Odd MPG on return journeys

[ARW]

So I am working in Leicester a lot at the moment.

I always do a MPG check on the journeys.

Now I always get a better MPG on the way back than on the way there.

It cannot be load. Sometimes I come back with more than I set off with.

The speed it always the same - true 80MPH on cruise control apart from the bit in Sheffield where there is a 60 limit for air quality so I drop to 66MPH.

Same route and same mileage both ways.

30.4MPG on the way down and 32.3 MPG on the way home today. The only difference was I put 75L of diesel in before setting off back.

Any ideas?

[Andrew]

Extra weight of fuel making the van nose-down, so more aerodynamic ?.

[Jethro_uk]

prevailing wind ?

Air temperarture ?

Tyre pressure ?

[Adrian]

In message <UaCnH.909090$ [e-mail], ARW [e-mail]> writes

First guess would be that it was an easier journey on the way back (fewer, shallower climbs), but I'm not sure if that applies to the M1 between Barnsley and Leicester.

Adrian

[Andrew]

Leaving early in the morning and the engine has cooled down overnight. Returning in the evening it is still warm from that morning's journey? Poor mpg as it warms up enough to bring the total journey mpg down a bit.

As an experiment try resetting the mpg meter after fifty miles or so.

[Andy Burns]

Between Leicester and Nottingham I generally get lower mpg on the way there and better on the way back, I've generally put it down to the mixture of long shallow slopes and shorter steeper slopes in one direction vs the other (it's hardly the Himalayas but there is about a

200m range of heights)

[ARW]

Cold engines have the poor MPG.

[Andrew]

Exactly 30.4mpg when it is cold in the morning and 32.3mpg when it is warmer on the way back.

[polygonum_on_google]

I often find this happens. Apart from things like difference of elevation of end points, and wind, I suspect the actual "profiles" of the routes have an impact.

For example, a mile of strong climbing, followed by five miles of gentle downhill might be very different to five miles of gentle climb followed by one mile of steep downhill.

[ARW]

6 to 8 hours on a drive should give the engine time to reach ambient temperature. 3 miles driving at most to reach an engine temperature of 90 deg.

[Harry Bloomfield, Esq.]

ARW was thinking very hard :

Both are terribly poor mileages, if its a car derived van type.

[The Natural Philosopher]

In my youth we took a friends mums car - something like a 50s ford escort maybe, or a hillman - cant remember - up north, and back. Got nearly 40mpg on the way, but only 34mpg on the return journey. We then realised there had been a reasonably stiff southerly breeze...

80mph ±20mph breeze takes the airspeed from 60 to 100 mph. A hell of a difference to a van shaped object.

[Dave Liquorice]

If that bit of the country is anything like this bit today you would have had a fairly hefty tailwind on the way back and headwind going.

[NY]

My car takes a lot longer to reach steady temperature than it used to - according to the temperature gauge, at least. On a day when the ambient temperature is about 10-15 deg C, and with the engine cold from overnight, it typically takes about 8-10 miles of mostly 60 mph driving before my gauge is showing its normal temperature - conveniently designed so the needle points vertically when it's at the normal, steady temperature. Being a diesel it will produce less heat than if it was a petrol, all other things being the same, but I do wonder whether the thermostat is playing up slightly. Would a duff thermostat mean the car takes longer to reach normal temperature than it used to?

[NY]

How reproducible is your mpg normally? If there is a journey that you do frequently, always in similar driving conditions, how much variation in mpg do you get? In other words, is a difference between 30.4 and 32.3 for out and return within the normal variation that you might get?

[williamwright]

You're less stressed so speed up and slow down more gently.

Bill

[Roland Perry]

In message <rnnba8$hqg$ [e-mail], at 22:02:13 on Sun, 1 Nov 2020, NY [e-mail] remarked:

In a traditionally designed "proper drivers' car", with lots of mainly analogue needles, the optimum operating conditions are when they are all vertical. So you can see at a glance if any of them are 'off'.

[Brian Gaff (Sofa]

Different type of diesel? Just like Petrol its energy density can change as it ages or with different processes at the manufacturing stage. However you need to find out if it works in reverse if you refuel the other way around.

More likely traffic hills and stuff like that. Brian

[Harry Bloomfield, Esq.]

NY presented the following explanation :

That depends on the failure mode. Some fail passing a little water all the time, others fail by opening sooner at a lower temperature than designed temperature.

Some temperature gauges are deliberately programmed to show precise optimum temperature, even if some way above or below the genuine temperature. My own gauge behaves like that.

My fuel gauge is programmed to show a level, which depends on present MPG. With low MPG the gauge goes down, with better MPG it can go up. Start the car on a cold morning and it might show almost empty, drive a quarter mile and it shows a much healthier level.

[Roland Perry]

In message <rnof7u$8fg$ [e-mail], at 08:15:57 on Mon, 2 Nov 2020, Harry Bloomfield <?.?@harrym1byt.plus.com.invalid> remarked:

That sounds like a "range" meter. Which many cars have, but not as the legacy fuel gauge. When I'm on a long trip it's often the case that the "range" meter steadily increases, the further I go.

Typically starts off at about 300 miles, and can creep up to 500 if driving the first 100 miles carefully.