Toyota Prius Forum banner

1 - 15 of 15 Posts

·
Registered
Joined
·
381 Posts
Discussion Starter #1
Was just out checking the odo, and noticed that Trip A currently reads 327.0 while the MFD reads 326. I always reset them all at the same time after a refuel while parked before I start it, so perhaps it's just an extremely slight timing issue. I may switch to Trip A in the morning and watch to see if the MFD "catches up" as I roll forward out of the garage. 8)
 

·
Registered
Joined
·
1,434 Posts
There's always that slight delay while the system 'registers' the full tank and then resets the mileage and mpg on the MFD. You could easily have driven a couple tenths of a mile before that happened. I bet your MFD will switch over almost immediately. If it showed it tenths it would probably show XXX.8 or XXX.9 miles.
--evan
 

·
Registered
Joined
·
1,002 Posts
efusco said:
There's always that slight delay while the system 'registers' the full tank and then resets the mileage and mpg on the MFD. You could easily have driven a couple tenths of a mile before that happened. I bet your MFD will switch over almost immediately. If it showed it tenths it would probably show XXX.8 or XXX.9 miles.
--evan
I think he's saying that he manually hits 'Reset' on the MFD when he refuels, at the same time as he resets TripA. I do the same thing. And, yes, they are sometimes off by 0.1.
 

·
Registered
Joined
·
1,563 Posts
I manually reset both as well, then drive off... After 'some distance' the MFD will detect fuel in the tank and reset itself again, hence the discrepancy. Mine's off by .2 right now, and some day I'm going to hit the car in front of me because I'm staring at the MFD waiting for the mileage to flip :)
 

·
Registered
Joined
·
26 Posts
Is there a way on the '04 to disable the automatic reset on the MFD? I'd like to be able to track milage over a longer distance than one tank of fuel.

I've been told that the '03 and earler did not reset automatically.
 

·
Registered
Joined
·
181 Posts
i manually reset both when i get gas and they are never the same. for one thing one is in tenths, the other isnt, but even then they are never the same.
 

·
Registered
Joined
·
73 Posts
odo

Not that this is on topic but when we left for the Yukon with 600km on the odometer I set trip "B" to 0 to keep track of how many kms the trip would be.

Sitting at the ferry terminal shortly before arriving back we checked the trip "B" and saw that it was at 36kms.

I guess the trip "A" and "B" only register to 10,000 since the odometer was showing 10,636.
 

·
Registered
Joined
·
141 Posts
GaryM said:
Is there a way on the '04 to disable the automatic reset on the MFD? I'd like to be able to track milage over a longer distance than one tank of fuel.

I've been told that the '03 and earler did not reset automatically.
You are correct that the NHW11 model ('01-'03) did not automatically reset its mileage display on the MFD. However, it is of limited usefulness, because it is not completely accurate. I have let mine go for a long distance (10 tanks) and compared it to the number I computed from odometer and pump totals, and I found my MFD reads some 6% higher than actual.

The reality is that the method used by the MFD to measure gas consumed is inherently inaccurate. I've read posts by more knowledgeable Prius drivers who believe it is simply summing the time the fuel injecters are open and assuming a constant amount of fuel is squirted through them when they're open. Toyota apparently recognized this inaccuracy and added the auto-reset feature so people wouldn't try to use the MFD for serious long-term measurements.

If you want to really know your lifetime mileage, you will have to write down the number of gallons of every fill-up, and do the math yourself. :(

Douglas (2002 Silver, Wisconsin)
 

·
Registered
Joined
·
181 Posts
i think your hypothesis on how the mfd is calculating fuel is wrong. if that were the case, then how would the car determine fuel economy while not moving? or going up hills? or down hills?

i personally think that the mfd is very accurate. this past summer a seattle tv station went out and had several local gas station's pumps calibrated and found that 11% were inaccurate. of those that were inaccurate, the ones that shortchanged the customer outnumbered the generous pumps by a 4 to 1 margin. The state of Washington estimates that 20,000 gallons of fuel a day is paid for but not received.

this stems from a state calibration team that numbers 9 people. they strive to inspect each pump every three years, but i have yet to see a pump that has been inspected that recently.

the newest inspection sticker ive seen sept 2000 the oldest, jun 1999.
 

·
Registered
Joined
·
1,215 Posts
Here we go again, with the whole gas pump accuracy thing.

This teevee station...did they ever say just "what" constituted innacuracy in their little test?

It's obvious that this isn't an important issue to people. It shows, because there is no enforcement. No penalties. No testing. No accountability.

Too bad. It probably won't change until gas reaches $5.00 per gallon. Or maybe not until $10.00 per gallon. But we need accuracy now!

<sigh>
 

·
Registered
Joined
·
181 Posts
yes they did state what constituted inaccuracy.

that was not getting what they paid for.

also the state admitted it was a problem but because of budget cuts and lack of personnel they simply could not maintain the 3 year inspection mandates.
 

·
Registered
Joined
·
141 Posts
DaveinOlyWA said:
i think your hypothesis on how the mfd is calculating fuel is wrong. if that were the case, then how would the car determine fuel economy while not moving? or going up hills? or down hills?
I didn't say it was my hypothesis, but I'll try to support it, because it makes sense to me. If we assume the car is calculating fuel used by summing the time the injectors are open (given some constant fuel flow rate through the injector ports) and calculating miles traveled by wheel revolutions, none of your objections seem to matter. At any time or under any conditions, accumulated mileage = fuel used since reset / miles traveled since reset.

The injectors are still squirting fuel into the cylinders at a constant flow rate when you're not moving, and the wheels cover the same distance for every revolution whether you're going uphill or down.

If you're looking at instantaneous mileage, there are two exceptions:
1. When the speed approaches zero, the change in wheel revolutions between two samples becomes extremely inaccurate. The Prius solves this by removing the instantaneous mileage display below about 7 mph.
2. When the fuel usage approaches zero (i.e. stealth mode), the mileage approaches infinity. The Prius solves this by pegging the instantaneous display to 99.9 mpg.

However, neither of these exceptions are inherent to the method of measuring fuel flow, and although the first exception is dependent on the method of measuring speed, I believe any other method (e.g. measuring the Doppler shift of radar returns from nearby stationary objects) would have similar inaccuracies near zero.

Am I missing the point of your objection?

DaveinOlyWA said:
yes they did state what constituted inaccuracy.
I think BIF's point is that your quote doesn't indicate how inaccurate were the pumps. For example, if 11% of the pumps had errors of 1% or 2%, this is much less than the 6% discrepancy I've measured between the pump and the MFD, so it wouldn't be a significant factor, and we could conclude that the MFD's error must account for most of the discrepancy.

Douglas (2002 Silver, Wisconsin)
 

·
Registered
Joined
·
7,166 Posts
I found my calculated MPG to MFD MPG about 2.6%. Its probably closer to 2.7%. Originally thought it was 3.6%, and filled the tank based on what the adjusted MPG was, and the tank ended up almost topping off.
When I adjusted to 2.6%, it remained pretty much at the same point for the next 2 fillups.
I reduced the subsequent fillup by .25 gallons so I wouldn't be so full. Now I can't see where I am in the tank, but still past that first click, by about 1 gallon.
 

·
Registered
Joined
·
181 Posts
sorry cant explain any 6% discrepancy.

i dont remember what the rate of inaccuracy was.

when i first got my car and noticed regular fluctuations between pump and mfd calculations, i started tracking the pump i was using and now im to the point where i have nearly eliminated the difference between the two readings

my lifetime pump average is 53.61mpg
my lifetime MFD average is 54.19 mpg.

i used to average plus or minus 4% in difference between fillups. granted most of that is different levels of fillups. but my overall trend of having the tank average be 2 mpg below the MFD has been eliminated to the point that now, i average .4- .6 mpg above the MFD.

at the rate im going, i feel that by next summer both lifetime averages will be the same.

also, that stat should be 20,000 gallons a MONTH pumped but not paid for.
 

·
Registered
Joined
·
381 Posts
Discussion Starter #15
Lifetime mpg across 10 tanks:
  • MFD = 47.37[/*:m:2uhtjiwp]
  • calc = 45.67[/*:m:2uhtjiwp]
It might be an interesting exercise to take an "approved container", mark it off at 1 gallon, take it to a pump and see where 1.000 gallons on the display ends up in the container in relation to the mark.
 
1 - 15 of 15 Posts
Top