General Instantaneous Miles per Gallon

[Mick F]

Hi guys,
500 TA 85hp Lounge.

Just fiddling about with the trip computer thingy yesterday and I selected the instant MPG readout.

Very interesting to say the least!
Our other car is a Clio and that only outputs average MPG since you last reset, so having an instantaneous MPG readout is enlightening to say the least. Clio's long-term average is in the low 30s MPG, and some weeks we only get 28MPG.

Yesterday, we drove from Saltash in our 500TA via Callington to where we live in Gunnislake (Cornwall) and I watched the readout as we drove home. For some of the time on the A388, we bowled along and doing anywhere between 40mpg and 80mpg depending on the slope, and at times it went to full-scale at 99.9mpg.

Going uphill - 5th gear, 50odd MPH, it dropped to the low 30s, but the last and final hill up to our gate, it dropped to 11mpg. Mind you, this is a 1in4 and very narrow and in first gear driving very slowly.

Last time I looked, our average economy was 38mpg. This doesn't surprise me in the least considering that we live in a hilly part of the country.

Greetings from Cornwall,
Mick.

 

[Mercky]

Hi guys,

500 TA 85hp Lounge.



Just fiddling about with the trip computer thingy yesterday and I selected the instant MPG readout.



Very interesting to say the least!

Our other car is a Clio and that only outputs average MPG since you last reset, so having an instantaneous MPG readout is enlightening to say the least. Clio's long-term average is in the low 30s MPG, and some weeks we only get 28MPG.



Yesterday, we drove from Saltash in our 500TA via Callington to where we live in Gunnislake (Cornwall) and I watched the readout as we drove home. For some of the time on the A388, we bowled along and doing anywhere between 40mpg and 80mpg depending on the slope, and at times it went to full-scale at 99.9mpg.



Going uphill - 5th gear, 50odd MPH, it dropped to the low 30s, but the last and final hill up to our gate, it dropped to 11mpg. Mind you, this is a 1in4 and very narrow and in first gear driving very slowly.



Last time I looked, our average economy was 38mpg. This doesn't surprise me in the least considering that we live in a hilly part of the country.



Greetings from Cornwall,

Mick.

I wouldn't say it's particularly accurate but using it is a great way to train yourself to drive economically

 

[UFI]

Last time I looked, our average economy was 38mpg. This doesn't surprise me in the least considering that we live in a hilly part of the country.

A lot of people find their best economy comes on trips with lots of hills. Going up the engine is at it's most efficient (BSFC), and going down the car uses no fuel at all (which is even more efficient).

 

[Mick F]

Here, we disagree.

I'm a keen and frequent cyclist, despite living in Cornwall.
If you were to ask any cyclist, he'd tell you that hilly terrain is hard work despite having downhills too.

A cyclist spends most of the time and effort climbing hills and no effort or time at all going down them. Calories are burnt hugely climbing hills, a few very are burnt going along the flat, but none at all going down. No matter how much you'd like it, the two sides of a hill cannot cancel each other out.

Riding round here - I do nearly 100miles a week - I ride 120ft per mile of total ascent. Ascent is the hard part, descent is simple and easy.

TTFN
Mick.

 

[Sweetsixteen]

No matter how much you'd like it, the two sides of a hill cannot cancel each other out..

Agree. And this principle applies whether you're walking, cycling or driving,
Far better economy is seen on the flat than with the same mileage in hilly terrain.

Crossing hilly Wales from coast to M4, my consumption shows around 64mpg. From M4 right up to the M25 this average continues to rise. It often reaches mid-70mpg by the time I hit the M25. Both parts of the journey are around 100 miles.

Doing the journey in reverse, I often see as much as 80mpg by the time I leave the M4 and enter Wales, and this steadily drops from then on, until I reach the Welsh coast, when the readout shows mid-70's again.

Or maybe it's just the Welsh air!

 

[306maxi]

Yeah, the problem with hills is that if there's a bend on the other side and you have to brake and you lose that energy you put in.

 

[babyferrari]

Sorry but you are going to have to explain that one to me. Going uphill the engine is at its most efficient and I think you also said in an earlier post that it is also more efficient when accelerating hard. I am no physicist and I am not saying you are wrong but something doesn't make sense. It may well be more efficient i.e more energy produced per the amount of fuel used but the total energy used to get up a hill must be more than that used to go on the flat and accelerating hard has to use more energy than not. Just because an engine is running more efficiently does not mean it is not using more fuel?!? And using more energy has to mean using more fuel. Doesn't it?
I am very happy to be told I am wrong and genuinely want to understand. Please enlighten me!
Thanks

 

[babyferrari]

Sorry, meant to quote UFI in my previous post. It was referencing his post about hills!

 

[babyferrari]

Have to agree with sweetsixteen and Maxi.

 

[UFI]

I'm a keen and frequent cyclist, despite living in Cornwall.
If you were to ask any cyclist, he'd tell you that hilly terrain is hard work despite having downhills too.

A cyclist spends most of the time and effort climbing hills and no effort or time at all going down them. Calories are burnt hugely climbing hills, a few very are burnt going along the flat, but none at all going down. No matter how much you'd like it, the two sides of a hill cannot cancel each other out.

Riding round here - I do nearly 100miles a week - I ride 120ft per mile of total ascent. Ascent is the hard part, descent is simple and easy. .

I have a 7.5kg hand built carbon frame bike with Zipp 303 carbon wheels and full Dura-ace electronic groupset (Like a robotized manual for our car friends - if you're wondering 'why?" it means I can shift at full attack uphills without the chain slipping).

Although cycling is actually a great way to think about overall energy expenditure, a human rider is a terrible analogue for a IC engine.

A human is not at it's happiest when asked to give 80-100% of the energy they can produce at a given speed. The human then still has to recover on the decent ready for the next accent, which the ICE doesn't. He still has to breathe, which the ICE doesn't (in fact the ICE switches off all fuel and takes a nap).

The final problem for the descending bicycle is that it has in a full racing tuck position a .83Cd which stops speed building on the decent like it otherwise would.

Yes, you can recover the energy put in climbing (unless you need to brake as Maxi points out, in a 500 you can pretty much tackle any corner at close to the speed limit ). If you remember Newton, your car at the top of a hill has stored kinetic potential, which is released on the decent (much like a hybrid).

UNLIKE a hybrid, which stores energy via gen-sets and batteries at maybe a 30% efficiency, the hill+ heavy vehicle is a 100% efficient energy store (energy cannot be created or destroyed - every ounce of energy used to get up a hill is still there to get you back down). Hybrids work on a very similar principle, in that they add load to the engine when it's not working hard to get it closer to that 80% BSFC point and convert it (inefficiently) into electricity for later use.

Let's look at a made up example just to illustrate my point. You have a hill, 1km of climb, 1km of decent. You average 30mph up but use nothing coming down (DFCO all the way), your average works out to 60MPG, not bad I'd say.

The main issue with hills is you can end up with too much stored kinetic potential and then have to waste it using the brakes, that's where a little technique and familiarity of the terrain comes in.

 

[UFI]

Sorry but you are going to have to explain that one to me. Going uphill the engine is at its most efficient and I think you also said in an earlier post that it is also more efficient when accelerating hard. I am no physicist and I am not saying you are wrong but something doesn't make sense. It may well be more efficient i.e more energy produced per the amount of fuel used but the total energy used to get up a hill must be more than that used to go on the flat and accelerating hard has to use more energy than not. Just because an engine is running more efficiently does not mean it is not using more fuel?!? And using more energy has to mean using more fuel. Doesn't it?
I am very happy to be told I am wrong and genuinely want to understand. Please enlighten me!
Thanks

The threory of BSFC is a hard one to get your head around.

Yes, you're using more fuel, but it's for a shorter amount of time than accelerating gently. Which means you get into your top gear sooner and then you can enjoy low cruising consumption for longer. It's definitely counter intuitive.

In short, if you need power to accelerate anyway, you may as well be getting it as efficiently as possible, and doing it for as short as possible. At BSFC an ICE can be up to 40% efficient. This is not to be confused with simply always driving around at 80% load all the time (which would not be very good at all - side note, in my six speed diesel with very tall gearing I actually do cruise at around 80% load).

Does it all work? My two daily drivers (one petrol, one diesel) are both beating the official fuel cycle figures by 40% (lifetime avg). The petrol is yet to be broken in and I'm still tweaking my technique, more is to come from that one (on the other hand the diesel has been leaking fuel and is now being repaired).

Further reading:

[ame]http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption[/ame]

http://www.autospeed.com/cms/title_Brake-Specific-Fuel-Consumption/A_110216/article.html

And yes of course, climbing a hill still uses more energy that driving on a flat, but it's more efficient energy and it's stored as kinetic potential (see above).

PS, this doesn't make tyre spinning, full throttle redline launches economical. Each each engine has different BSFC points, and manufacturer graphs are hard to come by. You can find your own BSFC numbers if you're willing to hook up a duty cycle meter to an injector. In my old MB, 1800rpm used less fuel (outright) than 1700rpm or indeed 1900rpm. Needless to say 1800rpm became my target cruise speed.

PPS, don't confuse 80% load with 80% throttle. If you let the clutch out at idle, the car wil creep along at maybe 30% load. Point it up a hill (without touching the accelerator) will see the load shoot up potentially all the way to 100%. Conversely if you're accelerating down hill, you could have a high throttle percentage but a relatively low load. Load is a measure of how much torque the engine is producing for that specific RPM. If you accelerate at say 50% throttle load will drop as the revs pick up. Load can easily be measured with an OBD connector and smartphone app, Scangauge etc. I can't even drive a car without a load gauge anymore, I just feel naked.

PPPS, the 80% load figure is based on some cars tendency to go into open loop at 100% load. In my own car's I've found 100% load has always been pretty safe. My Renault is the only one that likes to go open loop, but even then it takes 100% load plus a bit more throttle. The Wrangler will go open too, but only at speeds over 90km/h. I've never seen the Twinair go open loop once warm. Worth noting thought that enrichment may happen before open loop for cars with wide band O2 sensors.

 

[logger]

..PPS, don't confuse 80% load with 80% throttle..

Especially don't confuse load with throttle if you car is a twin air and not fitted with a throttle

Talk about out baffling with BS! At least you describe it as a "theory". I understand what you are saying. I routinely apply similar concepts flying large aircraft. ie climb at maximum thrust, cruise at most efficient level and descend at idle to minimise fuel burn. When fuel consumption is the only consideration it makes sense. When there are other costs to consider it does not.

However I will take a very long time to be convinced choosing a hilly route as compared to a flat alternative will save on fuel (all other things being equal). Then of course there are numerous other costs and considerations apart from fuel economy. Component wear and tear for a start.

Mick, seems I have the same cars as you. I sometimes look at the instantaneous MPG read too. I do find all the fuel consumption info on the TFT instrument cluster overly complex. With two pages for each trip plus the instantaneous one totally five pages it is too much. My VW also has a instantaneous MPG readout. Interestingly it transforms to fuel flow (ltrs/hr) when stationery which the Fiat does not.

 

[UFI]

I routinely apply similar concepts flying large aircraft. ie climb at maximum thrust, cruise at most efficient level and descend at idle to minimise fuel burn. When fuel consumption is the only consideration it makes sense. When there are other costs to consider it does not.

What other costs? Keeping on the ground for now, fuel is the biggest 'natural' running cost for a vehicle. The vast majority of people can't match the official fuel consumption figures, by beating them by 40% over a car's lifetime that's 5700 litres of fuel saved and at a $1.65 a litre that's $10,000+ dollars in my pocket depending on how much fuel prices rise (could well be double that). I'd have to wear out a LOT of components not to come out ahead (bearing in mind components wear no matter how you drive.

I said the theory of BSFC in the same way there's a theory of gravity. If it wasn't real, it wouldn't be in thermodynamics text books (at least those that deal with engines).

 

[logger]

The biggest cost to in owning my Fiat 500 will be depreciation, by far. Way more than I will ever spend on fuel for it.
That aside, my point is that you do not seem to be costing in additional wear and tear induced by your modified techniques. If you are counting pennies why not include ALL the costs you mentioned elsewhere you have incurred in your pastime. I recall you said you do it for the challenge and not to save money, having spent thousands on ECU remaps and so on. Extra tyres, wear and tear induced by launching off like a maniac every time. Drafting trucks, coasting right up to intersections in the hope of lights changing, staring at load gauges instead of looking out the window. I don't doubt for a minute you save plenty of dough on your fuel BTW.

Hilly routes over flat ones for fuel economy. I am not convinced I am afraid.

 

[Mick F]

Thank you for an enlightening few posts, UFI.
(I ride Campagnolo Chorus 10sp triple on my Mercian Vincitore BTW.)

Your explanation is no doubt correct in theory, but in the real world, hilly areas also have corners, some of them tight. If I could straighten out all the roads in Cornwall, my fuel consumption would be the same as someone living in Flatlandia.

It's very rare that any stored energy at the top of a hill can be released fully because you have to brake and ruin the speed advantage. Often, we have to start going uphill with zero run-up, and speed limits make you brake going down.

It ain't so simple round here.

Regards,
Mick.

 

[UFI]

The biggest cost to in owning my Fiat 500 will be depreciation, by far. Way more than I will ever spend on fuel for it.
That aside, my point is that you do not seem to be costing in additional wear and tear induced by your modified techniques. If you are counting pennies why not include ALL the costs you mentioned elsewhere you have incurred in your pastime. I recall you said you do it for the challenge and not to save money, having spent thousands on ECU remaps and so on. Extra tyres, wear and tear induced by launching off like a maniac every time. Drafting trucks, coasting right up to intersections in the hope of lights changing, staring at load gauges instead of looking out the window. I don't doubt for a minute you save plenty of dough on your fuel BTW.

Hilly routes over flat ones for fuel economy. I am not convinced I am afraid.

Depreciation is of course the biggest cost in owning just about any new car, but it's a fixed cost you can't do much about. I consider this an 'artificial' cost much like rego or purchase tax (maybe someone has a better name for it?). But assuming a 200,000km life and $1.65 (6l/100km)for premium works out to $19,800 in fuel. Even with a modest increase in fuel costs, fuel outruns depreciation for a lifetime owner. For our low mile scraping European friends, cars here usually make 200,000km without issue. I know people with 400,000+ on their cars.

I only spin (really I'd call it 'slip' -again tyres produce peak traction at a few percent of slip) my tyres after the clutch is fully engaged and only because the current tyres after seven years have 60+% tread but are beginning to perish (this only applies to my 2.5TDi). My Jeep tyres are 10 years old and again still have 50% tread but should really be replaced on safety grounds. For me, tyre wear is not an issue what so ever.

I had a clutch last me 300,000km, and I've had cars fall apart but still run/drive perfectly fine.

I don't draft trucks BTW, and certainly never at less than 2 seconds following distance.

In a previous post I wrote every thing I've ever done to a vehicle in pursuit of MPG, my total spend has been a lot less than someone modifying their car for performance (I have a Skyline too), and I can make use of the mods far more often. Right now, both my daily drivers are less than six months old and so basically stock my 40% gains are purely from technique.

I've never caused an accident and the load gauge is easy to read at the same time as the speedo (I keep mine in my line of sight and dare say it's less of a distraction than looking at the factory instruments. Once you get a feel for your car you tend to use it less.

coasting right up to intersections in the hope of lights changing

Perhaps you misunderstand this technique. If I see a red, I lift off early, I try to maintain a modest speed towards the light possibly with a downshift, prehaps 50% of the time it means I don't come to a complete stop. It's far safer and more mechanically sympathetic than the average Joe who races to the red light and slams on the brakes a few car lengths away. It also promotes smoother traffic flow to those behind (they don't need to stop either).

As to the original topic of hills, I only wanted to illustrate that (especially known hills) needn't be evil for your MPG. It's unfortunately impossible to test this theory in real life.

 

[UFI]

Your explanation is no doubt correct in theory, but in the real world, hilly areas also have corners, some of them tight. If I could straighten out all the roads in Cornwall, my fuel consumption would be the same as someone living in Flatlandia.

It's very rare that any stored energy at the top of a hill can be released fully because you have to brake and ruin the speed advantage. Often, we have to start going uphill with zero run-up, and speed limits make you brake going down..

You have to do the best you can with what you have. No doubt the serverity of upgrades/ downgrades, length between them, corners, could skew the results one way or the other.

On known roads you can (traffic permitting) crest your hill at a speed that hopefully doesn't result in excess speed at the bottom of the next decent.

 

[ahmett]

The biggest cost to in owning my Fiat 500 will be depreciation, by far. Way more than I will ever spend on fuel for it.
That aside, my point is that you do not seem to be costing in additional wear and tear induced by your modified techniques. If you are counting pennies why not include ALL the costs you mentioned elsewhere you have incurred in your pastime. I recall you said you do it for the challenge and not to save money, having spent thousands on ECU remaps and so on. Extra tyres, wear and tear induced by launching off like a maniac every time. Drafting trucks, coasting right up to intersections in the hope of lights changing, staring at load gauges instead of looking out the window. I don't doubt for a minute you save plenty of dough on your fuel BTW.

Hilly routes over flat ones for fuel economy. I am not convinced I am afraid.

Depreciation? I'll give you guys a heads up. My Fiat 500 1.4 Lounge Dualogic with Leather has 97,000 kms on it and is 4 years and 2 months old. (09/10 registered)
I bought it for 17,695 euros (admittedly if i bought it today i could probably buy it new for 16,000 with the better discounts around in crisis hit Greece).
They just offered me 7,000 euros as a trade in price, which i find reasonable given the mileage of the car.

 

[Sweetsixteen]

This efficiency argument is where tech-theory and real-world results don't agree.

My regular 200 mile run, with half of it hilly and the other half flat, always shows much better fuel consumption on the flat stretch, even though average speeds for both parts of the journey are similar. Read into that what you will, but it is a real-world result.

Obviously an engine has a 'sweet spot' where peak efficiency is reached, but it also has to lug a heavy hunk of metal around. Getting to that 'sweet spot' in the most efficient way is where the difference lies. When manufacturers calculate their claims for economy, they try every trick in the book, but they do not accelerate hard from one speed to the next during their tests. They use the maximum permitted time between designated speeds (check out the EU test specs), because they know that hard acceleration uses more fuel than gentle increases in speed. Taking the engine's efficiency as the only parameter ignores the many other factors that also influence fuel consumption.

And I have gone totally off thread. Sorry.

 

[UFI]

Not all manufacturers cheat. In the US a few auto makers have been fined for cooking the books by the EPA and have to mail a cheque annually to the affected owners, for the discrepancy between their reported figures and those tested independently.

Of my six cars from six different manufacturers, only Fiat have cooked the books.