Odd MPG on return journeys

I'm with the wind affect group. Prevailing wind is southerly rather than northerly. At motorway speeds effective frontal area and speed are the key factors. Would be interesting to see if things changed with a northerly wind direction. Minor factors would be amount of non-motorway driving especially when cold and how much traffic conditions affect your speed. Weight would be more a factor for stop start driving.

That sort of difference is precisely what I get 'upwind' versus 'downwind'

The other vast difference is stop start.

Especially from speed.

Without regenerative braking every stop from say 60 mph is an energy loss of 1/2 mv^2, which for a 2 tonne car or van is ~ 720 kJ

Diesel burnt in a car or van generates about 11MJ at the wheel per litre, so you use about 0.06 litres of diesel to get back up to speed again.

Every time you stop from 60mph It costs you 8p to get up to speed again.

Let's look at it another way, say the car will cruise at 35mpg.

Over 70 miles that's 2 gallons, but if you have to say slow down and stop for 30 roundabouts you can add 1.8 litres to that. 0.4 gallons, so your mpg will fall to *29 mpg*.

Obviously that is an extreme case, but it is why these days I am taking the longer road to town that has almost no traffic and fewer corners to slow down for.

And some fail stick wide open.

That doesn't actually make much difference.

If you accelerate your car to 70mph then it takes the same amount of energy (as in gives the car the kinetic energy of going along at

I seem to remember that BMW actually said that accelerating as quickly as possible to cruising speed gave best economy because the engine was worked near maximum efficiency for longer.

Of course you lose out a bit on consumption because you go a bit further at the higher/cruising speed where consumption is higher.

Also of course fierce acceleration and deceleration do wear things out faster and increase your costs that way.

What I remember from many years ago was Porsche saying to accelerate quite briskly to 30 mph - but not "as quickly as possible". That could imply wheelspin and other things that would worsen fuel consumption. (Maybe not in a van - but quoting BMW does rather suggest a somewhat more powerful motor.)

I've seen a car blow it's head in winter from a removed thermostat (little cowboy trick I've spotted enough times to spoil someones sale :) )

Car was whizzing along in sub zero temperatures and the (naturally anti- freezeless) water froze in the bottom of the radiator cicruit, preventing the engine being cooled. Coldest day of the year the car overheated and blew the head gasket.

Roland Perry formulated the question :

No, it is definitely what is called a fuel gauge. I do have a separate digital display fuel range which I can pull up.

The Natural Philosopher explained :

That uses a BMW 2L engine, those stats do become tired and begin to open early. A common fix was to install by bodging a second stat, forced into the hose from the head.

The engine temperature is reported to the ECU and the ECU controls the behaviour of the auto-box. So it will work better.

How long is the journey? On my trip London to Aberdeen (540 miles) I often try 95 octane one way and 97+ the other. To see if there is any difference in MPG (I don't notice any difference performance wise, but the car says to use 97+ where possible) And have never had a definite result.

Bosch engine management controls the coolant temperature gauge. So it will register bang on the middle (or wherever normal is) over a quite wide range of actual coolant temperature. To avoid panicking those who don't understand how an engine works. On some versions, you can get the OBC to show you the true coolant temp.

In message <rnp2l5$oij$ snipped-for-privacy@dont-email.me, at 13:47:16 on Mon, 2 Nov 2020, Harry Bloomfield <?.?@harrym1byt.plus.com.invalid> remarked:

Yes, but set up by the manufacturer to be in effect a "range" meter.

Dave Plowman (News) used his keyboard to write :

I can call my actual, as measured temperature up onto the digital panel, with a sequence of trip reset button presses. It provides lots of data, if you have the patience - temperature, engine coolant temperature, battery volts, fuel contents for both sides of the tank, etc..

Presumably you can get to 90mph before 90 degrees C !

Remember that the van was either travelling at 80+ mph or accelerating hard to get to that speed :-)

Well pretty much every run from home to Leicester and back has shown the same thing.

Of course. But I slow down for the 40MPH speed camera.

Perhaps there are more speed restrictions on the return route.

Yes that's right. My diesel gauge never moves from the vertical.

Bill

Fifty years ago the normal cure for a faulty thermostat was to either remove it or put a nail through it.

Bill