Toyota why is the mpg readout not accurate?????

^ Groan... Toyota has managed to make a mpg meter that is quite consistently inaccurate, with a strong positive bias. In my experience, the error is in a very narrow range, between 6 and 8 % positive. About 2 % range.

So why not just drag that back to the median, so the 2 % error range falls roughly either side of spot-on? Let me guess.

 

My first fill-up resulted in the actual mpg being 14.1% greater than the displayed mpg (53.6 to 46.0). After that, the difference settled into the CONS always being greater than actual, so I assume the tank had been topped off (or something) by the dealer and more gas than I put in was actually used. After that, the variance ranged between 3.0%-9.9% (0.7-4.3), always filled using the slowest speed until the nozzle shut off. Unfortunately, only 3 of the fill-ups were at the same station (the 1st, 2nd and last), so much of the variance could be from different flow rates, shut-off settings, altitude, etc. And while the average mpg is currently only 46.3, that also varied widely, 37.4-54.7. Now, my results might be a bit skewed too because I don't generally run the tank to empty when traveling. We stop every 2-2.5 hrs regardless and I fill-up based on where Gas Buddy says gas is cheaper along the route. So, our average distance between fill-ups this trip was ~294 miles (177.2-473.8).

What this tells me is that I can't count on getting a specific mpg on our trips like I could with our van where I consistently got ~24 mpg between fill-ups regardless of distance, speed, etc. Admittedly, we didn't generally use the van around town, so the average mpg was mostly highway miles, but the Prius seems to be affected to a much greater degree by terrain and speed than the van was. When I first started planning our trip, I had expected to make it to Moab UT (459 miles) before we needed to stop for gas. However, Gas Buddy showed that I could save quite a bit by stopping in Tuba City AZ and then Grand Junction CO. That decision was proved right when mileage was below 40 going uphill to Flagstaff. After that, I came up with alternative stops along the our routes and diligently watched the mpg to make sure we stayed on track. None of this is a problem for us because we like to stop often, but it is something I had to get used to when planning each subsequent day. It also made me make sure I filled up before we took off for a mountain drive.

 

There are two good fuel flow metrics:

• Injector timing and engine rpm
• Mass flow and fuel-air ratio

Sure there are precision fuel flow meters but they are not necessary for our studies.

Bob Wilson

 

FWIW -- Just about ANY decent (cheap) add-on MPG-meter provides user-adjustment/calibration for both MILES and GALLONS...certainly Toyota could come up with a simple MPG±X input to the MID-computer to provide SIMILAR adjustment of MPG values for the Prii.

 

Maybe not at any given moment, but if the fuel gauge is set to display based on average reading, short of being on a sustained upward or downward angle, it should read fairly true if it's designed to accurately reflect the quantity that the tank is designed to hold.

 

Actually no. Tank level is typically a float. I would guess accuracy is +/- 10%. This is part of why "E" means 2 more gallons.

 

What I take from this: your van is a very steady gas consumer, whereas the Prius has much more gas saving potential, is more opportunist. In a good way.

 

I'm sure they could, but don't, because the convenient (and rosy) way for owners to assess their mpg is via the dash display. And they want happy owners, proud of their mpg, likely to relay the results to others. So they intentionally push it as far as credulity will stretch. And then some, as this thread attests.

The numbers are still great without the BS factor, but integrity takes a back seat to the bottom line I guess.

 

It is interesting. We just purchased a Subaru Outback to compliment our Prius. The Outback too, over-estimates mileage. However, according to the Subaru Forum, there is a way to manually adjust the readout. I would be surprised if Toyota didn't have a similar method.


From Subaru:

There are 3 parameters that can be changed via the Combination Meter System. If your Fuel Economy display is high or low (after manual calculation) you can adjust it. To enter the Dealer Customize Mode you MUST do everything within the prescribed time as described below.

3. DEALER CUSTOMIZE MODE
The combination meter can be customized with dealer customize according to the following procedure.
CAUTION:
Perform the steps described in 1) through 4) within 10 seconds.
1) Within 3 seconds after turning the ignition switch to ON, set the lighting switch to tail light or headlight position.
2) Press the trip meter knob five times.
3) Turn the lighting switch to OFF, and press the trip meter knob five times.
4) Set the lighting switch to tail light or headlight position again, and press the trip meter knob five times.
5) Move on to the “DEALER CUSTOMIZE MODE”.
• When the dealer customize mode operates, the LCD displays each adjustment screen.
• The dealer customize mode consists of three setting screens. {Avg.F/E Correction Screen}, {Ambient
Temp. Correction Screen} and {Clock Adjust. Screen} is displayed cyclically in this order every time the trip
meter knob is tapped.
• Holding down the trip meter knob while each setting screen is displayed can change the setting value.
NOTE:
When ignition switch is turned OFF or the vehicle is driven, the customize mode is cancelled automatically.
No. Customize mode Initial value Correction range
1 Avg.F/E Correction Screen (initial value+/-0%) Correction Range+/-10%
2 Ambient Temp. Correction Screen (initial value)+/-0°C (°F) Correction Range+/-3°C (°F)
3 Clock Adjust. Screen (initial value on) Correction range on or off

 

I also have a relatively new Subaru, a 2014 Forester, replacing a previous century Legacy.

My Forester's MPG display also reads high compared to the odometer. But I also found that the odometer reads significantly low (compared to GPS and highway milepost markers), seemingly a compensation for the class action lawsuit over warranty fraud a few years back. After accounting for this new odometer error, the displayed MPG is almost right on.

Have you checked for this factor?

 

Absolutely true! My only point was that the Prius makes it a little harder for me to pre-plan economic fuel stops using Gas Buddy. I know a lot of folks don't care much about saving $.10-$.20 or more per gallon, but it's become a pet-peeve of mine over the years and I like to reward service stations, particularly those along freeways, who don't gouge their customers on the price of fuel while saving myself some lunch money in the process. Fortunately, there was only one time on the trip where I had to stop earlier than planned and pay just a little more, though it was only $.06/gallon and around $.50 total. Yes, I know that sounds anal, but it's become a game to me and I'm going to stop anyway, so why not stop a few miles one way or the other and save a few dollars over the long haul? I'll be driving 10 miles to have lunch with a friend today and gas prices vary from $3.099 to $3.359 along the route, so where do you think I'd buy gas if I needed it? It also varies by the same amounts from one exit to the next near where I live. Gas Buddy users would know which exit sells gas for $3.099 vs the $3.359. If nothing else, my obsession is good for a laugh.

 

You reminded me to check the Scanguage manual and it has two ways to adjust the speed and fuel consumption. Sad to say but our 2010 with Sumitomos tires is less than 1% off but the 03 prius is over 6% off. I'll test with it.

Bob Wilson

 

I plotted a hundred data points and fitted a linear equation. Look at the regression value. It is plenty precise. I think it is easily fixed if they want it to be accurate.

 

You plotted 100 data points for YOUR car or for a bunch of cars?
Also, that might be the worst graph I have ever seen. You apparently "calculated" a lot of stuff. I have no idea what the X or Y axis represents. I am going to assume that one is your listed MPG(which is calculated) and one is your measured MPG(which is actually measured, and not calculated).

Look, I totally believe that Toyota and most manufacturer's fudge the numbers to make them look better. My argument is that they do this for entirely innocent reasons. Namely, they didn't want a bunch of people bringing in their cars when they weren't getting EPA MPG. Also, there is some variability in MPG between vehicles for numerous reasons. If you shade everything to look a little better you get fewer people complaining about a non-issue. People who are stupid enough to totally trust the car computer for MPG are stupid enough to bring their car back to the shop and complain when it gets less that 45 MPG because they drive with a lead foot.

 

IF Toyota can stuff BlueTooth and WiFi into the INFO-tainment systems in their vehicles, they can/should -- at least -- make the dashboard information display (MID) readouts accurate!

There are government required standards on SPEEDOMETER and ODOMETER (milage) already, so why not equivalent requirements on MPG, since the manufacturers hype & sell these vehicles upon their *milage* values? Plus-or-minus 2% should be easy.

 

Makes me wonder if the readout calculation isn't designed to mimic the EPA testing calculations. I don't even know how they do EPA testing calculations, so I could be way off-base.

As for "stupid", my brother bought a Sonata Blue because he wanted a car that gets 40 mpg and still looks "sporty". He has never gotten 40 mpg and might still get a check from Hyundai based on how much lower his mileage is than the EPA numbers. We all know the Ford C-Max saga of the claimed 47/47/47 that has since been reduced significantly because they finally did EPA testing on it vice (legally) using results for the Focus. My brother didn't complain, but he did join in to get his checks. And truth be told, I don't know if he uses the displayed mpg or calculates his actual mpg.

 

And just what are those government standards for the speedometer and odometer? The only one I've heard about in the US is -0% on the speedo, only in some jurisdictions. No + limit here, while the EU has a very generous tolerance.

I don't believe for a moment that the government imposes a +/- 2% accuracy requirement on either. In fact, I don't believe the MPG bias we are seeing is even outside whatever legal accuracy requirement you may find for those other meters.

 

FWIW, the SAE J1226 standard for American vehicles is ±2%...of the speedometer's FULL-scale value; this article has some (though scant) information: Your Speedometer is (Probably) Wrong » Scott Bradford: Off on a Tangent

 

And just what government agency writes SAE standards?

 

GAO? When they buy Govt vehicles? Or perhaps trial lawyers?

Bob Wilson