mileage versus tire pressure

By the same token, your findings of only a 3% improvement are equally questionable. A commute with a large number of traffic lights will necessarily have extremely high varience. Do some statistical analysis (margin of error) on your three data points and I think you'll see that you can draw no conclusions whatsoever without another few hundred commutes added to the data.

That's not to say that I think the improvement is above 3%. I'm just saying that if you ask 5 people on the street whether they're voting for Bush or Kerry, you're not going to get an "average" answer that means a lot.

(I presume you were referring to my post with the "10 mpg improvement", since nobody else has posted something similar recently. But I made no such claim.. I claimed that hit a solid new "record MPG" for my commute, which was about 5mpg higher than my previous average.)

 

Although there are 18 traffic lights, and I seem to hit everyone of them on my normal commute, I only hit 2-3 each way on this one. The reason is that most of our lights response to traffic and if there isn't much this early so they're usually green along the main route. There's one set of lights I always hit. Coming off the freeway section and then after I've made the left hand turn under the freeway about 100 feet further. This 1/4 mile section is always taken either coasting or ev mode only. And I caught those lights on all 6 commutes. I probably should record the number of times I "lost momentum" waiting at a light on each run.

Yet you are correct that the more lights I hit the more inconsistant my data might be. After this first set of runs I now know that 6:45 isn't early enough to effectively normalize the effect of traffic. It's got to be 6AM. The fact that the second day was much warmer probably affected things a lot too. I've traveled this route hundreds of times and during my normal commute to work I see a range of mpg values of 55-70.5! I didn't see that wide of a range on this test because I made an effort to minimize as much variablily as I could. If we were doing a design of experiments I would have to control all of these variables exactly of course or do enough experiments varying the ones I thought to be significant. But you're right, the road is not a lab, it's the real world. So all I'm attempting to do is to take some data in the most controlled fashion I can. I the absense of any data we're left with speculation and heresay right?

As flawed as my data might be, don't you think it's at least as good if not better as my saying I raised the pressure in my tires today and hit 70.5 on my commute to work, my very best result? What's that worth in the effort to make a decision about air pressure values? I don't believe as you suggest that my testing is just an "opinion" but I do agree that unless I can do a better job, which I will try to do, of controlling the other variables affecting the results, my experiment is not ready for prime time yet! It's just a little more controlled observation. So make some reasonable suggestions if you would like. I'm going to continue to refine my technique and collect data however.

One thing that's going to screw this data up big time I've found is cross-checking the computer with a fill-up. The bladder VS particular pump VS temp error is going to really affect the result. I had to fill-up at a different pump this morning (big mistake but the station was backed up) and I "think" the shut-off was late. I can't prove it of course but unless I believe that the computer predicted 56 and I actually got only 43 I'm stuck with just recording the computer results, as incorrect as we know it can be sometimes.

I'd like the test to be longer and use more fuel so small fill-up differences would not affect things. But I also need to minimize the temperature swings, effect of wind which usually comes up later in the day, and of course traffic. That means doing the test during a period of minimal swings in these "other variables". So I'm going to continue with at least another test next weekend starting at 5-7 am (instead of 7-9 am) on sat and sun and see how it goes. Any suggestions will be appreciated. I really don't think this is total waste of time. The more meaningful test might be for someone to set the pressure low and drive for a full year starting with new tires. Then install new tires and do it again at higher pressures. Then you've got the condition of the vehicle, road improvements to consider, etc.

I do believe that increased pressure will improve the gas mileage and that 3% seems reasonable. I also realized that the FEA company I consult for does a lot of work with tire companies and Toyota as well. So I'm going to try to learn as much as I can about this issue from experts on the subject if I can. That may take awhile and you know how the engineers at these companies are restricted from sharing data. But I'll give it a try.

 

No, I don't think so.

One is an anecdote.

The other is an attempt to look like there's been some sort of valid experimental data collected, even though in fact that data is statistically meaningless.

According to my poll, 80% of the population favors Kerry, so I think he's going to win in a landslide.

For what it's worth, the only way to get a reasonably consistent test without an EPA treadmill would be to do a particular run on a particular stretch of highway at a known speed, at reasonably similar temperatures, with the A/C totally off. For example, cruise control at 55 along a 20-mile stretch of highway at a low-traffic time. That wouldn't show up braking effects, etc., but you could have a fixed schedule to vary the speed as long as there weren't other cars around to get upset by the strange car braking to 30 mph in a 55 zone, then accelerating to 65.

Anything else will be utterly swamped by random variation without a huge number of trials (at least hundreds). The best answer you'll get is something like this:

"I can state with 95% certainty that the MPG rating along this route at 42/40 tire pressure is between 47 and 74, and with 35/33 is between 45 and 71."

 

Jason, I re-read your first post. Although you were not trying to peform an experiment, you used the term honest-to-goodness 61.5 mpg for what.....for one 18 mile trip? So now you're attempting to discourage my honest effort to try to replicate your results in a more controlled and repeatable manner? It's only your opinion that it's not well controlled enough to yield good results. You don't really know what efforts I've employed to control the variables. You're just saying it can't work without for example asking me if I think I can improve on the variability.

Why not offer to do your own experiment and share that data with us so we have more than anecdotes? There are no quarter mile flat sections of highway around here, let alone 20 miles. So if you have the terrain for a test like you suggested then why not do it?

This response was going to go into how I'm attempting to control things that were not as well controlled in my first runs. But I think I'm going to just let that go for now to defuse this. So I'm calling your bluff in a friendly way...if you can contribute to the understanding of this, please do so. Show me your data!

I do have a clear understanding of all the variables that come into play with this and my first attempt was an effort to control the significant ones I thought I could, look at the results, and then do a better job the 2nd time around which I will do. I did point out that the results were preliminary right?.

I did get 70.5 mpg on my normal 11 mile commute with 42/40, but I also had as low as 55 mpg. Of course the variables all came into play on those 2 weekday commutes. I saw a much tighter grouping of the data on my early morning weekend runs which is an encouraging sign for me inspite of your less than positive comments. And it did shed some light, for me anyway, on why the commute home is so much lower than my commute to work even if you throw out the traffic. It's mostly the effect of uphill drive home. Getting 42-44 mpg coming home during the week is not as bad as I thought it was. I saw that there's the potential to approach 80 mpg on the commute to work and I didn't realize that. I thought that 70 was about the very best possible but I did hit 78.8 mpg in my test with 42/40 tire pressure. The 5 min warm up might be the limiting factor here. My best of 50 mpg during the first 5 min might not be good enough to break 70 mpg!

Anyway, I'm going to continue to refine this experiment and you can either continue to criticize it or offer some data of your own.

 

I already gave (above) an exact example of what I thought would constitute a useful experiment. I have too much to do in my life to run the experiment myself, and it really doesn't matter that much to me -- I'm happy with my tire pressure the way it is, whether or not it really increases my mileage.

I'm a little curious why you're interesting in MPG experiments anyway... Your original point wasn't about the MPG, but was rather that overinflation might screw up the wear pattern on the tires. If you found conclusively that your average MPG went from 55 to 60 with the inflation change, would you leave it that way permanently?

 

Last night I increased my tire pressures to 38/36.

After 65 miles of driving today this is what I experienced:

. When driving below 60 miles per hour the mileage showed better than I've been seeing over this past few weeks.

. When driving 75 to 80 miles per hour on the freeway, the mileage showed worse than I've been seeing.

. Overall round trip average mileage turned out to be about 1 mpg less than I normally see.

. There seems to be less feel to the steering than there was when I was running 35/33 psi.

I'll keep you posted.
Ken

 

I want to thank FredWB for stating his findings, and maintaining real professional poise.

I think it's unfortunate that some folks don't seem to appreciate objective testing. And even when you say "This what I found, this is what I think, but more testing is necessary" there's somebody to say "You're jumping to conclusions, you can't say that". What conclusions?

I know it's engineers that design modern marvels like the eifel tower, my cell phone, the Golden Gate Bridge, and the prius. I know how much study, time, discipline are behind it. It's the Toyota engineer's recommendation that I follow for tire pressure unless someone like FredWB proves otherwise.

I'm getting 52mpg hiway w/ tires at 35/33, at about 65mph, in an '04 Prius.

 

I’ll have to agree with 04preius that’s it’s good to have a methodical person like FredWB doing these studies for us, even though he is extending his tire pressures well beyond what any research source on safe tire pressures would recommend. After all, virtually every resource available recommends running tire pressures at car manufacturer recommended pressures. There are, though, those few expert sources, AAA being one of them, who are willing to state that you’re probably alright going as much as 4 psi above car manufacturer recommended pressures, but no more, regardless of maximum allowable pressure stamped on the tire.

Personally, I’d like to see FredWB run his experiments at pressures within that +4 psi window. The results could then be transferred to those of us who choose to not deviate excessively beyond manufacturer recommendations.

 

Gee, I wonder who you could possibly be referring to. It's obscured so subtly I can't tell.

I appreciate objective testing. But I think it's unfortunate that more people don't understand the scientific method and basic statistical analysis, myself.

Actually, if you look back at the original thread ("Tire Pressure!"), I think you'll find that *I* said "This is what I found, this is what I think, but more testing is necessary", and FredWB replied that I was wrong, jumping to conclusions, damaging my tires, etc. So it goes both ways, eh?