Cheap petrol & diesel – official MSE guide discussion

MSE_Kelvin

Hello there,

Sadly, after 14 years of active service, the original Cheap petrol & diesel thread is now closed, so I've created a new one here, which is specifically to discuss the following guide:

Cheap petrol & diesel

If you've any questions or any new tips, please post below.

Cheers,

MSE Kelvin

born_again

>>Fuel is heavy, so by filling the car up you're adding quite a weight. The less fuel your car has in it, the more efficiently it drives. If you fill up slightly more often and put less in (to 1/2 or 3/4 full), it'll make the car run more efficiently.<<

But at the rate prices rise at times, It can end up costing more to top up again, rather than filling tank up to the brim.

A ltr of fuel is around 740 grams so 30 ltr = 22.2 KG

I bet most people carry that much junk in their car anyway.

Ebe_Scrooge

I have to agree with you, born_again, this one has always seemed a bit extreme to me.  Yes, of course, in theory every extra ounce of weight will make a difference.  In a Formula 1 car where fractions of a second on a lap can make a difference, then it's worth considering (OK, that's primarily to do with performance rather than fuel economy).  But for an ordinary car, I bet you'd be hard-pushed to even measure the difference it makes.  Added to which, an ordinary car weighs - what, 1000KG or so?  So the 10/15/20 KG difference between a full tank and a quarter-full tank is negligible.  The old chestnut is true - you'll save far more by having your tyres inflated correctly and having a gentle right foot.

Another consideration, which not all drivers realise, is that any electrical equipment will affect fuel economy.  The bottom line (and perhaps slightly over-simplified) - electricity is generated by burning fuel.  So any time you switch on something that draws current, you're burning more fuel.  Something like the heater fan or the radio will draw very little current, headlights a bit more, heated seats/rear windscreen will draw a reasonable amount.  Sit with your car idling, switch on the heated rear screen, you'll probably notice the engine speed drop ever so slightly.

Yes, the real-world difference in fuel consumption is likely to be tiny, but it's worth bearing in mind.  Most things you can't change anyway - if it's dark, you need your headlights!  But don't, for instance, leave your rear screen heater constantly switched on if you don't need it (lots of cars will switch it off automatically after 15 minutes or so, for this very reason).

Grey_Critic

Ebe_Scrooge said:

perhaps slightly over-simplified) - electricity is generated by burning fuel.  So any time you switch on something that draws current, you're burning more fuel.  Something like the heater fan or the radio will draw very little current, headlights a bit more, heated seats/rear windscreen will draw a reasonable amount. 

Just like leaving the TV on standby - it costs about £10 a year by doing so.

scrappy_returns

Grey_Critic said:

Ebe_Scrooge said:

perhaps slightly over-simplified) - electricity is generated by burning fuel.  So any time you switch on something that draws current, you're burning more fuel.  Something like the heater fan or the radio will draw very little current, headlights a bit more, heated seats/rear windscreen will draw a reasonable amount. 

Just like leaving the TV on standby - it costs about £10 a year by doing so.

That's not the case with modern equipment. Still a waste though

born_again

But think of all that wasted energy & expelled C02 required to walk to the TV & back to the settee 🤷‍♂️🤣

Herzlos

born_again said:

>>Fuel is heavy, so by filling the car up you're adding quite a weight. The less fuel your car has in it, the more efficiently it drives. If you fill up slightly more often and put less in (to 1/2 or 3/4 full), it'll make the car run more efficiently.<<

But at the rate prices rise at times, It can end up costing more to top up again, rather than filling tank up to the brim.

A ltr of fuel is around 740 grams so 30 ltr = 22.2 KG

I bet most people carry that much junk in their car anyway.

The tiny percentage of fuel saved would likely be wasted by the extra trips to the petrol station anyway.

w12ee3e

Is it me or does the colder weather affect fuel economy ever so slightly? Even without running the heater. Very subjective of course but back in the summer my average MPG was about 5 higher. Air colder / more dense perhaps?

Ebe_Scrooge

w12ee3e said:

Is it me or does the colder weather affect fuel economy ever so slightly? Even without running the heater. Very subjective of course but back in the summer my average MPG was about 5 higher. Air colder / more dense perhaps?

In a nutshell, yes, you'll use more fuel in winter.  There are many factors, here are just some:

1.  You're using more electrical equipment (see my previous reply).  You'll have your screen demister on more, you'll have your headlights on more, heated seats if you've got them.

2.  The cold weather itself.  When the engine is cold, the fuel/air mixture is enriched (proportionately more fuel in the mixture).  In the olden days (or if you have a petrol lawnmower or similar) we pulled out the choke lever, these days it's done electronically.  But every time you start the engine from cold, it's running on a richer mixture until it warms up - which takes longer in cold weather.

3.  Cold weather starting.  When you start your car, it probably doesn't fire into life as readily as it does on a warm summer day.  So you've depleted the battery more.  That extra energy has got to be paid for by the alternator working harder/for a longer time to recharge the battery a bit more than usual (refer to point 1!)

4.  Rain - the roads tend to be wetter in winter.  Have you ever hit a big puddle on the side of the road, and felt the car pull to the left?  That's because there's more drag on the left side.  If the whole road surface is wet, you've got more drag overall, hence it takes more fuel to maintain a given speed.  And exponentially more to accelerate.  And the roads tend to wetter for a larger proportion of the time in winter than they do in summer

Each of these factors in themselves probably add just a very tiny (or even insignificant) amount to your fuel consumption.  But added together, and if they're all present every time you go out during the winter, they add up.

Jenni_D

Is there not also a (small) volumetric issue, given that volume of a liquid changes with temperature? (Otherwise why would fuel pumps be calibrated at a set temperature)?

Petriix

The best way to save money on diesel and petrol is to avoid buying it altogether. My EV is running at 1.1p per mile over 10k miles this year. That's saved me about £1000 towards the cost of upgrading my car.