Anyone get BETTER mileage than the trip computer estimate?
 

I've always heard the MPG readout on a trip computer is wildly optimistic, with the real figures coming in around 1-2 mpg lower than what's indicated. Well, I only have a few fill-ups logged so far, but going by Fuelly's calculations, I'm averaging about 3-4 MPG higher than the trip computer says. The trip computer has been at 17.2 to 17.5 MPG for the last two weeks. I've only seen better than that on long trips, but never in my daily commute.

The figures I'm entering appear to be correct, but something just seems off to me. I wouldn't be too shocked to get 1-2 MPG better. I live in the DFW, TX area, and my miles are at least 80% highway, but it's stop and go a good bit of the time in the afternoon.

If it's true, I'll take it! But like I said, something seems off here.

17 MPG, what are you driving a tank? ;) Like you say, 90% of the time, the computer reads 2 to 4 MPG over the actual figures. The only thing I can think of is that you didn't fill the tank right to the top, or it could have shut off early, I know you guys in the US with your huge tanks have automatic fuel pumps that shut off when the tanks almost full.

A short fill would make your Fuelly results look higher than they are for that single tank. Are you filling up at the same station and pump? To the same auto click off?

Are you using any advanced hypermiling techniques? The car might ignore any distance racked up while coasting with the engine off for the calculation.

My Hyundai is terrible - last tank, trip computer said 70 mpg, real mpg 60! By comparison, our Honda said 44.2 and i got 44.2 at the pump.

As far as I can tell, I'm getting a complete tank. I stop after the first click every time. I've been filling up at the same station, but not necessarily the same pump every time.

I know I'm not intentionally hypermiling, but I'm pretty easy on the gas. No hotrodding or anything like that. Heck, that thing doesn't have enough power for hotrodding, anyways!

I'm trying to think of ways that you or the car's computer got bad data. Otherwise, it would seem the dash mpg is simply reported low. Possible, but not likely.

Previous work done, or tires slightly different sized than OEM, would effect both calculations. A short fill is the usual reason for off results, and if that is the case it will become apparent with a couple more a fills logged.

Perhaps you are getting blessed by a fuel fairy.:)

Hi,

I don't hink you have bad data.

The car is working out the MPG from fuel flow, throttle position, engine speed and load, it puts all those values into a pot and comes up with a number. Now these values are fairly accurate but not 100%. There are a lot of variables that can change the calculation, I'm actually surprised that they can be as accurate as they are.

You are working out the MPG from the mileage travelled and the amount of Fuel added. Unless you can gaurentee that the amount added is exactly the same as the amount used then there is going to be a discrepency.

Using mileage travelled and amount of fuel only requires two parameters to be accurate, your milometer in the car and the guage on the pump. The guage on the pump has to be within a certain tolerance by law, and I believe your milometer is also governed by certain laws to be within a certain tolerance.

Over time your calculations, using the mileage travelled and the amount of Fuel used will be as accurate as you can get. I would use these figures in preference to the ones the car calculates.

You can check your milometer against a satnav over say 10-15 miles, that will give you an idea on how accurate it is.

Can't help with checking the pump though :)

In summary, stick with your own calculations, using mileage and fuel used has been shown time and again to be more accurate than the MPG guages in the car.

Oliver.

The tires I have right now match the stock size, so that's not the issue. So far, I can't think of anything I'm doing wrong. Like others have said, more fill-ups will tell the tale.

bryan1980,

Don't panic, you don't have an issue and you are not doing anything wrong.

The car guage works the same as a Scanguage, it takes various readings from the ecu and tries to calculate MPG. The car manufacturers define the algorithm to get the closest number to actual MPG, the key word being closest.

If you get a Scanguage, there are various settings that you can adjust to get the Scanguage to read as close as possible to your manual calculations.

The most accurate way to measure MPG is to use the distance travelled and the fuel used. The cars guage, like a Scanguage is only ever going to be a best effort, albeit very close to the real value.

The value to be gained from either the cars guage or a Scanguage is not the absolute accuracy of average MPG over a tank of fuel, it is the instantaneous readouts and the ability to see how changing your driving changes the values, hopefully for the better.

Don't expect the car calculations to match your own calculations, they will always be different.

Compare like with like, so for example if you set your trip MPG each day and today you got X mpg going to work and then tomorrow you get X-1 going to work, then you know you have improved.

Similarly for you own tank calculations, if on the last tank you calculated Y mpg and now on this tank you got y-1 then again you have improved.

Oliver.

My trip computer is normal 2-3 MPG optimistic.

Correct for incorrect miles due to tire being some percent different than the trip computer was calibrated for. Maybe possible differences due to short trips & many cool downs for a given tank of gas. Possibly, the use of 100% ethanol-free gasoline instead of 10% ethanol blends may cause differences.

Not quite the same, but my mpguino under reports my mileage by a bit. I decided not to recalibrate it, so I'm just nicely surprised each time. It's off by just the right amount, actually: if I put in a bit of effort, I can get a trip (and sometimes tank) average of over 50mpg according to the computer. This translates to about 53mpg at fill-up.

I'm finding the trip computer to be pretty optimistic in my new Seat Leon. I'm keeping a close eye on the discrepancy as it seems to be getting worse with time and the car has only done 2000 miles! Last fill the difference was 4.5mpg :confused:

Well, I think we're coming back down to earth. With the latest fuel-up logged, my average is 17.1, with the trip computer reading 17.4.

I'd love to find a station with 100% gasoline, if only for my lawn equipment. There are some around here, but they're all out in the sticks.

I suppose there must be a number of different "trip computer" systems out there, but you need to make sure that you're operating yours correctly. For instance, my system's "Cruising Range" number is based upon the "Average MPG" number, and you really need to comprehend that you need to RESET the "Average MPG" setting each time you fill up, so it can calculate both the Average MPG and the Cruising Range for each tank of gas -- otherwise, both numbers are going to be relatively useless.

So, make sure you understand how to operate your system, to begin with, and also make sure you comprehend WHY you should operate it one way or another, to get the results you want from it.

I don't believe my trip computer operates that way. It's a pretty simple one compared to some out there.

Yes, that's what I was trying to say -- there must be a ton of different systems out there, and they are most likely very different from one another.

Yes, my trip computer is quite simple compared to the ones in newer vehicles. The trip odometers are the only things I can reset. What I really wish it had was an "instant MPG" readout. You can use those as sort of a "driving coach" to optimize fuel efficiency in all situations.

I have found in practice that instant MPG displays are next to useless. The only time they give you any useful data is on flat ground at perfectly constant speed (i.e. with cruise control on). Any wind, slight incline, or the slightest acceleration or deceleration will give you bad data. There's no way to judge how efficiently you accelerate, coast, no way to get any data at all unless you are on flat level ground at constant speed.

I had hoped that the scangauge was smart enough of a tool to have figured this out... but it was mostly useless. It has instantaneous MPG and trip MPG and no other options. The rolling average is what's really useful, and a combination of trip MPG and tank MPG is very useful as well. I sold my scangauge after a year or so... waste of money.

My first tank chasing the Scangauge live MPG figure gave me a decent increase (69.5 mpg compared to 55 and 60 on previous fills, same route) - but at the cost of speed!
My car has a live bar topping at 60 mpg, but now I can focus on rolling along at 90-120 mpg on some sections of my route. These figures are just guesstimates, as they are provided by my Hyundais computer, and it is always wrong - but give me a useful sense of bad, good and better styles of driving, gears and speed. My car told me to expect 74.8 mpg when that 69.5 was the true figure.

The ScangaugeE has a graph that, I think, is a metric of how efficiently you are driving. The Ford hybrids have the leaves for the same, and other new cars have something for the same purpose.

Instant MPG is almost useless on its own. The Scangauges, and others, have other data streams to use in conjunction with it. I default to Throttle Position Sensor, but there is also Engine Load and Manifold(or Boost) Pressure to use in conjunction with the instant MPG to see what you are doing, if anything, effects the car's efficiency,

Any feedback is useful, instant MPG is proabbaly one of the most useful if used correctly.

This is from another well respected Hypermiling SIte:

Vehicle fuel economy feedback is possibly the most important fuel saving item one can add to their car or truck if it not already equipped. Most CleanMPG’ers see an approximate 15% increase once an add-on ScanGauge for 1996 and newer vehicles is installed. Some of the more advanced highway techniques including SHM for Toyota Hybrids and SAHM for many Honda’s make this device almost a necessity.

How else can you understand how you got the average MPG over a 10 mile journey, if you don't which parts of that journey you got high MPG and which parts you got low MPG?

How do you know when your car has shut off the fuel, if it does that, when using engine braking coing up to lights? For example, on our Skoda I know that until the engine is warm I shouldn't use engine braking as it uses more fuel than in Neutral, I can only know this from the instant MPG guage in the car.

The more information you have, the better you can make decisions on how best to drive in different circumstances to get the best MPG.

In my Skoda I would use the instant figure all the time, occasionally checking the trip average. The idea being that I try and keep the instant below the average, which will in turn bring the average down as well.

Hope this helps.

Oliver.

But how accurate are the instant MPG readings? I find the slightest adjustment to throttle can result in the figure dropping to stupidly impossibly low figures. In neutral going down hill it reads 99.9 MPG, in gear coasting down hill it reads 0.00 MPG.

That is quite true. My wife's Chrysler Town & Country minivan has an instant fuel economy bar, and it's quite comical to see it rise and fall as a result of every different throttle position! But, as long as you don't take the information as gospel, it can still be helpful to a certain extent.

Also, I found out that I can indeed reset my MPG readout. There's only one knob to control all of the trip computers functions on my car, and you use it to cycle through the functions (trip A & B, DTE, avg. mpg, and vehicle system information) as well as to reset. I had wrongly assumed that the reset function only worked in the trip odometer mode. So, I reset it after today's fuel up and we'll see what happens over the next week.

That's because your car is using DFCO(decelerating fuel cut off).
While in gear, the ECU shuts off fuel to the cylinders while coasting. While in neutral, some fuel gets burned to keep the engine spinning.

Just because the car DFCOs while in gear doesn't necessarily mean more fuel will be saved from coasting in gear on a trip. While in gear there will be engine braking slowing the car. Great for reducing brake wear when you need to slow down or simply the speed in check. Not so great if you end up burning more fuel than saved in order to reaccelerate after the coast.

That said, instant MPG can fluctuate rapidly and wildly at times. I don't look to it to see what my fuel consumption actually is, but to see if it gone up, down, or stayed steady. That can tell me if a change I've made or in the trip conditions is good or bad, or if I simply lost focus and let my foot get a little heavier on the pedal.

Mine is optimistic too! Several times during a recent 5000+ mile trip, my MMI indicated 40.2, and 40.8 mpg, but real world miles/gal math showed a high of 38 and change. Was really primed to break that 40 mpg barrier, but no joy.
Tenderfoot

2 to 3 MPG is perfectly normal, that seems to be the optimistic average of most on board computers.