Odd digital fuel gauge performance

quixotequest · 01-18-2008, 12:01 AM

Like others I have seen my 48-52 MPG normal averages drop to the low-mid 40s in the coldest times since the onset of winter, though on a couple recent tanks the MFD hasn't registered that far below my normal averages. But I have noticed some odd behavior: whereas my fuel gauge would generally drop off the first mark at around 110-120 miles, hit mid tank in the 200 mile range, and log over 400 miles before getting too low on the last mark, for the last 3 tanks I have been dropping off the first mark at 30 miles or less --one at only 5 miles-- and hitting the last mark on the fuel gauge by 300 miles give or take. Meanwhile, in contrast, on these recent tanks the MFD has reported my fuel economy at 43-47 MPG.

A few tanks before that I was seeing the first mark drop at about 65-75 miles. When I was in for a oil change they replaced the air filter for me citing that might be the reason. Anyway, it has just continued to get more inaccurate since then.

When refilling, with a single top off, and then computing MPG by hand "actual performance" has been within a couple miles, give or take, of the computer's projected economy. In other words, even though the gauge says I'm getting low in fuel, I really am not. The tank refills after adding between 6 and 6.5 gallons. (Before it has taken about 8.5 gallons to refill when the gauge has shown at a similar mark. The reduced fuel economy isn't great enough to account for this difference.)

This discrepancy with the gauge is driving me nuts. Is there some sort of reset or recalibration I should have performed? The car has less than 16K miles, so this seems very odd to me if it is failing.

Any thoughts or feedback is appreciated. Thanks!

firepa63 · 01-18-2008, 10:44 AM

There is a bladder in the fuel tank. In cold weather, the bladder will not expand as much as in hot weather, therefore you can't put as much fuel in during cold weather. Also, your fuel consumption is higher in cold weather due to additional energy required to warm the ICE and keep it warm.

**efusco** · 01-18-2008, 10:48 AM

Unless you manually push the 'reset' button on the consumption screen the mpg you're seeing is a lifetime average...that Lifetime average will change less and less the more miles you go. If you want to see individual tank averages (for any '06+ Prius) you have to manually reset after each fill up.

quixotequest · 01-18-2008, 12:43 PM

I do press the reset button after each fillup. Even though the tank is filling up with a lot less gas than I expected, if I take that amount and divide it into the miles driven the result is within a couple miles plus or minus what the MFD is reporting. Aside from a few anomalous tanks, over the lifetime of the car this method has been pretty consistent.

What drives me nuts is the gauge is getting more and more useless as a predictor of whether I really need gas or not. The actual results are telling me I have a lot more gas in the car than the gauge says I do. My current tank looks like its gonna start blinking at about 285 miles. Meanwhile the MFD is telling me I'm getting 47 MPG. And since the MFD has ben reasonably trustworthy it irritates me the gauge is getting less and less reliable as a predictor of a need for fill-up.

I hoped it wasn't just the bladder-related issue, but perhaps some sort of recalibration needed to be performed.

Quote:

Originally Posted by efusco

Unless you manually push the 'reset' button on the consumption screen the mpg you're seeing is a lifetime average...that Lifetime average will change less and less the more miles you go. If you want to see individual tank averages (for any '06+ Prius) you have to manually reset after each fill up.

Codyroo · 01-18-2008, 03:33 PM

At the end of the day, the only way you know how much gas you have in the tank is knowing how much you delivered to the tank during the previous fill-up (this is assuming that the gas pump is delivering an accurate amount of liquid).

This may not be the answer you want to hear (we all want our gas gauges to be "accurate" so we can best plan out our trips) but it will address any concerns about running out of fuel during any particular road trip.

01-18-2008, 12:01 AM	#1
quixotequest Member Join Date: Jul 2007 Posts: 37 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Odd digital fuel gauge performance - reset? Like others I have seen my 48-52 MPG normal averages drop to the low-mid 40s in the coldest times since the onset of winter, though on a couple recent tanks the MFD hasn't registered that far below my normal averages. But I have noticed some odd behavior: whereas my fuel gauge would generally drop off the first mark at around 110-120 miles, hit mid tank in the 200 mile range, and log over 400 miles before getting too low on the last mark, for the last 3 tanks I have been dropping off the first mark at 30 miles or less --one at only 5 miles-- and hitting the last mark on the fuel gauge by 300 miles give or take. Meanwhile, in contrast, on these recent tanks the MFD has reported my fuel economy at 43-47 MPG. A few tanks before that I was seeing the first mark drop at about 65-75 miles. When I was in for a oil change they replaced the air filter for me citing that might be the reason. Anyway, it has just continued to get more inaccurate since then. When refilling, with a single top off, and then computing MPG by hand "actual performance" has been within a couple miles, give or take, of the computer's projected economy. In other words, even though the gauge says I'm getting low in fuel, I really am not. The tank refills after adding between 6 and 6.5 gallons. (Before it has taken about 8.5 gallons to refill when the gauge has shown at a similar mark. The reduced fuel economy isn't great enough to account for this difference.) This discrepancy with the gauge is driving me nuts. Is there some sort of reset or recalibration I should have performed? The car has less than 16K miles, so this seems very odd to me if it is failing. Any thoughts or feedback is appreciated. Thanks! Last edited by quixotequest; 01-18-2008 at 12:36 AM.

01-18-2008, 10:44 AM	#2
firepa63 Senior Citizen Join Date: Jul 2005 Location: SE Pennsylvania Posts: 436 My Car: 2009 Prius Package: #5 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Re: Odd digital fuel gauge performance - reset? There is a bladder in the fuel tank. In cold weather, the bladder will not expand as much as in hot weather, therefore you can't put as much fuel in during cold weather. Also, your fuel consumption is higher in cold weather due to additional energy required to warm the ICE and keep it warm.

01-18-2008, 10:48 AM	#3
efusco Troll Slayer Join Date: Nov 2003 Location: Nixa, MO Posts: 14,703 My Car: 2004 Prius Package: #9 Nominated 1 Time in 1 Post TOTM Awards: 0 Friends: 26	Re: Odd digital fuel gauge performance - reset? Unless you manually push the 'reset' button on the consumption screen the mpg you're seeing is a lifetime average...that Lifetime average will change less and less the more miles you go. If you want to see individual tank averages (for any '06+ Prius) you have to manually reset after each fill up.

01-18-2008, 03:33 PM	#5
Codyroo Senior Member Join Date: Oct 2007 Location: Pleasanton Posts: 257 My Car: Other Non-Hybrid Package: N/A Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Re: Odd digital fuel gauge performance - reset? At the end of the day, the only way you know how much gas you have in the tank is knowing how much you delivered to the tank during the previous fill-up (this is assuming that the gas pump is delivering an accurate amount of liquid). This may not be the answer you want to hear (we all want our gas gauges to be "accurate" so we can best plan out our trips) but it will address any concerns about running out of fuel during any particular road trip.