Lying Cars (Dash vs Reality)
Discussion
paulrockliffe said:
DKS said:
gizlaroc said:
I may be wrong, but I reckon it is to do with how they calculate the fuel used before up to temperature that throws the obc so much.
I think I get what you're saying, but in that case why does it need to calculate how much extra fuel the engine needs when cold? It knows exactly how long the injectors are open for.Bonefish Blues said:
Computer generally says 60ish, calculations generally say 50-51.
Volvos of that generation seem to be pathological liars - Mrs BFB's is exactly the same.
I would have been happy if my old Volvo was merely a pathological liar, it was a complete and utter fantasist Volvos of that generation seem to be pathological liars - Mrs BFB's is exactly the same.
. Real life average of 42 mpg over 2 years and 45,000 miles, computer read of of 58 mpg!I'm another sad case who records everything, having spreadsheet that go back to 1996. All of my cars have been optimistic (where they've had a trip computer), although most of the VAG brands have only been 2-4 mpg over.
My petrol Toyota is optimistic too, but I'm not entirely sure by how much yet as it resets periodically. I think it's about 5 mpg over.
My old Superb 1.9TDi would be 4-5 MPG out. The Honda Civic (petrol) we have is also around 3-4 MPG out (circa 10%). On both these cars the speedo also over reads by roughly 10% and when you get over 50MPH its a constant 5mph over-read.
My last volvo and my current volvo have been better. The old V40 actually under-read MPG whilst its speedo was bob-on all times. The S60 seems to be 2 mph over-read and MPG is mostly accurate, though sometimes produces a higher figure than I calculate.
Most remember that I don't think there is a direct fuel metering device, the ECU basically has a torque demand and converts that into a required fuel mass and that goes round on the CAN for all the devices that need it. The injectors have flow rate calcs (using applied voltages, pressure differentials between fuel rail and cylinder/manifold, fuel temperature, trims for individual injectors open times etc) and the injectors are fired for the right time. I just assume that the calculated fuel mass is what is used as the "fuel used" data.
I could be wrong though. But if they can eliminate an actual flow rate sensor then I guess they would!
My last volvo and my current volvo have been better. The old V40 actually under-read MPG whilst its speedo was bob-on all times. The S60 seems to be 2 mph over-read and MPG is mostly accurate, though sometimes produces a higher figure than I calculate.
Most remember that I don't think there is a direct fuel metering device, the ECU basically has a torque demand and converts that into a required fuel mass and that goes round on the CAN for all the devices that need it. The injectors have flow rate calcs (using applied voltages, pressure differentials between fuel rail and cylinder/manifold, fuel temperature, trims for individual injectors open times etc) and the injectors are fired for the right time. I just assume that the calculated fuel mass is what is used as the "fuel used" data.
I could be wrong though. But if they can eliminate an actual flow rate sensor then I guess they would!
It would appear that i am the only one who puts in fuel to the point the car is full and then just drives until the little needle has sunk to the point i know it really means i should fill up in the next 50-75 miles or risk the red light of doom appearing.
As a rule i know that if the Porsche has done 300 miles from the last stop it is about to need fuel, the sooner the better. The Golf, 400 miles, or when it says i havbe less than 80 on the readout.
I have no idea what MPG the Porsche does. Sufficent i think is the anwer. The Golf tells me it does 42ish over 3000 miles but i have no idea if this is a random VW number or reality.
As a rule i know that if the Porsche has done 300 miles from the last stop it is about to need fuel, the sooner the better. The Golf, 400 miles, or when it says i havbe less than 80 on the readout.
I have no idea what MPG the Porsche does. Sufficent i think is the anwer. The Golf tells me it does 42ish over 3000 miles but i have no idea if this is a random VW number or reality.
Rude-boy said:
It would appear that i am the only one who puts in fuel to the point the car is full and then just drives until the little needle has sunk to the point i know it really means i should fill up in the next 50-75 miles or risk the red light of doom appearing.
As a rule i know that if the Porsche has done 300 miles from the last stop it is about to need fuel, the sooner the better. The Golf, 400 miles, or when it says i havbe less than 80 on the readout.
I have no idea what MPG the Porsche does. Sufficent i think is the anwer. The Golf tells me it does 42ish over 3000 miles but i have no idea if this is a random VW number or reality.
Golf has a 55 litre tank, so I would guess you put around 12 gallons in at each fill up, or £56 worth of fuel?As a rule i know that if the Porsche has done 300 miles from the last stop it is about to need fuel, the sooner the better. The Golf, 400 miles, or when it says i havbe less than 80 on the readout.
I have no idea what MPG the Porsche does. Sufficent i think is the anwer. The Golf tells me it does 42ish over 3000 miles but i have no idea if this is a random VW number or reality.
400 miles from 12 gallons is 33mpg.
My e46 320d is accurate to within +->1mpg over a tankfull, for the 40,000 miles i tracked, and while I haven't checked the trip meter the average speed readout matched GPS average, so I figure,the trip is correct.
And both my old 325tds e36's matched to within 2mpg most of the time. I don't know why newer ones would be so far out of whack, unless they are chipped or running heavier or different size wheels or tyres, because BMW seemed to have it figured out perfectly ten years ago, and reasonably well 20 years ago!
And both my old 325tds e36's matched to within 2mpg most of the time. I don't know why newer ones would be so far out of whack, unless they are chipped or running heavier or different size wheels or tyres, because BMW seemed to have it figured out perfectly ten years ago, and reasonably well 20 years ago!
I reset my average MPG every time I fill up (I am also one of these sad people who records all of his fill-ups) and it tends to be within about .5mpg of my calculations. Things like how empty the tank is when the fuel filler cuts off and how much dribbles out of the pipe into the tank will always vary I expect, so it's never 100% accurate in either case.
DickP said:
My company car claims on the dashboard that it is averaging around 55mpg, and has said approximately this since I got it earlier in the year.
I thought though it would be good to just check what it actually is doing to see its accuracy. I presumed a slight variance may happen of a couple mpg. Reality is the car averages betweeen 46 and 47 mpg. That's a difference of over 10%! I guess I was naive to think that the car would be able to produce a genuine figure? Are lots of manufacturers playing this game as well, fooling their buyers into thinking the car is doing better than it really is?
Have you tried resetting it and re-testing it's accuracy?I thought though it would be good to just check what it actually is doing to see its accuracy. I presumed a slight variance may happen of a couple mpg. Reality is the car averages betweeen 46 and 47 mpg. That's a difference of over 10%! I guess I was naive to think that the car would be able to produce a genuine figure? Are lots of manufacturers playing this game as well, fooling their buyers into thinking the car is doing better than it really is?
I say this, as you may not know what average mpg it is actually displaying. If it's a life time (or since last fully reset), then it may still be accurate if you haven't owned the car from new.
GroundEffect said:
It calculates it using the same speed as displayed on the cluster. What did you think would happen when they are by law forced to over read?
You say this but on my old (mk2) Mondeo if you set the cruise speed via GPS to (for example) 60mph and then reset the average speed it would (after a short while) also display 60mph. All the time the speedo would be showing about 63-64mph. So the fuel computer *knew* what speed the car is doing even if the speedo is being optimistic.
I'll add that the (calculated) mpg on that thing (and actually on the current one) was within 1mpg (and to the pessimistic side) of the calculated mpg.
gizlaroc said:
Golf has a 55 litre tank, so I would guess you put around 12 gallons in at each fill up, or £56 worth of fuel?
400 miles from 12 gallons is 33mpg.
Would be about right i suspect, only a few MPG out here and there. To be honest the daily drive is 8 miles there, 7 back (go different way home so I drive the roads in the directions I like the most) and is not an MPG friendly run. On the other hand weekend before last we were on a 2 hour drive there and 2 hours back drive so big variations in driving style and road meaning that over 1 fill up there could only be 300 miles of 'grunt' work, yet the next two could be 400 mile+ tanks.400 miles from 12 gallons is 33mpg.
As said i have not bothered to work out the maths much so might have a go in the next month or so just to see if it is as 'out' as your fag packet maths suggests!
Fastdruid said:
You say this but on my old (mk2) Mondeo if you set the cruise speed via GPS to (for example) 60mph and then reset the average speed it would (after a short while) also display 60mph. All the time the speedo would be showing about 63-64mph.
So the fuel computer *knew* what speed the car is doing even if the speedo is being optimistic.
I'll add that the (calculated) mpg on that thing (and actually on the current one) was within 1mpg (and to the pessimistic side) of the calculated mpg.
When I mentioned this before some explained that even though the speedometer is set to over read by approx. 5-10% on most cars, the odometer always had to be calibrated properly, taking into account tyre sizes allowed and wear etc. So the fuel computer *knew* what speed the car is doing even if the speedo is being optimistic.
I'll add that the (calculated) mpg on that thing (and actually on the current one) was within 1mpg (and to the pessimistic side) of the calculated mpg.
A that has done 100k miles has to say it has done 100k miles I guess?
I would guess MPG is calculated from odometer and not speedometer?
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