Mathematical Help Required

[Zak33]

As some of you will know, if Zakky starts two or three semi-related threads in a day, he's up to something.

Who's good at Maths?

Because I propose a whole new system of Car Benchmarks for all of us to start touting. One day, maybe, it will be the defacto stuff used.

Here goes.

Miles Per Gallon in relation to Speed.

Ie I want a way to calculate the FUEL USED IN RELATIONSHIP TO THE JOURNEY TIME...and it needs to be taken into account on long-ish journeys

IE Miles per Gallon Per Hour, or similar.

Does that make sense? If you drive faster, and get somewhere quicker, but use more fuel, you get a chance to rate cars on their ability on both speed and fuel use

What do ya think?

And how do we do it?

[dkmech]

I know what you mean, i did think about it before. I'll sort it out after f1 qualy or in an ad break.

[Honoop]

Ok I dunno if Im gonna be of any use seeing as Im an accountant not a mathemetician but Im guessing that to get anywhere near you would need to know:

Size of ur petrol tank, how many miles you get per tank (on average) and your average speed.

So for Example say you have a 25 litre tank and you get about 350 miles (averagely) out of it. Thats 0.071 of a litre per mile.

Then you need to figure out how much fuel you use per minute to be able to convert your speed into a fuel consumption (I think). So if avg speed is 50mph divide that by 60 mins and per min you do 0.833 miles a minute.

So take your average litre per mile and divide by your mileage per min:
0.071/0.833=0.085 litres of petrol a minute. Multiply by the time of your journey (considering consistent speed) and you have your fuel consumption.

I think that this is a way of working out what you wanted Zakky. Not entirely sure how accurate it would be.
Obviously replace your speed to that which you would be doing but I think it would work.
(This has taken too much brain power!!)

Sorry if Im just being a bit pants!

[Zak33]

thats pretty close actually

What we need is a LARGER NUMBER the better the car performs. I want a really fast car, than literally hlaves your road time but doubles you fuel bill to get the same score..roughly.

We then need 3 categories, just like now on fuel consumption data

Urban
Extra Urban
Combined.

Then when someone buys a 1.9 DTi 150 Golf and it trashes the oppostion Honda VTEC cos while the VTEC just pips it to the post, the VW uses 2/3 the fuel , I wanna see a Mile per Hour Per Gallon score thats relative.

Then we can compare cars fairly. The punter still has the choice of getting a petrol car, its just usefull data to know that maybe a diesel of the same model IF ITS FAST ENOUGH, is worth thinking about.

Another good example.....TiG's Skoda Octavia VRS....its a fast car (180 bhp) and yet if he drives it calmly, he gets good consumption.

The PROBLEM with this whole thing is HOW DO WE COMAPRE DRIVING STYLES?

Do we have a "ragged it like a street racer" figure, and a "drove like a Nun" figure for each?

For my use, I would like to do the same route to and from work for several weeks, in my own driving style, in several cars. I would like to time it, both ways, and then sort my fuel consumption and speeds.

Cos my Corsa CDTi 16v is currently doing over 50 mpg and its being beaten stupid by me daily. And if I ease up on it, the 65 mpg is easy as pie.

And if I had a Corsa 1.8 SRi and could overtake more cars every morning and shave 5 mniutes per journey, twice per day, 6 days per week, that ONE HOUR of my life for other things, while overtaking safely and yet using loads more fuel

Make sense?

[Honoop]

Ok, yeh - so if you all work out your average fuel consumption depending on your driving and do the calculations it should give you what you want.

[Big RICHARD]

What about the current load being carried in the car, ie, is the boot full of bricks or is it just the (very slim female) driver and the rest of the car (including glove box and door pockets) empty?

Which gear are you in? Are you doing 30 mph in 1st everywhere or in 5th doing 40 mph?


Etc etc etc......

Too many variables tbh...

[Zak33]

no...there isn't...its normal driving.

When large magazines have a fleet of cars (such as Auto Car, and What Car) they keep diaries of the fuel consumption. They have long termers.

If you are carrying a load, it is pointed out, but none of the governement trials that are Legal Requirements have too many variables. You deliberately get rid of variables for deep science.

But if any of the guys in my other thread had commented on the load in their cars, it might have changed things...but they didn't. We all drive with some stuff....I am looking for a good average cross section of all your cars and we can then see the truth.

[TiG]

what i think you are actually saying is what is the efficency of an engine at a given speed.

As speed is obviously also an energy (although different type) - all you are really talking about is conversion rates of energy to be honest.

50 Litres of stored chemical energy in the petrol/diesel. converted to velocity the losses you get at speeds are influenced by drag etc etc. All of which would be calculatable.

As much as this is pretty good thing to dicuss its probably best done in person with some proper maths.

TiG

[RocketmanX]

You ask some decievingly simple, but good, questions that always have a complex solution Zak

Knowing automobiles to be the unique indiduals that they are, even between models, makes etc... the question seems a bit tough to answer. Unless each individual car were to be tested. Knowing the variables like gear ratios (final drive, overdrive ) and things like the differeing gears gone through to get to full cruising speed, would all have an affect on the net outcome. Even torque, HP numbers would play a factor, mostly through the getting up to speed part.
This of course doesn't bring into account things like AC on or off, windows up/down, tires, options, etc...

Very good question as we all know over here, I'm sure there as well, when the speed limit was raised to 65 MPH nationaly, it was a big argument about fuel economy and a lot of vehicles were found to be more efficient at 70 than the old speed limit of 55 mph. A big variable and it's a real good point to know about each car Zak.

Could be done, just a bit more than the ratings we have here now. Not sure if you guys have mileage displayed on the window sticker when buying a new car over there? Of course the ratings are at 2 specific speeds, not exactly enough info to plot a graph The "City" and "Highway" ratings are always below what anyone drives, 'cept maybe that old lady with the left blinker on for 3 miles going 15 in a 45

A standard would be a must, all options on or not, tire size per each model etc.... I don't think the auto makers want another test to perform. Except considering the fuel price over there I'd imagine Ford would jump on board.

Good question Zak, no answer though really as I don't feel calcualtions would be accurate enough via mathematics. Cars (as a whole compilation of mechanical parts) are too uncertain and complex for formulas.

[dkmech]

Surelly the relation between standard economy figures and those at higher speeds is a fairly simple affair. You check the formula for a couple of cars and you should be able to predict with reasonable accuracy what the mpg is. As all it is - the revs are higher so more instantaneous fuel usage but faster speed. You could really just compare standard mpg and then take into account how dynamic a car is and your magic formula is there.

[Lowe]

Also.. what about load on the car? Someone driving up hills with a packed car will get a mega low figure won't they..

[dkmech]

Yes, but you want to just rate the car in general, with a certain assumed usage condition. And then you can compare them and choose what you want. And if you then choose to load it with cheese and drive it to the market its your choice.

[Lowe]

True, I'm just trying to make a point that it'll be very hard to take figures on when there are soooo many variables...

[Zak33]

What it boils down to is this:

Saturday night I went to Devon from Buckinghamshire. Its a 220 mile journey roughly, first bit is across country roads, then dual carriageway, then motorway for ages, then tight lanes.

I took a Corsa 1.3 CDTi ....

I caned the hell out of her on the way down....done in 3hours and 5 minutes.

I drove like a nun on the way home on Monday, 3hours 45 minutes.

I used one tank of fuel in total, including to and from work . Obviously I used loads more getting there, but I saved 40 minutes. 40 minutes as a percentage of 185 minutes (3hours 5mins) is 21%

If I calculated the fuel consumption for both directions, I could work out if the car was 21% more efficient on fuel on the slow journey.

Cos 21% was saved in time on going fast down there.

I wanna be able to make a decision on that car, then I'll use another car another time...maybe a 2.2 Petrol Signum, and cos that car will do the fast journey with less of my right foot on the loud pedal, it MIGHT work out more efficient!

DO you all see where I'm coming from?

If the same route was used, a long route, clear of traffic on the whole, which it is, I could decide which car to use for a fast journey time!

Or which car to use for SIMPLY good fuel use. I know the Corsa 1.3 CDTi is good around the A and B roads, but to do a really fast Motorway trip its not running at its most efficient

With results and data like that you could choose the right car for your journey type.

[RocketmanX]

Being from a family that's been in business for 4 generations I get a different perspective on things because of my Father's influence.

The way I was told to look at a situation like that, consider that I own my own business where my time is money but that's true for everyone actualy, is that how much was the time saved worth? That is how much was the extra fuel relative to what you would be paid in that amount of time? Was the time savings worth more than the price of the extra fuel???

Not exactly car related but considering fuel prices in the UK I'd say you guys have no choice to look at it that way unless you were filthy rich.

Zak, what you need is a Hybrid car. We have several Hondas? running around town. You know the type I'm talking of, uses gas in town (but minimal power and phenominal gas mileage, but on the highway reverts to battery power at cruising speeds hence major fuel savings on a trip like you descibed. They also use the braking power and gas engine when used to charge the batteries so no need for a charger.