Inaccurate Mpg

August 30, 2011

The trip computer on our 2008 XTR consistently shows around 43 mpg. This is mainly short journeys in town, as the other half is on maternity leave and is usually only out and about to go to the shops or visit her pals.

Anyway, using a 'Fuel log' application on my smartphone, i have calculated (or i should say the application has) that we are actually only getting around 39 mpg.

Is this a ploy by Mr T to pull the wool over customers eyes? Ive heard that most trip computers are inaccurate. My question is ...... why? Surely in this day and age they can make accurate computer systems for cars? :censor:

August 30, 2011

This is just a stab in the dark,but would not that mean more sensitive sensors,like flow meters, sensors for maybe air and fuel temps as they enter the engine for instance? Surely that would mean more expense re sale price?

Personally, I would sooner have things displayed like oil pressure,charge rate and on my auto,trans fluid temperature instead of incorrect info that some peeps can work out with a pencil and paper or with more slightly sophisticated means.

I now await correction from more informed members,without resentment. :thumbsup:

Del.

August 30, 2011

The trip computer on our 2008 XTR consistently shows around 43 mpg. This is mainly short journeys in town, as the other half is on maternity leave and is usually only out and about to go to the shops or visit her pals.
Anyway, using a 'Fuel log' application on my smartphone, i have calculated (or i should say the application has) that we are actually only getting around 39 mpg.
Is this a ploy by Mr T to pull the wool over customers eyes? Ive heard that most trip computers are inaccurate. My question is ...... why? Surely in this day and age they can make accurate computer systems for cars? :censor:

The fuel computer on our 02-reg 5-dr is very accurate whilst the one on the 54-reg 3-dr is about 20% optimistic. Re the former it's obviously no problem and now that I know the error on the 3-dr, I can take it into account, i.e. I can still monitor mpg generally by keeping an eye on how much, if any, the reading changes by.

Overall, I think that with any make of vehicle, mpg read-outs are hit or miss. The reality is that I reckon that this feature is a by-product of the amount of electronics/computing hardware there now is in the average car and as such it isn't maximised specifically to be accurate. They can produce a "near enough" mpg read-out at no real extra cost so they do since it adds value as far as the punter is concerned & makes the product seem that little bit more sophisticated. I would guess that the majority of people either don't know they have a fuel computer or, where they do, they believe it as long as it doesn't seem substantially different to the mpg that they think they're getting (i.e.so long as it isn't ludicrously inaccurate). Consequently there is no incentive (or pay-back) for manufacturers to make the equipment more accurate.

Personally I think that the mpg read-out is nice to have but anyone who is serious about it will do a physical check now & again anyway. I would put more value in having accurate real-time read-outs for other parameters like charging amps, oil presure & temp, oil level (my old Peugeot 406 had an electronic oil level indicator that was very useful), etc. In fact all the things that upmarket & performance cars used to have as a matter of course. Now however, like so many other things in life, driving has been dumbed down to the point where you get a warning light if something major goes wrong and a standard instruction to approach your dealer when it does happen. If we had real-time info on key systems then failure might be avoided in the first instance.

I suppose it's just a reflection of the fact that a car nowadays is just another appliance, like a fridge or a TV. There's little pride in ownership, much less in the ability to drive expertly which would include some understanding of the mechanics involved in getting your boom box from A to B.

August 30, 2011

The trip computer on our 2008 XTR consistently shows around 43 mpg. This is mainly short journeys in town, as the other half is on maternity leave and is usually only out and about to go to the shops or visit her pals.
Anyway, using a 'Fuel log' application on my smartphone, i have calculated (or i should say the application has) that we are actually only getting around 39 mpg.
Is this a ploy by Mr T to pull the wool over customers eyes? Ive heard that most trip computers are inaccurate. My question is ...... why? Surely in this day and age they can make accurate computer systems for cars? :censor:

Accracey varies from car to car with the computer readings,its not an exact science,though why not in this day and age.just tyre wear will mean the distance covered per rev of the tyre will reduce actuel distance travelled.but the odometer does not allow for this.so if your computer was spot on to what the system thinks the car has travelled it will not be a true figure.long gone are the days of 4 galls for a £1.00 so you did bother much about consumption.well i didnt,different now though.

August 30, 2011

So, for a basic calculation, if we take say a 17inch wheel with a 225/65 tyre (sound familiar?), the diameter is going to be (17*25.4 + (225*.65*2)) mm = 724mm.

Shall we say we have 10mm tread at the start of the tyre's life and down to 2mm at the end?

So, about 16mm off the diameter?

So %age change is about (16/724)*100 = 2% ish, so 2% about 1mpg-ish error @ 40mpg?

and that's at the end off the tyre's life, it will be less while wearing down to that. Oh, but then there is a thing called rolling radius....but isn't that based on a constant vehicle speed?? Oh, but then as the vehicle speed increases the tyre tends to inflate anyway......

No, I can't see it being down to mainly tyre wear.

People also have some odd ideas about "accurate computers" too. The whole system will be designed to be as accurate as it needs to be. No more and no less. ECU internal component value tolerances will very often be 1%,5%,10%, depending on what they are used for. They will have tighter tolerances only where they really need to be, because greater accuracy costs ££££ and every penny does count, especially when you are producing these things in volumes into six or seven figures per year.

No, it just sounds like pure and simple "tolerance stack-up" to me, even though Japanese manufacturing is not supposed to understand the word "tolerance". Some mpgs read high, some read low. Mine reads about 7% low.

Plus bear in mind, other things besides tyres actually wear. For the diesel (and probably petrol too), the fuel injected volume is based on sensed fuel rail pressure and time one of the injectors is open. (Keep a known aperture size open for a known length of time at a known pressure and you can calculate fuel volume through it). So, injectors may wear, rail pressure sensors may drift, target rail pressure may drop within acceptable limits.......blah, blah, you get the picture. :thumbsup:

I don't think there is any deviousness at work here.

Maybe they could offer the facility of a manually entered "correction" (fudge) factor, but that would probably also confuse a lot of people.