Car's computer agree with fuelly?
 

I've noticed my car's computer is giving me mpg higher than I get with fuelly.
I'm confident fuelly is calculating right. It's not rocket science. Maybe I should start double checking the calculation?
I could understand the computer being off. I also think calculating a tank like this on fuelly is dead correct...can't be off.

EDIT: Just an example....most recent...
Computer said I got 34.8 mpg. Fuelly says 33.4 mpg.

There is a big topic I started about this here:
https://www.fuelly.com/forums/f8/dash...ual-17936.html

A cars computer will never be 100% accurate, they are nearly always a few percent optimistic. There's no need to check your calculations, fuelly is the best method of gathering data.

Thanks....That was an interesting informative read.
I do have to admit. As long as the computer is consistent. It is doing a bunch to help me with mileage. I'm very thankful for it.
If I just goof off driving. I've gotten only 30.6mpg. I'm confident I'll get ~34 this next fill up.

Yes regardless of the accuracy, it's still a good tool to use whilst driving as a reminder to save fuel. It becomes a game after a while, you'll start chasing numbers if they're there right in front of your nose!

Hehe.... chasing numbers.. I can be an obsessive cuss...Doh!
I've already decided on Pirelli P7 tires this year. They should gain me some mileage. .... hehe

There are plenty of tyre topics here do, always pays to do a bit of homework, I've learnt alot since being on this forum. Here, our tyres are rated A to G for economy, so it makes choosing easier.

If you're really in to numbers and mpg, consider this: One thing to consider after your new tires are mounted is how the true diameter vary from tire to tire and how this variance affects the accuracy of your odometer (trip meter), and therefore, sometimes, the difference in mpg is much less or more than what it seems.

If you care and if you can, you can check your worn, existing tires mounted on your vehicle against a GPS a few times, and then do this again once the new tires are mounted. From my experience, there will most always be at least some error and some differences from tires to tires, and also from my experience, the odometer/trip meter check versus a GPS is usually a very consistent result.

Currently, I'm commuting in an F150 with the all new 2.7 Ecoboost engine. I have the original factory tires Michelin LTX 245/70R-17s. Every single time I've checked the trip meter against the GPS, the result has been 1 mile vehicle-recorded loss for every 54 miles, or roughly 1.8%. I've checked about three times before my 1st rotation and once after. All four have given the same variance. In my case, if the GPS is the more accurate of the two, then I can multiply my fuel-tank miles by 1.018 before dividing by the pumped volume to refill and my actual mpg is overly pessimistic w/o considering this common error.

What I do is add 1.5% so that I have a conservative record, however, I am not very physics or tech. minded, so what I can't tell you is how accurate GPSs are with respect to tracking miles, but I can say that mine is very consistent as long as it does not lose satellite reception.

I realize that with regards to my current vehicle, there is not that much error caused by tire diameter throwing off the accuracy of the trip meter. However, in my last vehicle, I once mounted new tires, and the error went from 8% to 1% as to what I had to subtract from the refill miles. W/O accounting for this error, I would have thought that the new tires caused a 10% mpg loss, but the actual difference was far, far less.

One last thing to consider... it is my experience that new tires will usually hurt mpg even beyond the extra diameter that new tread might add versus bald tires, because all that tread and newness will give lots of grip, and also, in the 1st thousand miles or so, there will be a break in period, so if you're unhappy with the first few recorded tanks, don't get too discouraged, as it may not be long term.

Interesting.... I do have more than one GPS. Might do some obsessing on that. hehe
My choice of tires I got mostly from Consumer Reports. I don't want to get into dedicated summer and winter tires. My car calls for high performance, all season, V rated (speed rating), tires. I want to get the V's to match what Toyota designed the suspension for. The Pirelli's have a lower rolling resistance than the others listed. I would love to get the Michelin's they listed. They do steal a little mileage though. They track on grooves also. The Pirelli's don't do that.
I really don't think I want to get into searching for the best mileage tire. Sacrifice performance. Especially in winter.
Want to do well in mileage. But not overdo it.
I am considering getting the anti sway bars from Toyota. That's to try to up my speed on cornering. While not driving above mellow. Up the speed for better mileage. Don't have to accelerate from lower speed. Besides, I live in the Hills. Typical hilly, mountain, type roads. Lots of curves. People come from all over for Rally's here. Just to drive around the Hills. Most well known is the Sturgis Bike Rally.
I have noticed here on fuelly. The 2014 Corolla's lean toward better mileage. I don't know exactly why. We just seem to do better on average than other years. I like that. Am happy I bought used from that year.
Originally, my brother had offered to grab me one of those good year end deals in the Sunday paper in So. Cal. on a Corolla. As time went on, he backed off. I ended up telling him; "I don't want a wanna be xxx's car". That offended the donkey. He bought himself a new Accord last year. I know how he drives. He can't be getting the best mileage that Accord can offer. I want to walk all over his mileage as best I can.

Just for fun, before filling up the tank, I started to record mileage calculated by the car's computer. The computer automatically resets after a fill-up so this should be directly comparable to the values one can calculate manually (or via fuelly). Will try to attach a plot of the result, based on 10 fill-ups -- which indicates the car's computer is poorly calibrated.

(actual mileage in MPG) = 0.76 x (mileage reported by computer) + 6.8

So, for example, when the car's computer says 35 mpg, the actual mileage is more like 33.4 mpg.

I wonder if there is a way for owners to adjust the car's calculation?

Link to plot: https://docs.google.com/spreadsheets...4&format=image

I think trip computers are calibrated using 100%(ethanol-free) gasoline. Additions of ethanol(E10-10% ethanol, E85-85% ethanol) & the accuracy of the trip computer over-estimates mpg. 114 octane ethanol needs high octane, high compression ratio(16:1) ethanol engines to release its energy effectively. Used(not burned efficiently) in a low 87 octane, low compression ratio(9:1 to 11:1) gasoline engine, ethanol fails to deliver high mpg.

Interesting thoughts. I am thinking my computer is about 1.5mpg high. Pretty consistent so far.
I'm starting with the regular E0 tomorrow. Looking forward to it.

Just my opinion, but I don't think ethanol has any reason to do with the cars computer calibration, as you'll get similar discrepancies the World over, in most cars of all makes and models using all different types of fuel.

Ahhhh.....That's the problem! ... The all different types of fuel.
Accuracy fuel...
Japanese.... Saki!
French.... Wine!
English... Tea!
USA..... Beer!

Mileage is a pretty simple calculation: (miles driven) / (gallons consumed). The energy content of the fuel does not matter -- except that one will typically drive fewer miles per gallon for a lower energy fuel.

There are various places where errors can creep into the mileage calculation. Consider the "miles driven" part. The car should know exactly how many miles were driven from the odometer (by counting rotations of the (front) wheels) which feeds directly into the car computer. Assuming you are using standard sized tires. Using larger or smaller tires will make a difference how far one rotation will take you. The same problem can occur for seriously under inflated tires of standard size.

Then there is the issue of how many gallons of fuel were consumed. Two ways that might be used to measure that would be: 1) reading the fuel gauge; or, 2) using an inline flow meter. My guess is that Toyota uses data from the fuel gauge. Maybe the fuel gauge overestimates the fuel left in the tank? Past experience suggests that fuel gauges are non-linear. That is, the needle does not move uniformly from F to E over the course of a tank. Unless the mileage calculation takes this into account, an error will be introduced.

Finally, if the gas tank is not filled up completely during a fueling stop, the calculation for that tank will suffer. That is totally the job of the owner. Except in Oregon, where only gas station attendants are allowed to pump gas!

It seems that folks who report discrepancies observe that the car computer _overestimates_ the mpg. Perhaps this is an intentional and clever marketing ploy. While there are legitimate sources of error, it is a bit surprising Toyota did not do a better job with the mileage computer.

Miles should be the accurate part of the calculation. The thing to consider when thinking about it is that the cars that computer mpg usually also show an "instantaneous mpg", along with "average mpg"
The instant one shows 0.0 mpg when not moving, to 99.9 mpg when you let off the gas completely over like 20-30 mph.

From what I can tell, the average is calculated from the instant over time. Errors come in, with the biggest example being that zero mpg time, with no miles driven. The computer has a bunch of seconds with a zero reading(where the car went zero miles, and used a varying amount of gas per second) a bunch of seconds with 30-40 mpg, and a bunch with high mpg(coasting). Add them up, and divide by the number of total seconds, and you don't actually get the real overall mpg, even if the computer is measuring fuel use accurately, because your measurement interval is a time, not a distance.

The other issue seems to be how the fuel use is measured. It is measured by the activity of the fuel injectors, and the manufacturer doesn't bother to make it give an accurate readout, because they want customers to believe they are getting real good fuel economy. They would rather err on the side of displaying high mpg than low. It is marketing.

On filling the tank completely, all that matters is that it is filled to the same level each time, and any errors in filling will balance out if you calculate over multiple tanks of fuel.

My observations with my Versa are that it consistently reads about 2 mpg higher than actual mpg.

My old Hyundai i20 was the most untrustworthy swine when it came to reporting the mpg with the on-board computer. Often it would claim around 75 mpg, but deliver 65 mpg - recently it promised 72, but delivered 55 at the pumps! Hyundai seem to be far more inaccurate than most, but even our Honda CR-V can be as much as 4 mpg wrong.
I would have to say, as most people don't work out brim to brim mileage, it must benefit all manufacturers to have over optimistic read-outs, as these are the figures most friends and colleagues will share, over inflating a brands economy - false advertising! I believe in America, official mpg figures are pretty much what cars will do. In Europe it is a joke - my old i20 was rated at 94 mpg highway, my almost exclusively highway based lifetime mileage was 64 mpg!

Guys and gals, this article explains everything you need to know, no more speculation required! ;)

Why Your Trip Computer Isn’t Giving Accurate MPG Readings (And How to Fix It) – Feature – Car and Driver | Car and Driver Blog

Awwww Geee! I like speculating that my Toyota is more at home with Saki! :coolsmiley:
Seriously....
I do like the consistency of my computer. It does appear to be dead on consistent. I need to find out if there is an adjustment to calibration.

On my 2008 Ford Escape Hybrid the trip computer is usually +2 MPG high. I just look at the display and subtract 2, and that's usually right where my calculations land after taking into account my odometer offset and doing the calculations manually.

Well I just fuelled up after 722.2 miles on that tank, and the computer read 64 MPG on the nose as I pulled into the forecourt. After refilling and adding the fuel up, fuelly said I got 63.2 MPG so the computer was VERY accurate given the length of time and distance between refuelling. I worked it out to be 1.2% optimistic, I don't think I've ever had a more accurate reading on the trip computer.

I'm off to read the article, but for now, I'll relate that my 02 Maxima reported 3-4 mpg better than actual, for the entire 145K miles I owned it.

If I was really working on economy (65mph on flat highway, and keeping it at 65), I'd get a reported 35mpg, and an actual 30.5 or so.

I will be interested in seeing how my new car does.

That sounds pretty normal, a consitant indicated reading from the cars ecu. I'm a bit confused as to why my last tank was VERY accurate, but could have been a small discrepancy with the fuel pump as my filling methods are very consistent.

My car's computer tells fiction when reporting fuel economy numbers. The indicated values are always wildly optimistic. I created a detailed fuel consumption spreadsheet years ago, and I started populating it along with Fuelly when I got my current car. Fuelly matches my spreadsheet's values, so I'm confident it's right.

Note that when you view a specific vehicle's Dashboard, it shows the fuel economy over the last 10 fuel-ups. When you use "Research Vehicles", it shows fuel economy since Day-1 for each contributing vehicle, same as in your badges.

The used 2013 Ford Edge I just bought is the first vehicle where the dash read out was the same as Fuelly. :coolsmiley:

I wish I had created a dummy record for each car - to plot the fake results of the onboard computer.
Running alongside the real car, it would show the difference between true MPG and the one the manufacturer wishes you to believe the car does.
It should show the implied saving this trick gives, as I'd need to enter fewer litres to save the (almost always) higher MPG.
As the Prius has an onboard lifetime figure, I have a rough idea - let me check!

I found out that my Outback has a dealer adjustable setting that I'm going to have changes to be more pessimistic, that is, the readout is going to show lower fuel economy than it is now. I'm going to start by having them change it to 10% worse fuel economy. I think that will get it very close to actual values.

On this, and on the Maxima I sold, I also have an Excel spreadsheet that calculates my real fuel economy, and where I record the ECU reported fuel economy. I'll have to see if I can make a plot of that.

For the Toyota works out around £40 over 7,000 miles. My old Hyundai was far more inaccurate - often nearly 20 MPG out, and usually around 10. For an accurate car, maybe a £1,000 swindle over its life, for an outright lying gas guzzler over £3,000 misrepresentation of how economical it is?
As just a tiny percentage of drivers seem to work out true MPG, the cars "over confidence" combined with the optimistic advertised urban/highway figures must create a reality where most people think their cars are far more economical than they are.
I bet they sleep better than I do, ha ha ha!

My last tank read 66.5 MPG, real was 62.9 MPG, I make that about 5% optimistic, or to put it the other way 95% accurate. Unless you use the same pump every time, and fill the fuel up the exact same that is until you can see fuel sitting in the filler, then your results are going to vary every time. I'm am very consistent with my filling methods, almost ocd.

As stated elsewhere my Seat Leon's computer was about 8% optimistic, so I had the calibration adjusted to be more accurate. I was a bit enthusiastic so the computer is now about 1% pessimistic. I'm happy enough for that, it encourage usa to try to get better mpg figures.

As a side note, this correction can be done on all VAG group cars.

I wonder if it was part of the emissions scandal they purposely made thier computers read higher MPG'S? I wonder if that can be done for other cars too?

Last car I owned actually was pessimistic on the highway by around 1 - 2% depending on speed. In the city depending on how many short trips I drove it was optimistic by up to 5%.

Surprisingly most younger people do not know how to calculate fuel usage especially L/100KM

Have not had a chance to check out present car on a long trip.

I put fuel in the Honda yesterday, trip computer read 66.9 MPG, actual was 68.24 so the actual figure was 2.3% higher than the computer reading. Like Bluerover says, not all trip computers over read.

But very rarely! The Prius has been both very close, and way off. The last 2 tanks were just under 70 mpg, it guessed just over 70 mpg. However, it has also guessed 70 when 62 was the real figure. I must have had 50 fill ups in the i20, only a couple of them I remember stating a lower guess on the computer. I have to say, when it comes to L/100KM though it may as well be pints per furlong - my head only computes in miles per (imp.) gallon...

Generation II (2004-2009) Prius owners have another issue that really affects onboard computer versus "real life" calculations. Our cars are fitted with flexible fuel bladders inside the fuel tanks. Depending on the flexibility of the bladder at any given fill the tank capacity may vary by as much as 3 gallons, even when filling at the same station, same pump and same rate of fill. My onboard mileage will usually be off +/- 2 or 3 mpg but it has been off by as much as 6 mpg.

I glance occasionally at the onboard readings to get a sense of how I'm driving at the time but I don't expect specific accuracy. The same with actual calculations at the pump. I depend on averages over multiple tanks to check the health of the car. I don't get excited about any individual fill.

I just take the fuel fill receipt and record the miles traveled. You can travel miles without consuming fuel if you EOC. You can consume fuel without travelling any distance if you allow your engine to idle. I love EOC and hate idling. Fuelly just allows me to publicly display my mileage and keep track of that mileage over time, while confirming my math is correct, a skill I acquired when calculators weighed 20 pounds and weren't allowed in school.

To check your difference between two different tires if you replace them, or even if you just want to know.

Mark a point on the tire and the pavement, roll the car until the mark covers one revolution precisely, then mark the pavement again. Measure the distance between the two marks with your preferred measurement type.

A 25 inch diameter tire will roll right at 3.1416X25 inches which is 78.54 inches (per revolution). A miles is 63,360 inches. 63360/78.54=715.6 revolutions per mile.

Lets say you put slightly larger diameter tires on your car. They roll 705 revolutions per mile. Since they roll fewer revolutions per mile your speedo (and odometer) will register lower at identical speeds. To calculate the difference divide 715.6/705=1.015. Multiple the distance recorded on your odometer by 1.015 and you get the true distance.

Works fine or everything but ice, metric, US, Imperial, just require adjustments.

Just found something that may very well effect the computer being off.
Been using my dash readout of MPG. It only goes to 99.9. That's all. The bar readout on the computer screen only goes to 60mpg. I take it the computer is using the numbers same as the dash readout.
For example; It reads 99.9 whether I'm coasting in drive, or coasting in neutral. We all know the car is doing better mileage in neutral.
Still, mine is very close. It's also very consistent. Very accurate as far as working with it. Great for motivating people to get better mileage.

Just to contribute a data point on computer-indicated vs actual fuel economy, for my latest fill-up, yesterday (664 miles, combined city/highway driving):

• Computer says: 6.0 L/100 km = 39.2 MPG US = 47.1 MPG IMP
• Actual: 6.38 L/100km = 36.87 MPG US = 44.28 MPG IMP

The computer was about 6% optimistic in this example. I'm driving a 2015 Audi Q5 3.0 TDI.

The car doing better in neutral depends upon the deceleration fuel cut off behavior of the car.

As for my dash display, the computer ignores engine off coasting miles for the MPG calculation. So it is lower than Fuelly.

Keep in mind that you're assuming the display's value is the number being used in the computer's calculations. It might not be. It might just be a display limitation. When you're using no fuel and your vehicle is moving, you are effectively getting "infinity miles per gallon". Most people would not recognize the "infinity" symbol -- the number "8" on its side -- or understand "infinite". The 99.9 could well be just a programmer's decision to show the highest possible value on a 2.1 format display.

From a physics perspective, car manufacturers simply do not want to put in an accurate and expensive fuel flow meter, when "pert near" is good enough for most folks. Ditto with fuel gauges, in which you can drive 200 km (120 miles) and still show full, drive 1/2 of your known range while the gauge shows 2/3 full, etc. Again, it's an inexpensive "pert near" solution that serves the purpose of giving a relative reading (e.g., my tank is less than full, and more than empty) rather than an absolute (and accurate, and correct) one.

Yup Steve... Agree with your putting things in perspective. Like the term "pert near" good enough.

I'm just rationalizing the difference, somehow.
The tank I have now. Even though the calculation came up a little bit lower than my best. That's 0.7 mpg lower. My computer said it's the best I've gotten. It makes me feel good, if nothing else. Reference to my fuel thread makes me feel even better.