Playing with fuel efficiency

[anitah]

Charlotte's (2008 Barcelona Red Metallic pkg #6, I bought her a month ago) tank has now been filled twice. I was puzzled and disappointed by the first fill-up, which gave me actual mpg of only 42.3, even though the MFD said 49.2. I've since learned that the MFD can be reset at any time, not necessarily coinciding with fill-ups. So, either the previous owners reset it and logged some high-MPG miles just before I bought her, or, more likely, they'd NEVER reset it and 49.2 was the car's lifetime average. If the latter, then the number said nothing about the current tankful, half of which was driven by them and half by me.

However, I reset that gauge the first time I filled up. And upon 2nd fillup, the readout said 50.6, but actual was better - 52.0!

And now I think I've found the secret for my short commute (7 miles), normally an mpg-killer. The recommended pulse-and-glide technique (accelerate to 40mph, foot off gas, light touch on gas pedal till no arrows show, glide down to 30mph and repeat) was giving me better-than-EPA, but not by much, because speed would quickly drop to much slower than ambient traffic and I had to step on the gas too frequently to keep up, plus I kept getting caught at red lights.

Today I kept her in a different mode most of the time. Guess I'll call it "engine mode" - orange arrows from gas engine to wheels, plus yellow to charge battery. This is NOT the dual mode where BOTH engine and electric motor are powering the wheels - that one comes into play on acceleration but uses lots of gas. I brought her up to 45-47mph (I should mention here that my commute is along streets & expressways, NOT freeway!), lightened up on gas pedal till "engine mode" arrows show as described above) and held steady, no acceleration even on inclines, just letting the car's speed decrease slightly to the crest of each little rise, then increase again on the way down.

I was surprised to see that even though the engine was running almost all the time, most of the time MFD was saying 60mpg or more. This mode also helped me make all the green lights, because I was keeping up with traffic better. The final 10 minutes of my commute today included some slower stretches where I could use battery-mode (yellow arrows only). The bars on the consumption screen were all the way to the top for those 10 minutes. By the time I got to work, MFD reported 54.7 mpg average this tank!!

This also explains why I did so well with cruise control on a long freeway trip a few weeks ago -- the car was probably in engine mode most of the time (at freeway speeds you really can't do pulse-and-glide - it's not safe, and anyway over 45mpg the gas engine keeps spinning even if you try to coast). In engine-only mode, the car loses a little on any incline, but more than makes it up on the next downhill. I was seeing it drop to about 45-48 on inclines, but rise anywhere from 65-80 on downhills.

Does this coincide with others' experience? What tricks do you use for short trips?

[qbee42]

How did you determine "actual" fuel mileage? If you are doing it by dividing the miles driven by the gallons put in during refueling, then your data is close to worthless. The 2008 North American Prius has a fuel bladder in the tank. The bladder causes a large amount of uncertainty in tank capacity, so refueling is highly variable. The only way to do accurate manual mileage calculations is to total the miles and gallons over a large number of tanks. I wouldn't trust anything less than ten fillings; more is better.

The 2008 Prius is a Gen II model. Most owners have found the MFD mileage figures on the Gen II fairly accurate - usually optimistic by about 2%. For the short term, use the MFD figures. If you want to do manual calculations, start recording your data for later.

Tom

[anitah]

I am recording my data for later, have already built a spreadsheet for it. I'm aware of the fuel bladder inconsistency, and that this is only preliminary data. I only mentioned the discrepancy as additional data points for those appreciate reports in full detail. But most of my post was about immediate MFD improvement I've gotten by ignoring the advice to pulse-and-glide in favor of another short-trip technique. My experience for the past month is that with every day of my commute, my overall MFD number decreased by 1/10th (due to the First Five Minutes phenomenon). If, as you say, the MFD is pretty accurate, then an increase on it from 50.5 to 54.7 in a single trip rates mentioning, don't you think?

-- Anita

Originally Posted by qbee42

How did you determine "actual" fuel mileage? If you are doing it by dividing the miles driven by the gallons put in during refueling, then your data is close to worthless.... For the short term, use the MFD figures. If you want to do manual calculations, start recording your data for later.