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

The only time I care about the mpg meter is if it shows a sudden unexplained decrease. The rest of the time, I do the math manually and record both when I fill the tank. If the meter is consistently x% high/low, I can deal with that. Even if it was accurate, I'd still do the computations myself to continually verify it. My van's meter was less than a mile off, but I still did the comps, out of habit I guess.

 

About 1 mpg off in a 20 mpg car is the same error percentage wise as the Prius.

That reminds me, the arithmetically challenged can just subtract 2.5 mpg for a pretty accurate result if the meter reading is in the range of 44 - 52. The error at the margin is about 0.2

The less lazy can deduct 5%. Error is about 1 in 400, good for half the error at the above margins, or about 0.1 mpg error for just about all Prius results.

 

^ my error's currently at 7.6%, cumulated over about 44000 kms calc'd. That is total distance and total liters.

There's another thread going here, a guy getting about 65 mpg at 65 mph. Has a pic of the dash display to substantiate it. Therein lies the frustration.

 

what is the frustration ?

 

Well, if his error is similar to mine, then his actually mileage is just over 60 mpg. Still very good, but it's frustrating having to do the Toyota fudge correction.

 

Imagine how the GM owner feels, listening to the Prius owner agonize over whether it is 61 or 62.

 

FWIW, the FE-display error of our 2014 Prius is very consistent (≈+5% high) compared to the FE-display error of our now defunct 2011 GM Chevrolet Cruze, whose error was both greater (≈ 7-9% high) and more erratic.

 

I have a 2014 Avalon Hybrid. Sometimes mine is way off. Other times not too bad. I just gave up with that. It is nowhere near accurate.

 

Boy oh boy... where did you get this ?? The problem is not the 5% but the 5% POSITIVE BIAS !!!!! MPG is quite 'accurately' always reads about 5% more !!!

I had a GenII prius for over two year with 35Kmiles and the mpg reading was about 2-3% off BUT above or below the computed mgp/per tank and it was well within 1% for long term ... so it is quite possible for toyota to be accurate

 

Choices:

1. Increase the tire diameter - this lowers the indicated MPG but it increases the risk of a speeding ticket. Use a GSP speedometer.
2. Combined GPS and fuel consumption app - use the OBD for fuel consumption and GPS for true distance.
3. A calibration unit - spoofs the pulses from the ABS controller to the displays to adjust the indicated speed and distance so you don't have to change the tires. Use a GPS speedometer to minimize speeding ticket risk.

Bob Wilson

 

Choices for what? For dealing with the dash-displayed mpg bias? Changing your tire size and installing assorted electronics to remedy this seems a bit extreme, lol.

I'm fine with just being aware that it's consistently bs'ing (around 7.5% in my case), and doing calculation with each tankful.

 

Truly, how many people want more accuracy than can be had from just subtracting THREE from the display reading ?
What a tempest in a teapot.

 

Well, I'm still ticked over the (apparently intentional) misinformation in the dash mpg display. But Toyota should be remedying it, I think Bob's gone off the rails.

 

Mendel,
The display inaccuracy might be intentional, or it could be explained as the simple result of different tyre diameters sourced as OEM by Toyota. Perhaps a little of both.

How full is your glass ?

 

Our error stays pretty consistent, both with the OEM 215/45R17 and snow tires in 195/65R15.

 

Calibrate by distance, using either distance road markers or a good GPS.

 

Our 2003 Prius has Yokohama 195/70R14 on the front and 185/70R14 on the rear. The 195/70R14 are too large to fit in the rear wheel well. These are 6.2% oversized to reduce the transaxle stirring losses. To deal with speedometer and odometer errors, I'm using a Garmin nuvi as a GPS speedometer and tripmeter distance recorder.

I'm also planning to measure the signal levels feeding the 2003 displays. I have an MSP430 that can read the ABS pulses and apply a calibration constant to generate accurate rotation pluses. Add a GPS mouse and the calibration constant can be automatically measured and updated.

Bob Wilson

 

Considering the success of the 'patch lost 8 MPG' complainers, I am not expecting anything. Tires are not really accurate sources for true distance or speed.

So 'off the rails'?


Bob Wilson

 

This is ridiculous. All onboard computers calculate MPG using a lot of assumptions. The car doesn't have a very accurate measure of how much fuel is in your fuel tank, so it does some quick math. It assumes the flow rate of fuel when the pump is operating at a specific speed. This is "generally" accurate. This is very useful for engine setup, and is within a decently tight tolerance for feedback systems. It then derives your total fuel consumption based on this instantaneous assumption.

In essence, your car is measuring fuel in the same way that most people measure flour in their kitchen. You fill a container and eyeball it(volumetric measurement). The actual amount of flour can be more carefully measured by weighing the flour(mass measurement). You get close enough, but any commercial kitchen is going to use a mass measurement.

If you want your car to have a better MPG reading, then you should go buy the following equipment:
1" Diameter Stainless Steel Compression Load Cell, 0-100 to 0-10,000 LB. Capacities
In-line Flowmeters
If we used these tools, we could more accurately determine your flow-rate. OR you could just do some basic math at the pump using the calibrated volumetric flow meter that costs hundreds of dollars that is attached to the pump and certified annually.