Fuel Efficiency and the Yaris

[kentborg]

Breaking 50 MPG

Scangauge II said I was at 52.6 MPG when I left the freeway. By the time I got home it was down to 52.4. After a morning trip through red stop lights in the city I bought gas and the actual MPG by then was just under 50--which was consistent with the Scangauge number. So I am convinced I was well over 50 on the highway.

The trip was from near Burlington, VT (Taft's Corner) to Boston (Somerville), with one stop for tolls and another immediately after to close the fuel fill that somehow was not latched. Dry roads (wet kills mileage), 2007, stock USA hardware, 1.5L engine, 4-door, 5-speed with 11K miles on it. No passenger (to add weight nor for me to annoy). Trunk was not empty. GPS with vertical speed displayed. External GPS and Sirius antennas adding a smidge of drag. Headlights on, AC off.

How did I do it? I am not sure. I am still learning. Every few miles I seem to adjust my strategy.

I have my gauge display showing:

MPG TPS
AVG MPH


Here is my current guess about what I was doing:

- Plan ahead so as to never hit the brake. (If I must brake for safety, yes, I brake.)

- Once I am on the highway, I never use anything but 5th gear unless I need to accelerate out of trouble, if something slows the traffic too much, etc. Low gears offer power and acceleration, but are not good for mileage.

- Try to go at least 55 MPH. First, going too slowly is scary, other cars don't expect it. Second, idling the engine buys me no distance, so even when coasting, I want to go fast enough to amortize that overhead.

- Don't invest unnecessary gasoline in going too fast, because the wind resistance will go up. If I know there is a long downhill that will get me going fast, I don't waste gas on cresting the top of the hill any faster than I need to to be safe in traffic.

- When I need a little braking (maybe on a long downhill) I take my foot off the clutch and watch MPG display 9999. Instead of wasting my speed on heating up the brake pads, I can use a little of it to run the engine at idle with no gasoline consumption. However, that 9999-display is not my friend unless I need a little braking. Otherwise I coast with the clutch down.

- When on the flat I pulse-and-glide: do a pulse throttle between 25 and 30 (the TPS number). Towards the end of my drive yesterday I was thinking a throttle of 25 is better, but I didn't have much flat left at that point. For the pulse I accelerate to about 63 MPH (because of display lag, I usually end up hitting 64), then I put my foot on the clutch for the glide and coast down to 57 MPH before hitting the gas for the next pulse (again, because of lags my speed will usually fall to about 55 before I speed up).

- Drive the hills. Watch the GPS vertical speed display to know if I am currently going up or down, look ahead to see whether the road bends up or down. Be strategic and try to use gas to go uphill and gravity to go downhill.

- Drive the traffic. Not just for safety, but for good mileage. I always need to know who is in front of me and who is in front of them and whether they are slowing or speeding up. I always need to know who is behind me, approaching or following or getting annoyed by me and looking to pass, and who is behind them. I need to know who is in lanes next to me and what they are doing. Anticipation is key and to anticipate I need to be very observant of the current situation. To get good mileage I need to be more aware of traffic not less. Watching my gauges is only part of the task.

- When going uphill the "glide" part of pulse-and-glide doesn't work so well, so if the hill wasn't too steep, I did "pulse-and-go-easy". The pulse would be a throttle of 25 and the "go-easy" would be maybe a throttle 20 or 21--but at that point I am really watching the current MPG and speed, by pulling off on the gas I can get a bigger MPG number. I won't see the 200+ MPG of coasting at high speed, but I can see my go-easy MPG go up to maybe 50-something. The pulse MPG would maybe in the low 30s (depending on the hill). That makes for an uphill average that isn't so bad. (And any downhill on the other side will still be great.)

- If the uphill is too steep, I do one long pulse, keep my speed high enough to not be dangerous, watch the current MPG, and frantically wonder whether there is something I can do differently to use less gas.

- Pretend I have a passenger whom I don't want to annoy, even if I am alone--smooth use of the clutch and gas saves clutch and gas.


Proviso: Pay attention to driving! When I wanted to find another radio station, my mileage fell--this kind of driving requires concentration; it is a good way to fight boredom on a long boring drive, it is not something to do instead of driving safely. Make sure you are comfortable with the operation of your car. If traffic or weather distracts, pay attention to the traffic and weather. Getting in an accident is terrible for your mileage.


My guess is that getting over 50 MPG on the highway is quite doable in good conditions, but it seems touchy enough that I think I am near the edge of what is practical.


A couple questions are bouncing around in my mind:

- What is the most efficient throttle setting for this engine?

Is throttle ("TPS") the right parameter to watch?, or should I be looking at load ("LOD")?? Is there a single answer to this "most efficient" question, or are there complications I am missing? I am thinking this is how hard to pulse, is that correct, what are the exceptions? Is there a practical way to answer this question by being "all scientific" and measuring actual engine efficiency?

- It seems I can spend gas to (1) move on the flat, (2) invest in additional speed, i.e., accelerate (which can be cashed in by coasting), and (3) invest in altitude, i.e., climb a hill (which can be cashed in by going downhill). Does this cover it? Am I missing something here?

- Given that I don't have a Prius, how can I get vaguely good mileage in the city?

As bad a braking is, I suspect that starting from a dead stop is the real killer. What is the best way to start from 0 MPH?

I'll try to remember to follow up with new numbers next I get in some highway driving.


Thanks,


-kb, the Kent who is wondering how much this kind of driving can become automatic and so let him again tune the radio.


P.S. I have only had this car for a couple thousand miles, I still need to check the tires. I plan to over inflate them a little, maybe up to 37 PSI vs. the standard 32 (is it?), it will be interesting to see how much that helps. ... Checking them now I notice they are currently not even inflated to factory spec, so I might have some significant room for improvement here.