Measurement of MPG vs Tire Pressure in Gen-3

View attachment MPG vs PSI tests.xls Well, it wasn't raining today, so I decided to try a controlled measurement of MPG vs PSI. Each trip covered the same 13.5 mi section of the Nonconnah Parkway on cruise control at 62 mph in ECO mode. Time chosen was between 2 and 4 pm when the temp should be max and fairly stable. Trip B on the MID was zeroed at the beginning of each trip after quickly bringing the car to 62 mph. Car had 1,800 mi @ start of runs. A/C at 78 deg, fan at 3 bars, inside air recirculating, fuel started at 7 bars, ended at 6.

Planned to also include mileages from the 3 return trips, but forgot to reset MID on on return trip and had to junk return data (there is an exit, traffic light and steep return ramp at the East end of the Nonconnah where I turned around, so can't mix two-leg data with continuous one-leg data). Could measure cold pressures for trips 1 and 3, but not 2, which was in the middle of the runs so had to interpolate it.

One other thing: the road was increasing in elevation, so the return trips had approx 4 MPG better performance. So the average is about 2 MPG *HIGHER* than shown in the plot (assuming the plot makes it through )

If it doesn't come through, I will try again after I figure out how to imbed an EXCEL file in the post - sorry.

- g

 

Any chance you can put your results out again?

BTW, I forgot to say what tires these tests were made on (dumb de dumb dumb)... the OEM version Yokohama AVID S33D P195/65R15.

I'm wondering why these tires show less improvement with increased PSI than earlier reports, where people say they got 5 MPG improvement or more. Couild it be that these low-rolling resistance tires already have taken out so much of the problem with their stiff sidewalls that there is little left to give? Well, uh, I guess another possibility is that I screwed up the data , (although I was pretty careful)... Hope some others will try this.

Cheers!

- g

 

Good idea. I wouldn't feel comfortable on Nonconnah Parkway at that speed, but there are some stretches of road in Arkansas I could try that have both lower speed limits and light traffic. I may try another exact repeat of the Sunday runs, which will also add the return leg data (assuming I remember to reset the MID !)

No doubt about that effect, you can watch the wheel rise slowly as you pressurize. I will try to measure that and put in a correction factor for changes in tire diameter.

Thanks for the suggestions and for your great work in the MPG vs speed studies!

 

I thought there were a few weaknesses in their protocols also. Not trying to be picky but they drove about 1,000 feet uphill from LA to Phoenix at high pressure and then the same 1,000 feet downhill back to LA at low pressure! This is a 2000 foot elevation advantage to the low pressure data. Is this a good way to measure a few % change in MPG?? Same question about wind.

It would have been so easy to fix this problem by driving halfway out and returning to LA then dropping the pressure and doing the second run out and in. Now elevation and wind largely cancel out (so does fun in Phoenix ). What if the weather conditions change from the first day to the second day? You can correct for this by running OUT-Low P and IN-High P followed by Out-High P and IN-Low P. They didn't try any of this.

I also thought I should post the whole data set I took last Sunday. The 55 PSI return data are missing (becasue I forgot to re-zero MID), but the other 5 data sets are available (as well as the averages of the lower and intermediate pressures). The advatages of showing everything are:

- full disclosure
- two pressures (27 and 40 PSI) now have both out and in data shown, so elevation and wind cancel out- The new data confirm the earlier data: there's a stronger PSI effect on MPG and both of the in runs show higher MPG.

Will try to do better next time.

Thanks for all your helpful comments!

 

Now there's a mouthful worth remembering! How many times have you read where driver A says "I did X and I saw Y..." and then driver B responds with "I did X and I didn't see Y...?"

I'd bet that almost all of these cases come about because one or both people didn't do the above. The rest probably come from things like unknown differences in the cars, environment, etc. where it's more or less impossible to keep those things the same. I often make this mistake.

Just my 2c worth................:yawn:

 

Want some MPGs for free? maybe a few?

When you boost your tire tire pressure, two things happen that increase MPG.

- rolling resistance decreases
- the tire gets bigger

People have only been talking about the first thing. But because of Bob Wilson's reminding me of the second one, I spent the morning on the concrete floor of my garage measuring how the distance from the wheel axes to the floor changes as you change pressure.

Not knowing anything about this, I had to try something real dumb, I guess. Anyway, the idea seems to have worked (subject to someone's checking me). I put masking tape on the LF and LR wheel hubs at the centers. Then I scribed a horizontal line on each wheel (using a level) at (as I recall) 11.5" above the floor. Then I moved the car until the wheels rotated 180 deg and made another horizontal line on each wheel, again at 11.5" above the floor. I then drew the line halfway between those lines - this line should pass thorough the axis. I then measured the height of this line as I changed the pressure (using a $10.00 12V compressor which burned up just as I finished the job).

The results are in the attachment. I'm assuming the ODO and MPG are read from the rear wheel where the variation with pressure is less. Bottom line - the wheel radius is 3.1% larger at 55 PSI than it is at 25 PSI (the two extremes I have been using)

So, if someone had the pressure at 25 PSI and increased to 55 PSI and, say, the MPG went from 58 MPG to 62 MPG, the gauge is holding back on us. It should be increased another 3.1% to 64 MPG, because the wheel is bigger but the MID does not know that.

Most of us aren't at 25 psi now and we aren't going to 55 psi, althought there ARE people with concrete tires out there who could get +2 MPG or more from adding in this correction. But most of us could still easily get a 1 MPG boost or more for free. What the heck!

I realized that the front and rear radius changes tell you the front and rear loading, so I had made a poor man's auto scale and the results say that the 2010 Prius has a front to rear weight distribution of

59.2/40.8

actual numbers, I believe are

60.2/39.8

Maybe next time I can do better than a friggin ruler and a dying air compressor...

 

Thanks,

I think the Hobbit's idea sounds neat. May we presume, since Toyota recommends 33 PSI rear tire pressure on the OEM 15" AVIDs, that their gauges are calibrated to that pressure and diameter? Or maybe they also put in a 50% tread wear factor (which wouldn't be much)? Hmmmmm......

- g