30mpg

Mway at 70mph, plus 10 minutes at around that plus 30: 39mpg
Mway at 70mph, being very careful: 42mpg
Cross country and around half a tank on motorway at 70: 39mpg
Mway at 85mph indicated, slowing on big hills: 37mpg

I'm just finishing off the last tank, which included a long session driving with little regard for fuel economy, I'll update on that soon.

I'm very impressed with these results - this is in a 1990 BMW 318iS (1.8 16v) with 220,000 miles on the clock.

There's about a 5mpg improvement in reducing the speed from 85 to 70mph, which is a 13% improvement for driving around 13% slower.

Just for fun, I worked out that at 70mph it's costing 15p per minute and at 85mph it's 21p/min. Obviously at the faster speed less minutes are spent, but as I was sat on the road for 14 hours yesterday, I did wonder what the cost per minute was.

I'm a little confused that mpg seems to be inversely proportional to speed, not speed squared like I expected. I'm going to ponder this for a while.

klint

tomstickland wrote: »

Mway at 85mph indicated, slowing on big hills: 37mpg

I would be interested to see what would happen if you actually sped up when going uphill, and slowed down when going downhill. I think it would improve your average consumption for the same total journey time.

tomstickland wrote: »

I'm a little confused that mpg seems to be inversely proportional to speed, not speed squared like I expected. I'm going to ponder this for a while.

It could be because of fuel used to power your headlights, air conditioning or stereo. This is measured in gallons per hour, independent of speed, and thus the faster you go the more miles per gallon is attributed to these factors. This perhaps can cancel out the speed-squared relationship and make it look like proportionality.

tomstickland

klint wrote: »

I would be interested to see what would happen if you actually sped up when going uphill, and slowed down when going downhill. I think it would improve your average consumption for the same total journey time.

Most fuel economy gurus think that it would do the opposite. ie: reduce the mpg.

jeferey

I'm up to 53.3 mpg - not going above 60mph and gentle acceleration.

I need some new front tyres - anyone got any tips on best ones for fuel economy without losing safe grip?

AdrianHi

jeferey wrote: »

I'm up to 53.3 mpg - not going above 60mph and gentle acceleration.

I need some new front tyres - anyone got any tips on best ones for fuel economy without losing safe grip?

Yes, look up the latest Michelin Energy tyres just out this Spring. I first learned of them on MSE, was sceptical, but after reading reviews and independant tests they seem to be as good if not better than the best of the rest of the "touring" tyre bunch for grip in the wet and dry. You won't notice a huge saving on fuel, perhaps £80-£160 across the whole life of the tyre - every little helps.

jeferey

I've just come back from East Midlands Airport - constant speed of 50mph in the roadworks (cameras!) so I kept it below 60mph the rest of the time going down and back - I've hit 60mpg for the first time - fantastic.

tomstickland

I was reading What Car whilst waiting at the dentist. The Aug 08 issue has a report on mpg, including detailed measurements. All of the vehicles tested produced better mpg as speeds were reduced, with the best results at the lowest speed that they tested at, which was 40mph.

jeferey

tomstickland wrote: »

I was reading What Car whilst waiting at the dentist. The Aug 08 issue has a report on mpg, including detailed measurements. All of the vehicles tested produced better mpg as speeds were reduced, with the best results at the lowest speed that they tested at, which was 40mph.

I would agree with this Tom - I've started doing 40 on my way to the motorway on a 50 mph limit road (doesn't wind too many people up!) and my on board computer definitely starts ticking upwards.