whoa hold the phone, are you and let me get this correct as I'm hyperventilating here, are you saying that the 21 mpg I get isn't accurate its actually probably much less.....

 

Jesus titty ****ing Christ. Sell it, Sell it now.......

 

oh wait, I just remembered I can always fill it up with more at various locations around the country and nip in for fags when the old stuff runs out.....phew.....I thought i only got a tank and a half for a second

If you're not interested in fuel consumption, fair enough - but in that case, why read this thread, let alone post in it?

If, however, you are interested in your fuel consumption, I'd have thought that the most sensible thing would be to take the trouble to find out what your fuel consumption actually was, before joining in a long debate about it.

The weird people are those who post at great length about their fuel consumption and the precise effect - real or imaginary - on this of using various types of special magic (and invariably more expensive) fuel, but who have no idea of what their real fuel consumption actually is and have never bothered to measure it, preferring instead to waffle on about meaningless things such as "miles per tankful" or relying on the almost certainly spurious readings on their dashboard computer.

 

Can't say I've really experienced this. Pretty much every serious fuel economy thread I've seen has involved owners calculating their MPG properly......if they don't they soon get advised of the error of their ways! 

Stuarted,

Why not accept that modern electronics are reliable.

Therefore after establishing the accuracy of the "Trip" meter against "actual" miles per gallon.

Simply use the "Trip" function.

Because "actual" mpg's is only an approximation anyway.

%age error in litres of fuel  dispensed.

%age error in miles covered due to tyre wear.

%age error due to hotter tyres being larger diameter/rolling circumference

etc

etc

etc

.

PS

The Professor who taught us surveying at the Poly had a wee mantra,

to wit

" one ever only knows appromimately where any surveyed point is, one can not know absolutly"

Then the eccentric Maths Prof started about "approximately", being a meaningless term.

He only worked "within proscribed limits"

And was wont to state that a "Million" was a small number, or indeed an acceptable error, within proscribed limits!

regards,

Marcus

Edited 25 April, 201412 yr by dieseldogg

If you're not interested in fuel consumption, fair enough - but in that case, why read this thread, let alone post in it?

If, however, you are interested in your fuel consumption, I'd have thought that the most sensible thing would be to take the trouble to find out what your fuel consumption actually was, before joining in a long debate about it.

 

ill be serious for a second

 

whats the point in working it out are you suddenly going to sell your car if you find out its 3 or 4 mpg lower than what the trip computer reads?

 

every cars trip computer is optimistic, they are just there for a reference if you will. so I fail to see why you would bother doing it.

 

manufactures released mpg figures are optimistic as well and not what you will achieve on the road they are there as a reference to compare against other brands.....you take them with a pinch of salt.

 

will knowing your actual mpg change anything. whats the point as well because you cant replicate the same set of driving conditions again there is far too many variables, traffic, speed, acceleration rate, weather, tyre pressure, air temperature, car load.......so your actual mpg really means nothing does it you cant beat it as you cant replicate the same conditions. did you fill your car with a calibrated flow device? did you weigh or measure the contents of the initial tank, did you fill it up to the same point? I was a lab tech for 5 years and even I wouldn't bother my arse trying to work it out correctly as there is millions of variables that would effect a true reading even if you measured it manually.......anyone who doesn't know that is suffering from delusion

Manufactures figures are far from accurate. About 20% difference compared to what is generally achieved. Anybody who buys a car and expects to hit those brochure based figures I would guess would be more disappointed than not most of the time.

Sure I read somewhere that the testing dated back to the 70s bit is going to get a revamp. But still based on what they do now.

Please tell me a more accurate method of measuring average fuel consumption than that used by me?

From my earlier post - "Over more than 10,000 miles my average fuel consumption has been 43.75mpg. Yes, I do record every fuel purchase. The Maxidot shows 46.0 so is just over 5% optimistic and this inaccuracy has been consistent since I bought the car."

How did you record you had covered 10k miles? Both the mileometer and GPS are innacurate so I'm sceptical about any readings quoted here.

The cars reading of economy is the best way to compare fuel economy between Octavias as the software and method of calculation is the same in each vehicle. This way any errors will be the same in each car and as such should cancel each other out.

If you are going to go through ball-ache of calculating your actual, real MPG, you'd need to first make sure that the computer system is correctly calibrated in the first place. So, you'll need a good rolling road (that's calibrated) and VCDS, to teach the car what a mile is.

 

And then.....

 

will knowing your actual mpg change anything. whats the point as well because you cant replicate the same set of driving conditions again there is far too many variables, traffic, speed, acceleration rate, weather, tyre pressure, air temperature, car load.......so your actual mpg really means nothing does it you cant beat it as you cant replicate the same conditions. did you fill your car with a calibrated flow device? did you weigh or measure the contents of the initial tank, did you fill it up to the same point? I was a lab tech for 5 years and even I wouldn't bother my arse trying to work it out correctly as there is millions of variables that would effect a true reading even if you measured it manually.......anyone who doesn't know that is suffering from delusion

 

Once you take all that into account and realise how futile it all is chasing that extra 0.1mpg, aiming to get more than 600/650 miles to a tank is where you end up :)

Isn't the speedometer set to read between 5 and 7% higher than the actual speed you are doing? Or something like that.

I noticed that. When on motorway my car speedo says 70 my sat nav says 64?

At speedo 80 my sat nav says 74?

I'm not going to get into GPS, trilateration and timing errors but rest assured your speedo and your GPS are both innacurate to a certain amount. You don't need this accurate service, hence America does not provide the companies renting the GPS service from them, to provide you with definite and accurate speed/location data. This is reserved for military use.

Edited 26 April, 201412 yr by Bcc212

So if both speedo and GPS are inaccurate how on earth can police, speed camera et al supply fines for being over a maximum speed limit when we can't tell how fast we are driving in the first place?

Obviously I know they must give a certain grace for allowance and I'm not talking about doing 70 in a 40 zone but if my speedo and GPS have a 6 mph difference at 70 how can a traffic cop give me a ticket for being over 70 as technically I could drive a 76mph and claim my GPS states 70mph and obviously the speed gun/camera will no doubt read something different.

That's why you often here quotes of you can do 10% + 1 mph before being fined for speeding.

I.e. in a 40 zone you wouldn't be fined until over 45 mph.

I.e in a 70 zone you wouldn't be fined until over 78 mph.

Now people will say that if the police wanted to execute 0% tolerance they can fine you for 41 mph in a 40. With speedometers not being 100% accurate I doubt that would stand up in court.

Double post.

Edited 26 April, 201412 yr by Bcc212

There's tolerances of course but they're not 'that' innacurate. Basically if you're going 31 on your speedo you're probably doing 29-30. That's enough tolerance. This increases at speed. Wheel size/tyre size all play a part. But that's another story, hence the tolerances, as I'm sure skoda didn't calibrate my speedo any different to someone with 15" wheels.

Anyway, they use the Doppler principle which measures how long it takes for that radar pulse to return to the transmitter/receiver. Again I don't want to go into that, but when those signals marry up with a pre designated time frame it sends a signal to take a photograph. Lines on the road are used to accurately **** you off by proving the system works. Speed = distance (in ft markings) over time (takes 2 photos 1 second apart) quite simple really, and also allows you to dispute a claim by measuring it yourself.

Edit: other police equipment uses the same principle but doesn't take a photo. Usually filmed for reference or the roadside speed gun is calibrated at a pre determined date (week/month)

Edited 26 April, 201412 yr by Bcc212

So if both speedo and GPS are inaccurate how on earth can police, speed camera et al supply fines for being over a maximum speed limit when we can't tell how fast we are driving in the first place?

Obviously I know they must give a certain grace for allowance and I'm not talking about doing 70 in a 40 zone but if my speedo and GPS have a 6 mph difference at 70 how can a traffic cop give me a ticket for being over 70 as technically I could drive a 76mph and claim my GPS states 70mph and obviously the speed gun/camera will no doubt read something different.

Because all manufacturers make speedos that over-read. One that under-reads is illegal. If you speedo says 70mph, you are always going to be doing less than that.

A police cars equipment is calibrated by driving over a measured mile and the number of pulses from the gearbox recorded. This is done weekly as something as simple as tyre wear or pressure alters the result. Compare this to the satnav and both readings differ.

That's what I'm asking. So knowing that my speedo "under reads" then it would be plausible that someone could travel faster and argue that fact to get out of a speeding ticket. I mean I'm not overly concerned I usually just go up to speedo 70 and switch on CC but if at 70 there's a 5-6mph discrepancy then obviously another car would be travelling at a different speed thinking they are doing 70. If that makes sense?

No. The discrepancy is about 1-2 at that mph. It is true that you could travel faster than indicated speed. However, if you have a ticket, you have been going over 70mph, whether indicated correctly or not. End of story. Radar doesn't lie, especially when it's calibrated regularly.

Edited 26 April, 201412 yr by Bcc212

If the speedo is calibrated to read slightly fast (don't know the actual %) won't this mean the milometer records slightly more mileage than actual ?? Making fuelly, paper calculations, trip computers etc all slightly inaccurate.

Probably.

If you are going to go through ball-ache of calculating your actual, real MPG, you'd need to first make sure that the computer system is correctly calibrated in the first place...

.

Why?  What has the computer got to do with it?

 

.

Why?  What has the computer got to do with it?

 

 

You do know that the on-board computer controls the speedo, the odometer and the trip computer, don't you?

 

Without the on-board computer being correctly calibrated, you're just going to guess how far a mile is then? If you don't accurately know how far you've travelled, the 'm' in mpg becomes meaningless.

cept, per an earlier post, I checked all possible variables that I could, and before the days of GPS,

this involved counting motorway mile marker posts, as good a hobby as any during mundane motorway driving. 

cheers

m

You do know that the on-board computer controls the speedo, the odometer and the trip computer, don't you?

 

Without the on-board computer being correctly calibrated, you're just going to guess how far a mile is then? If you don't accurately know how far you've travelled, the 'm' in mpg becomes meaningless.

.

So?  The mileometer on my Octavia is just as accurate the the mileometer on any other car I've ownerd - they all read the same over the same journey.

 

Edited 26 April, 201412 yr by Stuarted

Seriously doubt it, based on 4 different cars over 40 years.

Of course tectonic plate movement could be a factor over this period.

One would require to make due allowance for that variable.

Edited 27 April, 201412 yr by dieseldogg

.

So?  The mileometer on my Octavia is just as accurate the the mileometer on any other car I've ownerd - they all read the same over the same journey.

 

 

Calibration by "it reads the same as the others"? Interesting concept I supose. Not very useful against the fact that speedos and OBCs are set to over-read.

 

If all these things were bang-on then there'd be no need to have a calibration system. If all these things are bang on, then why......

 

A police cars equipment is calibrated by driving over a measured mile and the number of pulses from the gearbox recorded. This is done weekly as something as simple as tyre wear or pressure alters the result. Compare this to the satnav and both readings differ.