Fuel trip meter give high reading

[madasafish]

Though I use Fuelly it only gives me a long term average. For some reason the same pump will cut off with wildly differing amounts of fuel in the tank. My 'brim to brim' figures have swung between 33 and 61 in the last couple of months.

If the car is on  a slope  - in any direction - the fuel cut off will be affected by that. If you are filling brim to brim and the list is the passenger side below the driver side ,then the fuel filler pipe will fill up before the tank - which will have air trapped in it... and thus contain less fuel. Ditto if the front of the is above  the back...

[culzean]

Just best to accept that there are inaccuracies in the system due to tyre wear and the fact that the tyre is flat at the bottom ( how flat depends on how much pressure inside, temperature and flexibility of sidewall ) maybe putting extra pressure in tyres like hypermilers do works because it increases tyre diameter and goes further per revolution of the wheel. Also speedo and oddometer get their data from same source = most likely the ABS rings on wheels,  but speedo readout can be scaled to show a different reading to the incoming data.

I know the ambient temperature, humidity affect MPG ( anyone who has ever had a two stroke engine knows that they performed far better in cold damp conditions than hot dry ones due to density of charge ) but colder air is more dense so harder to push the car through it.  I had a car once with a 'vacuum gauge' ( simply measured the vacuum in inlet manifold ) it was graduated from green through orange to red, the harder you pressed the loud pedal the further it went from the green area to the red,  and you could sometimes hear a sucking noise from fuel tank area when needle was in red area. 

Some people claim that driving slower to reduce fuel consumption means you will use more because the car engine is running longer to get from A to B,  they obviously have no idea about physics because the fuel consumption is not GPM ( gallons per minute ) but MPG ( miles per gallon ) so time is irrelevant in the calculation - only mileage and amount of fuel used is important, and losses are less at lower speed ( that is why the record breaking Jazz that went Lands End to John O'Groats on one tank of fuel did 40mph the whole way ).  To travel 2x as fast needs 4x the energy.

[BrummPopBang]

It's interesting reading all the different views. One thing I had not thought of checking was for a difference between satnav distance and the odometer.

I fill up each time to the fuel nozzle and record that on an Excel graph. I had a Peugeot 205 1.1 years ago. The fuel usage graph was a very shallow wave from summer to winter (hi/lo) varying about 7mpg and with a ripple on the curve of about 2mpg. As someone else mentioned, a drop in fuel efficiency proved to be a warning of binding calipers. Just an aside - I fitted a set of Girling calipers in place of the original (useless) Bendix. The first time I tried the brakes I stood on them like I did with the Bendix. Hell's teeth - I nearly went through the windscreen...

But the curve for the Jazz is just crazy, up and down 39mpg to 55; all over the place - no rhyme or reason.

I hadn't heard of Fuelly.com and was surprised to see an average for the 1.4 2005 of less than 40mpg. At least mine manages 42. Although I suspect the front O2 sensor is dodgy (re: the above fuel graph) and have another to fit.

Just to add to the problems I removed the intake a couple of years ago and it seem I may have created a combustion problem. The plan is to work through all the issues but somewhere in all this I think I have a life...

[sparky Paul]

The odometer is way more accurate than the speedometer. The speedometer, must by law, never show less than the actual speed and no more than 110% of the actual speed so this is built into the display to register between 100 and 110%. The odometer does not have this error.

I'm baffled now. How can the odometer be more accurate than the speedo if they are derived from the same signal? It used to be a single cable drive, but these days it's pulses from a gearbox sensor or ABS sensor. Miles driven is simply a function of speed and time.

There has to be some inaccuracy, to accommodate similar errors to the speedometer, and I'm not sure if it would be permissible for the odometer to read less miles than actually travelled...

[madasafish]

No doubt the speedo  - which is electronic - has a formula to ensure it overreads..for UK.

The odometer  I assume does not have one. (no legal need)

[sparky Paul]

No doubt the speedo  - which is electronic - has a formula to ensure it overreads..for UK.

The odometer  I assume does not have one. (no legal need)

Maybe not, but the signal is still derived from the same sensor as the speedo, with the same range of error.

Speedometers are designed to accomodate those errors, and still guarantee that they do not under read - for example, they may be calibrated to show 105%, to accommodate train errors of ±5%.

If the speedo has a range of inaccuracy of, say 100 to 110%, the odometer is still going to have the same 10% range of error as the speedo - even if that's ±5% on a calibration of 100%. What I'm saying is that the odometer may well be accurate, but it is just as likely to be affected by tyre size, tyre wear, etc., as the speedometer.

Does that make sense, or am I overthinking this? 

[JimSh]

The odometer is way more accurate than the speedometer. The speedometer, must by law, never show less than the actual speed and no more than 110% of the actual speed so this is built into the display to register between 100 and 110%. The odometer does not have this error.

I'm baffled now. How can the odometer be more accurate than the speedo if they are derived from the same signal? It used to be a single cable drive, but these days it's pulses from a gearbox sensor or ABS sensor. Miles driven is simply a function of speed and time.

There has to be some inaccuracy, to accommodate similar errors to the speedometer, and I'm not sure if it would be permissible for the odometer to read less miles than actually travelled...

Both odometer and speedo are presumably fed from the same sensors. If you think in terms of the old cable driven system the speedo can be made to over-read slightly (needle twisted slightly clockwise relative to scale)
Presumably in electronic systems there is some sort of electronic gismo converting the pulses to a display which performs a similar function of introducing a deliberate positive error in the speedometer. Or maybe the odometer just over-reads as well as the speedo?

Edit On second thoughts I think my second explanation seems more likely. As long as both over-read their would be no illegality.

[JimSh]

If the speedo has a range of inaccuracy of, say 100 to 110%, the odometer is still going to have the same 10% range of error as the speedo - even if that's ±5% on a calibration of 100%. What I'm saying is that the odometer may well be accurate, but it is just as likely to be affected by tyre size, tyre wear, etc., as the speedometer.

Does that make sense, or am I overthinking this? 

Our last two posts overlapped. I think we are both over thinking this and that the odometer will be over-reading by up to 10% as well as the speedo.

[sparky Paul]

I think we are both over thinking this and that the odometer will be over-reading by up to 10% as well as the speedo.

Well, that's what I would have thought at the start of this thread... but I haven't done any scientific study, you understand 

My last point was, whether the odometer's target is accuracy, or is affected by the over-enthusiastic speedo reading, both must be affected equally by errors introduced by different wheel/tyre combinations, etc..

[JimSh]

Yes. I don't think it's going to make much difference if the odometer reading is slightly high.
The original post was about fuel trip meter being optimistic anyway.
That introduces a further factor - the petrol consumed.

[John Ratsey]

As noted earlier, there's a legal incentive to have the speedo reading slightly high so that the driver can't blame the vehicle (or manufacturer) if caught speeding. I recall checking my HR-V against my sat-nav and found it read about 2 mph anywhere between 30 and 60 mph. That looks to me like the speed being measured accurately and then 2 mph added on as a buffer. It would be easy for the electronics to use the TPMS sensor pulses.

I've not found significant odometer errors in any recent (or older vehicles) for the same approx 100 mile trip I've done intermittently for many years. It's a simple task to add up the wheel revolutions and multiply by the circumference and there's no technical or legal reason to include a fudge factor in the calculation. However, as the tyres wear then the actual distance per revolution is reduced so the reported distance increases as will the apparent mpg. A quick guestimate suggests that 5mm wear on the tyres will create an over-reporting of distance by 1.5% with corresponding increase in the calculated mpg based on fuel used and miles travelled. That would explain why putting new tyres caused a small drop in the calculated mpg for my HR-V!

[Jocko]

From my house to my mother-in-law's is 37.4 miles according to Google Maps. My odometer shows 36.8. My speedometer shows almost 10% over what's displayed on my GPS Speedometer and my Dash Cam, both of which read the same.
This says to me that the odometer on my car has a minus error and the speedometer a plus error.

[John A]

Just about every bike / car I've owned the speedo over-read and the odometer was spot on.

[JimSh]

Lightbulb moment. Or slaps forehead and calls himself stupid beggar.
I was thinking about the "computer" on my bike. When I set it up I had to input the circumference of tyre.
There is a pulse from sensor every revolution of wheel.
Distance gone = no of pulses X circumference of tyre and speed calculated by distance/time.
If circumference figure is wrong both distance and speed are affected.
In car if speedometer and odometer are regarded as separate a larger value of circumference could be input to speedo to give an inflated distance figure to calculate speed from.
Both speedo and odometer would be subject to errors due to differences in tyre size and wear but distance as measured by speedo would be consistently larger than distance measured by odometer.

[culzean]

Both speedo and odometer would be subject to errors due to differences in tyre size and wear but distance as measured by speedo would be consistently larger than distance measured by odometer.

That was the 'scaling factor' I mentioned in earlier post, with electronics you can take the same information and apply a 'correction' ( ahem ) to it before it gets sent to destination / readout display. But there are still variables like differing tyre diameters, tyre pressure and wear as well as wheelspin that the ECU cannot account for.