Hypothetical mpg question fo a 90 mile journey.

Ultrasonic

londonTiger wrote: »

60-65mph gets you the best mpg figures. This is partly why lorries are limited to that speed.

Any faster and the drag pushes back harder and causes the car to use more fuel per distance covered.

Missed this earlier. That's not true I'm afraid.

Aerodynamic drag doesn't suddenly become a factor at around 60 mph. Virtually every car on the roads would get better fuel economy in top gear at 50 mph compared to 60 or 65 mph. There are exceptions, but generally speaking optimal efficiency for steady state driving is achieved at the slowest speed that a vehicle drives comfortably in top gear.

For paddedjohn's example of a van the aerodynamic drag effects will be larger than for cars, making the fuel savings of slower speeds larger. Not that I'm suggesting doing a 90 mile trip at 35 mph!

bsms1147

With the hypothetical scenario proposed, the car traveling at 30mph will use three times the fuel as the one travelling at 90mph, minus whatever things you want to factor in, such as air resistance.

I doubt the air resistance (and other unaccounted for factors) could treble the fuel-use of the 90mph option so that it matched the fuel use for the 30mph option.

vaio

bsms1147 wrote: »

With the hypothetical scenario proposed, the car traveling at 30mph will use three times the fuel as the one travelling at 90mph, minus whatever things you want to factor in, such as air resistance.

I doubt the air resistance (and other unaccounted for factors) could treble the fuel-use of the 90mph option so that it matched the fuel use for the 30mph option.

if that's true then it seems strange that (generally) fuel economy gets worse the faster you go and, during the fuel crises of the '70 & 80's most governments lowered speed limits to reduce fuel use

reeac

Norman_Castle wrote: »

I'm ignoring the variables because they are not given and are potentially endless. Is this a Luton van with a low torque engine on a windy day which only maintains 90mph with the throttle on the floor or an aerodynamic car on low resistance tyres fitted with an engine producing maximum torque at 2000 rpm and ideally suited to 90 mph cruising ?

Not a good tactic to ignore factors which aren't tightly specified. That way you'd ignore the effect of weather [notoriously variable, at least in the UK] on the fuel consumption of your house.

Norman_Castle

Ultrasonic wrote: »

If you were a newbie I'd assume you were trolling.

The difference in air resistance between 90 mph and 30 mph is a HUGE factor, you simply cannot ignore it.

I had a 1.4 Citroen which averaged 40- 45 mpg at 1500 - 2500 rpm in general city driving, 20-40mph, and 35-40 mpg at a constant 80-90 mph at 3500 rpm. It did not use three times as much fuel.
The difference air resistance makes depends on how aerodynamic the vehicle is.

Norman_Castle

reeac wrote: »

Not a good tactic to ignore factors which aren't tightly specified. That way you'd ignore the effect of weather [notoriously variable, at least in the UK] on the fuel consumption of your house.

The alternative to knowingly ignoring unknown factors is making them up, which is possibly a worse tactic.

My friend budgeted £200 to heat her home this year. Is that a reasonable amount?

LandyAndy

anotherbaldrick wrote: »

Put it this way. At 90 mph my 3 litre TDI in 6th would be turning over at 3600 rpm, it is cranking 2800 rpm at 70 mph. It red lines at 5000rpm at 125 mph.

:eek:Are you sure? That (25mph per 1000 revs) seems very, very, low geared for a big modern diesel. The 2.2 diesel in my Mazda6 pulls nearly 40 per 1000 in 6th.

WTFH

bsms1147 wrote: »

With the hypothetical scenario proposed, the car traveling at 30mph will use three times the fuel as the one travelling at 90mph...

Yes. If the test was carried out on a rolling road, then you're correct.

bsms1147 wrote: »

I doubt the air resistance (and other unaccounted for factors) could treble the fuel-use of the 90mph option so that it matched the fuel use for the 30mph option.

It would account for a lot, but exactly how much, no one can say without knowing the specifics of the vehicle, route and conditions in question.

anotherbaldrick wrote: »

Put it this way. At 90 mph my 3 litre TDI in 6th would be turning over at 3600 rpm, it is cranking 2800 rpm at 70 mph. It red lines at 5000rpm at 125 mph.

My car has a 2l engine. In 6th gear it does 56.19km/h per 1000rpm. My wife's car is also 2l, but in 6th it does about 52km/h per 1000rpm. I wouldn't be surprised to find an efficient 3l diesel transmission would be over 70km/h per 1000rpm, and if it was a van, it's possible the gearbox could be set for town use in 4th, but doubtful.
What age/type of vehicle is your engine in - 40km/h per 1000rpm doesn't sound particularly economical?

reeac

Norman_Castle wrote: »

The alternative to ignoring unknown factors is making them up which is possibly a worse tactic.

My friend budgeted £200 to heat her home this year. Is that a reasonable amount?

No, of course it isn't unless she's a masochist. Your friend should have referred to her own historical useage or asked a few other people for theirs. I say useage [in kWh of gas or electricity or litres of oil] in order to allow for the inclusion of increased fuel prices. Alternatively read a few of the many recent newspaper articles on fuel costs which quote typical overall household fuel bills of £1300-1400 p.a.

reeac

Norman_Castle wrote: »

I had a 1.4 Citroen which averaged 40- 45 mpg at 1500 - 2500 rpm in general city driving, 20-40mph, and 35-40 mpg at a constant 80-90 mph at 3500 rpm. It did not use three times as much fuel.
The difference air resistance makes depends on how aerodynamic the vehicle is.

You're comparing stop-start city driving with constant speed open road driving. Stop-start has a big effect on mpg.. My Jag. fuel computer shows 7 mpg. instantaneous when accelerating but does 25-26 mpg. overall.