Trip Display Overstates MPG

Congratulations...you are the 100th person to start a thread on this subject.

 

In other words, it is a well known issue. Who knows why they did this.

 

yes.. yes.... the car tends to be very optimistic with those numbers.. it's probably some number that was calculated with ideal conditions...

conditions that you would probably not typically achieve.

 

Probably a Toyota thing being overly optomistic on mpg. My Honda is dead on after dozens of manual calculations i now trust the dash.

 

Pretty much every car's OBD (that i've ever driven) overstates the MPGs.
Never believe the car. Always self-calculate.

 

Welcome to the club. This bias for inflated mileage is an annoying distortion, but tiny, compared to the assumption that electricity is free when computing mileage for plug-in hybrids. After all, the Chevy Volt only achieves 90+ miles per gallon of fuel if you don't count fuel consumed generating the electricity that charges the batteries. Toyota should fix the slightly biased mileage reading in the Prius and all manufacturers of plug-in hybrids and pure electric cars should include fuel consumed generating and transmitting electricity used to charge traction batteries. I'm not holding my breath waiting for either of these advertising gimmics to be remedied.

 

My ex-wife had a Dodge Caravan with the big 6 saying she was getting over 30 mpg in town. I calculated it at closer to 12. So, keep calculating, the car don't want to.
At least it's closer than that '02 Dodge.

 

Sticker MPG ratings should show the car's mileage at 15, 30, 60, 100, & 200 miles.
They should also show a cost/mile for plug-in hybrids, based on the national average of the cost of that paticular fuel (gas, electricity, CNG, etc.)

The way they show it now is very misleading.

... but oh yeah, my dash display is off an average of 2-3 mpg also. Oh well, I don't care. I can still see that I'm getting great mileage.

 

There is a sticky post in the fuel mileage section covering this. For what it's worth, the error will be fairly consistent (mine is 5.8% or roughly 3.6 mpg from my typical tank). Thus, if my MFD is reading 64.0 mpg, I know I'm roughly at 60.4 mpg (60.3 - 60.5 mpg). I know I have a ~12 gallon tank, thus can drive nearly 700 miles before I'm on fumes.

Knowing your typical offset will make calculating your tank mileage fairly easily. I would prefer the displayed MPG to be accurate, but it isn't.....

 

Of course you can verify that the amount of gas that you added in this fill up is exactly the amount of gas that you used from the last tank. Because, you know, all gas pumps are always right on track and there's no variance caused by the pump itself, the ambient temperature/humidity, etc. And if the amount added differs from the amount used by as little as a pint or so, it will skew your calculation, but not necessarily the calculation the car makes.

Just saying.

 

You'd think it would be easy enough to change (or fix depending on one's viewpoint) by some minor reprogramming of the Prius computer. Customer comes in says please change my MPG by X% up or down and they jack in and change it.

 

As already pointed out, this is the topic of very many threads. In fact, it is even the second sticky thread in the GenIII Fuel Economy Forum, with more replies than you can possibly read today:
calculated vs. computer MPG - Please post your results

 

Why would I assume that the hand calculation is dead on or even more accurate than the computer. There are variables in the hand calculation that lead to inaccuracies.

 

I think it's accurate in the way 1+1=2. I believe the problem is a "full tank" which varies from pump to pump which can lead to inaccuracies. I don't claim to understand why partial tanks (even minor ones?) cause this.

 

Coming from a second generation Honda Civic Hybrid, I the same experience: Honda accurate, Toyota inaccurate (in favour of the house). The mpg error with our Prius is around 7% I think.

One dead-obvious motivation: Toyota get's free hyperbole reporting of mileage, from reviewers who take the in-dash readings as gospel.

At least forum frequenters are mostly wise to it, but it is a frustrating issue. It would be in everyone's best interest, Toyota included, to fix this.

Apparently the new Prius C and Plug-In are more accurate.

 

A gal at work has a hybrid Camry and when I asked her what her mileage was, she said it was 35 mpg. I asked her if that was the displayed value or a hand calculated value and....well.....let's just say she's not interested in her mileage to the point of actually CALCULATING it. Suffice to say, you will end up with many more happy customers if your reporting is "optimistic".

I'm not saying this was the reason it is done, but it certainly plays into it. I think I'm in the minority when I calculate out my MPG's compared to the "typical" driver.

MaverickHiker, for the Prius C, they did tighten up the calculations, but early reports are showing a 3 - 4% optimistic report vs 5 - 6%.

Hoddy 4 - I've tracked my mileage over 2 years via Fuelly and a spreadsheet. For 45,000 miles, I'm showing a hand calculated tank average of 59.4 mpg. The computer displayed average would be 62.8 (I had to do some math on the spreadsheet to be able to calculate what the lifetime average computer displayed MPG is). Hand calculated will have some indeterminate errors that will cancel out over many tanks. The displayed value definitely has a determinate error.

 

I also don't know why you would assume something that can be checked.

A long term fuel log will average out the tank-to-tank inaccuracies.

That leaves uncertainties in the odometer, and in fuel pump dispensing accuracy. The former is easily checked against various external references, and at last check mine was running 0.2% low. The later is regulated by state weights and measures regulations in one direction, and in the other by retailer's efforts not to cheat themselves by giving customers free product. Together, these bounds are clearly tighter than the Prius GenIII computer error.