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# Vehicle Fuel Economy Monitor Adjustment

Make it accurate
 (+5) [vote for, against]

Basically every vehicle fuel economy monitor I've ever used reads high. Industry reps claim the thing is but a tool to assist the driving experience, but when they typically read 10-18% high these "tools" aren't very useful.

An option within the manufacturers menu should allow the user to correct the fuel economy indicated with the figure mathematically calculated over the last fill-up; successive such interventions should allow the circuit to learn and more accurately indicate real-world economy.

 — whatrock, Oct 21 2020

Fixable, if I owned a Bmmer. https://www.youtube...watch?v=PCh3iNBDPh4
[whatrock, Aug 17 2022]

 They're inaccurate for a few reasons. Firstly, calculating fuel flow is quite tricky, you could do it at the injectors by measuring duty cycle. This leaves you dependent upon accurate sensors for temperature & pressure in the fuel rail. Then you need a calibration table for flow Vs duty cycle. I wouldn't be surprised if the accuracy and precision of each injector was out by >5%. Then you multiply that error by any problems in temp/pressure measurement. Then you multiply that by the number of measurements made, which is 0.5 x number of injectors x total number of revolutions in the journey. So for a standard econobox car 0.5*6*50000 = 150,000 for a 20 minute drive.

 Other options would need something like a specific fuel flow meter in the main fuel line. These aren't simple, you get a stated 3% error, which will degrade with time, in a ~\$200 part. For a car it would need to be cheaper, and will still be subject to temperature induced density variations.

The main reason for that, and one you can do something about if the motivation is there, is because they usually don't include idling. I can set my car computer to measure the fuel economy of a 10min drive, then I idle for 5 mins at the end and the trip fuel economy stays the same. Sneaky.
 — bs0u0155, Oct 21 2020

A simple spreadsheet on your phone will do this much more efficiently, shirley ?
 — 8th of 7, Oct 21 2020

//calculating fuel flow is quite tricky//
The fuel has to come from the fuel tank; therefore measure that. Measuring at the cylinders/injectors is like measuring the position of each toe to get the distance you walk.
I would suspect, given the inaccuracy of car speedometers, that distance travelled is the more difficult; although these days GPS could do better than measuring wheel revs (apart from that pesky built-in error...).

 //fuel has to come from the fuel tank; therefore measure that.//

 Now you're in a real mess, because most fuel systems pump fuel to the engine where a fuel pressure regulator will portion the incoming fuel to either the fuel rail, to replace that the engine has burned, or return it to the tank via the return line. They run as a sort of endless loop with a variable fraction diverted off to the fuel rail for injection to the cylinders. You could measure flow after the regulator, but then you have to compensate for pressure/density/temp issues. Those vary pretty wildly given cold start-up/heat soak etc.

 There are return-less systems, but they rely on computer control*. That computer is largely concerned with maintaining pressure and if it cares about flow, it derives it from injector duty cycle.

*And are therefore not to be trusted.
 — bs0u0155, Oct 21 2020

 When measuring fuel economy you don't need a moment-to-moment analysis. Start at empty. (it doesn't have to be absolutely empty) Drive the car 400 miles in city traffic. Drive until empty again. Now calculate the total miles and add up all the fuel that was put into the car. That's the mileage, to within a very small percentage. If you want to adjust for different driving styles change drivers every 100 miles.

Or just get the actual numbers from 100 people driving that model over a thousand miles each, drop off the 50 least likely to be reporting actual mileage, drop off 10 outliers, and perform a regression analysis on the remaining 40.
 — Voice, Oct 21 2020

 // Start at empty. /

 No. Wrong.

 Start at full - brim the tank. That is an easy, highly repeatable datum.

 Drive the car, using whatever distance-measuring system you favour (but using multiple sources, i.e. GPS, GLONASS and the odometer is best) for an arbitrary distance.

 Brim the tank again. Calculate ratio of distance traveled to fuel consumed. Weep bitterly into an alcoholic beverage and curse the repressive regime that taxes fuel unreasonably. Plot the overthrow of the regime by violent means. Rebel. Kill them all. Leave the corpses dangling from lamp-posts as an example. Assume power and decree free fuel for everyone.

Job done.
 — 8th of 7, Oct 21 2020

 — Voice, Oct 21 2020

 //curse the repressive regime that taxes fuel unreasonably.//

Tell me about it. It's only just dipped below £1.60 a gallon. They should be paying me to dispose of that ethanol-tainted crap.
 — bs0u0155, Oct 21 2020

 No integrated vehicle display seems to factor in changes in altitude, and they can have a significant effect on consumption.

 Driving in mountainous terrain is likely to use more fuel than travelling a similar distance on level ground, so a metric which includes both dimensions would be more enlightening for the sophisticated user.

For substantial changes in elevation, such as from the California coast up to the Sierra Nevada, engine performance degrades noticeably, although turbocharged powerplants suffer rather less.
 — 8th of 7, Oct 21 2020

 Basically if the driver's math consistently shows 10% lower economy than the vehicle's math then the driver could access the appropriate vehicle menu and adjust the displayed value to reflect the actual economy, i.e. 400 miles driven / 12 gallons to refill the tank = 33.3 mpg, not the 37 indicated.

 The system would continue to measure fuel consumed per duty cycle, or whatever it does, but for the average economy readout figure it would apply the appropriate error correction based upon the value the driver entered at the most recent fill-up, thus the 37 in the vehicle's mind is adjusted (down, in this example) to 33 to reflect my math.

Most people simply drive until they need to refuel, whatever their economy, and I'm no different but a consistently true indicator helps as a vehicle health monitor: when the economy dips for no apparent reason there might be something that requires attention.
 — whatrock, Oct 22 2020

 True, a consistently inaccurate monitor would serve the same purpose regarding vehicle health but now we're using it solely for that alternate purpose because it doesn't work for the factory intended one.

If the head keeps sliding off the hammer I suppose you could use it solely as a projectile weapon but it was really meant for driving nails.
 — whatrock, Oct 22 2020

 // A consistently false indicator can serve the same purpose, as long as it really is “consistent” //

 That's partly correct. Consider two analog clocks, both set correctly at midnight; one is stopped, the other loses 10 minutes per day.

The stopped clock will indicate the correct time twce per day; the "losing" clock will be right only once in 144 days ...
 — 8th of 7, Oct 22 2020

 [+] If the car had an accurate fuel gauge, it could calculate the long term average consumption and compensate automatically. With current technology, it seems like an accurate capacitive fuel gauge shouldn't cost more than a few dollars per unit in mass production.

I like the idea of using GPS to compensate for inaccuracy in the odometer (and speedometer) caused by tire pressure and wear. // (apart from that pesky built-in error... ) // What built in error? If you're talking about selective availability, that got turned off in 2000 and is supposed to stay off. GPS accuracy is claims to be better than 16 feet. That's plenty good enough for this purpose, and most of the error will average out anyway.
 — scad mientist, Oct 22 2020

 //recalibrating the dashboard indicator after every fill up would be a waste of time// Could be totally automated. Fuel pump wirelessly transmit volume of fuel delivered, to car system. Car system calculates difference between dead-reckoning and measured values and adjusts conversion scale appropriately, Display then shows calibrated value.

Also if all displays would just show error estimates as well e.g. "46 +/- 3 mpg" we could all relax a lot more.
 — pocmloc, Oct 22 2020

Don't forget relativistic effects - as you go faster, your car, and your car's fuel, will get heavier, which may affect your fuel consumption calculations.
 — hippo, Oct 22 2020

 The manufacturer's calculation assumes consistent fuel quality, which is an unrealistic assumption at the stations* near Camp Teacup.

 Knocking, pinging, and 'bucking stallion syndrome' tell us which stations have water in the underground fuel tanks (by accident or design is unclear; research is ongoing), which have added corn (ethanol, whether advertised or not), and which have inadvertently filled the storage tanks with diesel (not really, but attendants lurking around just out of visual range make us wonder).

*We have accounted for condensation in the car's line or tank due to seasonal temperature changes, since the fuel never sits long enough between the 40C of summer and the -40C of winter.
 — Sgt Teacup, Oct 22 2020

 [kdf], I see this monitor thingy as something designed to assist me but which fails consistently and in the same direction and should be adjusted to serve me better. Idling, route, terrain, fuel quality, use of a/c, driving style, tire size, all the various things that define my per- tank driving situation could be summed up in one correction such that the displayed value is closer to the truth. Your driving situation in your vehicle might be different and so your user-inputed correction would bring your readout closer to your math.

 I have a .22 rifle that used to print high and left. Perhaps another user would have found it to be dead nuts on but for me it was high and left so I did a sight adjustment and now I'm in the ten-ring.

Like that.
 — whatrock, Oct 22 2020

 //Fuel pump wirelessly transmit volume of fuel delivered, to car system. Car system calculates difference between dead-reckoning and measured values and adjusts conversion scale appropriately, Display then shows calibrated value.//

 That's reasonable. The fuel pumps are supposed to be calibrated and temperature compensated.

 //I have a .22 rifle that used to print high and left. Perhaps another user would have found it to be dead nuts on but for me it was high and left so I did a sight adjustment and now I'm in the ten-ring.

 Like that.//

 That's fair enough. They're clearly overrating the figures to make people feel better about their car/driving rather than giving the best possible information. They'd have to be forced to change that.

A solution maybe to drive at a consistent speed on a totally level road in Nevada or somewhere. Film several hours of it as data collection. Then, brim the tank and state calmly and confidently "At the indicated fuel economy, I should easily arrive at the next fuel stop with a 7% reserve" Then drive off at the same consistent speed and indicated economy filming all the way. Then, run out of fuel 3% short of your target and over the next day or two, die of dehydration. That should leave you with a pretty winnable argument in court.
 — bs0u0155, Oct 22 2020

Yes, your legatees will undoubtedly welcome your selfless act when the class action pays out.
 — 8th of 7, Oct 22 2020

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