OT: mpg question

Much the same happened to me on a longer trip of 260 miles which I'd done a few times. Yes, I'm pretty sure it's the wind that makes the difference.

A pair of wing mirrors was once measured as using 2bhp at 60mph.

drag is proportional to airspeed squared. Drag times distance is energy lost to drag. Over 50mph most of the energy loss is drag. I cant be arsed to work out actual numbers, but its moire than plausible.

I remember many years ago my Triumph Spitfire - which used to do about

Before that a friend had some old Hillman and we drove North with a following wind at never more than 55mph, and south again into the wind. Again the mpg was significantly different.

North is up hill all the way, South is down hill.

My friend got it to 60 mpg with some very skilful and cautious driving away from motorways. .

My car (Peugeot 308 with a diesel engine) was the subject of a well-publicised tour around the UK by two "feather-footed" drivers who tried to use as little fuel as possible. They apparently averaged 90 mpg. I think I tend to drive fairly economically (gentle acceleration, keep to speed limits, lift off power early to lose speed rather than driving right up to a junction and then braking hard). But the best I've averaged on a tankful of fuel is about 60 mpg - and that may partly have been due to random filling differences (different pumps cut out at different points). The average over about 170,000 miles (*) is 54 mpg. It's possible that the car used for the record had a less powerful engine (if that makes it more efficient) and I couldn't find any mention of whether the drivers had a lower self-imposed maximum speed to use less fuel. Maybe they only did each leg of the journey on a day that the wind was predominantly behind them ;-)

(*) Mainly motorway, A road and country lanes, but almost no stop-start in heavy traffic.

well its actually the other way, north is down wind and south is into the wind. which was around 20mph+ yesterday and broadly southerly

I turn my map through 180deg when going North. Saves a small fortune.

I find motorways give me the best mileage - no corners or winding lanes.

In March I drove from Woking to Leicester in a Prius. Before leaving, the Satnav said the distance was 118 miles and the trip computer estimated the range available to be also 118 miles. Reaching the M1 I tucked in behind a sequence of HGVs and made the journey at 60-65mph. Reached home with an estimated 5 miles range still available.

Or all unnecessary weight removed from the vehicle. No spare wheel, no luggage, no back seat etc.

Depends on loads of things, not least the envelope where the engine produces the most efficient output - usually c. 3000-3500 rpm for petrol.

I once saved 25% on a 100 mile journey by doing 55 instead of 75.

You can tell how much of this climate "emergency" is bollocks from the fact that a rigidly enforced 65mph limit on motorways would save the UK quite a bit in fuel consumption. (Although not as much as not travelling at all).

I guess that a lot of people who think they are doing 70mph are actually doing 63 to 65mph because most speedometers read 10% high.

Risk of accident wise, boredom on a long motorway journey is possibly a higher risk than speed.

Just pull the nozzle out a bit and continue to fill - carefully. I usually manage to get 2 litres more in, doing that.

Depends on the wind speed but yes it could easily make the difference since the drag is proportional to the square of the speed that the air is going past your car. If you have the wind behind you then you actually get less drag than in still air and into a headwind more.

to Perth = k(V-v)^2 from Perth = k(V+v)^2

The increases in fuel costs don't affect me because I always put exactly £30's worth in.

Bill

And maybe only put in a small amount of fuel and keep filling up, to reduce the weight of fuel. 60 litres of diesel weighs about 50 kg which is about half a person (OK, half a fairly heavy person like wot I am).

My grandpa had lived through fuel rationing in the war and had been taught how to conserve fuel for the times when he was making an authorised journey. It's ironic that his technique of slipping into neutral as he was slowing down (which he still did to the day he stopped driving shortly before he died in 1999) would actually be counterproductive with a modern computer-controlled engine. I've proved it with the instantaneous fuel-consumption menu on my car: pressing the clutch and keeping the engine idling uses a measurable amount of fuel (which displays as around about 150 mpg) whereas lifting off the throttle and staying in gear (ie clutch not pressed) uses a mythical 999 mpg. That's because the computer can cut the fuel *totally* if the motion of the car on overrun will keep the engine turning, whereas with the car in neutral (or clutch pressed) a token amount of fuel is needed to keep the engine ticking over.

Interesting. I never knew that. I was taught to move into neutral in advance of the stop and coast to save fuel but when I did the Advanced Driving course, I was told the opposite (in gear at all times when moving).

I tend to let the pump cutoff, try again, cutoff - and regard that as "full".

But I'm well aware that there is quite a lot of variation in the cutoff points of different pumps. I once did three consecutive tankfulls, filling up at the same pump each time (at my local garage) and the consumption figures were more consistent than for other tankfuls which had been filled at a variety of pumps/garages. That suggests that the variation in driving conditions from one tankful to the next can sometimes be less than the variation in cutoff from one pump to another - which surprised me. I did have a reading which was very suspect: dramatically better mpg on one tankful followed by dramatically worse for the next tankful which makes me think that one pump cutoff way before my tank was full.

But in general, a lot of driving on country lanes, slowing down for lots of bends and then accelerating back to a typical straight-line speed in between, will use more fuel than a constant 70 on a motorway, which in turn will use more than a fairly constant 50-60 on a free-running A road. Town driving - lots of short journeys, lots of stop/start at T junctions and traffic lights, uses dramatically more.