Milege low on 2014 Prius

Yes, safety is your issue -- Sure you've been lucky so far, but the law of averages may catch you some day. Plus, you'll wear out your tires down the center of the tread prematurely. Take it from an old coot - Been there, done that. Happy motoring!

 

Oh boy, I can see the discussion now:
Today's radial tires are much better than the old bias-ply tires and even early radials.
They hold their profile better under pressure.
They don't heat up as much.
Recommended tire pressure is simply to maintain a cushioned ride and have little, if anything, to do with safety as long as max pressure is not exceeded.
Tons of drivers run 5 or more lbs extra with no additional center wear or handling problems, and have been doing so since 2001.
Adding air will result in a slightly stiffer ride and more road "feel". I just started running 40/38 vs 35/33 and I can tell the difference, but I prefer the slightly stiffer ride. Jury is still out on handling until we head for Glacier in late August.

I'm sure I've missed a few things.

 

what were your calculations? 600 / 10 gallons = 60 mpg.try a few tanks shutting off at the first click, using the same pump. i've run 42/40 for ten years, no issues.

 

ya, you can't do it that way. now that you've filled it to the top, you can fill it to the top again and calculate, but it's not recommended, you can damage your carbon canister. fill it to the first click and don't be so impatient. put 3 or 400 miles on her before refueling. 160 miles could be anything, it's not like the car gets 50 mpg for every mile, it's an average.

 

That makes it sound like there are sensors in the fuel tank and the one near the top does not go all the way to the top. That might account for the difference between computed and calculated. Depending on tank, pump speed, etc., you could be adding 1-1.5 gallons or more per fill-up, but actual miles divided by actual gallons still equals actual mpg. Instead of 500 being divided in both cases, you get the computer yielding 500/1o=50, while your calculation yields 500/11.5=43.5, quite a difference. Trouble is the computer missed 1.5 gallons in its computation. Plus, one fill-up doesn't really work too well with hybrids because too many variables exist even when driving the same road day after day. It might be hotter, traffic pattern is different, the A/C runs more, so the ICE runs more, etc. MPG is only good for that particular fill-up and, like so many things in like, has little bearing on what the next fill-up will get you.

 

While that's true, the first fill-up stopped when the pump shut-off, so by topping off, you not only added your extra fuel, but also made up for the difference with the initial fill. If you added 1/5 gallons extra, you mostly likely added another 1.5 gallons to the computation. You got 161/4.6=35, but the arithmetic should be closer to 161/(4.6 -1.5)= 52. In effect, you only added 3.1 gallons because you had to make up for the missing 1.5 gallons.

 

That only applies to the first fill-up because you don't know how much gas was put in the tank to begin with. If you top-off every time after that, subsequent calculations should be relatively accurate because now you know fur sure the tank was filled before.

 

The trouble now is that if you only fill to the click-off, you will be shorting that tank the 1.5 gallons, so the arithmetic will be off again. What you need to do is settle on either always filling up to the click or always topping off. I would opt for the click simply because it's safer. There is still going to be some variance because pumps have different sensitivities, especially on Fast. I run Fast and the switch to Slow when I think it's getting close to full. Since the Prius tank is so small, I just may run Slow all the time. MPG will not be exact no matter how you do it, but tracking it will show a trend and can be an indicator of things going wrong with the fuel system.

 

Sign up on fuelly.com and enter your mileage and gallons there! Then over the next few fills you will begin to see a pattern! Your mileage will even out, no matter how you fill.
Your mileage will be great! Enjoy your new car!
You know more than your dealer does!

 

My 2c...

- How you fill may affect the accuracy of the fuel gauge but has nothing to do with the accuracy of the mpg readout. It uses injector flow rate / timing information to compute fuel burned per distance and doesn't care how much fuel may or may not be in the tank.

- In general the mpg meter is extremely accurate / reproducible. It will likely have a small relatively fixed bias, usually in the neighborhood of 0-5% high, but taking that fixed error into account it will give almost exactly the right answer mile to mile, trip to trip, and tank to tank. If it says you averaged 50mpg over 161 miles, you can be reasonably certain that the real answer was in the range of 47-51 mpg.

- Calculating mpg by hand is inherently very inaccurate. There is no precise way to measure how much fuel is in the tank when you go to fill up unless you run it dry, which I would not recommend. Fuel pumps are reasonably accurate (though vary with temperature), but the auto stop feature on the pumps is very inaccurate. Overfilling to the point where you can see the fuel at a given level each time might help accuracy somewhat, but is likely to damage the vapor recovery system so also not recommended. If you are losing liquid fuel into the vapor system then you've also just introduced another error. Given the uncertainty in the total amount of fuel in the tank both in the "empty" state and the "full" state, there is a lot of variation in the calculated amount of fuel consumed per tank/distance. Since this variation is statistical in nature, the only way to eliminate it is by averaging over a large number of fill ups. With a large enough sample size, it will eventually be more accurate than the absolute accuracy of the mpg meter allowing you to estimate the specific bias of the meter on your vehicle.

- What is the max sidewall rating on your tires? My friends 2013 are 50psi, some may be lower. The tires are rated to be run continually at this psi, at max load rating, at max temperature, and max speed rating. As for accelerated wear, I've run 42/40 through several sets of the OEM tires (max sidewall 40psi) on my '05, and they still all wear out first on the shoulders with tread left in the center. If anything this indicates they are still under-inflated for the load they are carrying.

To sum up, I would air up your tires as much as you are comfortable below max sidewall. Always keep the front 2psi higher than the back. Drive on the freeway and see if the noise is acceptable. Go to an empty parking lot and try some avoidance maneuvers and see if you are happy with the handling. Then just get a few tanks under your belt and see how the average calculated (total miles over total gallons) compares to the average on the display. If its still significantly more than 5% off and not showing any signs of converging toward the displayed average, then I would start to look for issues.

Good luck!

Rob

 

I've asked that question 2-3 times and you're the first to indicate the computer uses a flow-rate system/meter of some sort to measure fuel usage. While I believe you are correct, I've yet to see any documentation that spells this out. Can you point me in the right direction?

Why would there be a 0-5% error? I totally get that computing over multiple fill-ups spreads out the variances to give a fairly good indication of overall mpg, regardless of pump methodology, but that should probably fit the same 0-5% variance if you fill up the same way all the time. If you track all the fuel used over 100,000 miles, the variance should be negligible no matter how you fill or compute. However, we do tend to use individual fill-ups to gauge how well things are doing, how far we can go, etc. For me, Fuelly and the CONS display will work just fine and I'm not going to worry about the discrepancies as long as those stay fairly stable too.

 

It has basically been stated here already, but BE CONSISTENT with your filling technique. I always let mine stop automatically and once it clicks off, you're done. Write down the miles traveled and divide by gallons pumped. The automatic shutoff will vary from pump to pump, so you have to keep a running tally. I just use a google docs spreadsheet and record the 2 key data points mentioned above, as well as the date, price, the "indicated" MPG from the trip computer, and the spreadsheet calculates the mpg for me, along with:

# of days since previous fill
% error of Indicated vs. Calculated
total tanks
lifetime average

Any variation from tank to tank is leveled out naturally in the lifetime average...

I did have one "oops" in my calculations that was caused by the pump simply NOT shutting off at all (fuel was splashing out of the filler neck until I manually clicked it off). Fortunately I was standing there and stopped it after a second or two. Oddly - that particular tank wasn't that far off (MPG-wise) from those immediately before and after...

 

I don't have any documentation to back it up, but there seem to be only a few ways to measure the fuel used (that I can think of):
1. Use the fuel level sensor in the tank (this can be ruled out by the fact that the sensor in the tank doesn't have the resolution needed to measure tiny amounts of fuel level variation)
2. An actual flow rate sensor in the fuel line.
3. Injector pulse length and fuel rail pressure (and probably a few other variables).

I think #3 is the most likely scenario. Injectors are very precise in fuel flow rate, and the computer likely has a map of pressure/temperature/timing that can calculate the amount of flow for each injector pulse. To calculate accumulated (trip) mileage, divide the miles traveled by the sum total of all pulses.

As for the "CONS" value displayed, some speculate it is just the way that Toyota "tunes" (inflates) the value to make you feel a little bit better about your mileage. My spreadsheet shows anywhere from 3.5% high to 9% high...

 

spiff72, I do the same, but I'm not sure why I really care how many days since last refill since I'm retired now.

 

If you fill up the same way, every time, say stopping when the nozzle first clicks off, you will have VERY minor discrepancies in your individual tank calculation of mpg. And your next tank or two will completely cancel out that minor error.

The main thing is that you fill the tank, take it from me. Well meaning friends, or wife, will screw it up, doing partial fills, not noting the data, and so on.

I've tracked about 45000 km's, pretty much since we rolled the car off the lot, new. Right now the mpg number I've gotten (since the outset) is 7.6% lower than the car claims.