So, here is the experiment: Putting more air in your tires than you are told by the manufacturer. This is pretty common, and unless you go crazy with it, not dangerous at all. In any case, the idea is this:

<strong>First: </strong>You figure out your average gas mileage and what PSI (pounds per square inch) your tires were running at.
<strong>Second: </strong>Pick another PSI somewhere higher that you would like to air up your tires to, and drive three tanks of gas and record you MPG each time by the standard fill-up and reset your trip odometer method.
<strong>Third: </strong>Figure out if there is any difference is fuel economy or handling and other such things. Then report it in this thread and I will crunch the numbers. After a sufficient amount of people report I should be able to come up with so solid numbers on the effects.

<strong>Note: </strong>Do not fill up your tires more than 10 PSI above the max. load printed on the tire. PSI should always be measured cold, that is also how the manufacturers so it (when the tire heats up it gets higher). Always fill up at the same pump and station, due to things like gas detergents and pump errors. Also check out this link given by chasgood about how to check tire pressure for more info on that:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1&currentpage=29

Here is the information you will need to report after your three tanks of driving:

PSI (before/after):
MPG (before/after):
Make/Model/Year:
Manufacturer and Type of Tire:
Size of Tire:
Tire Ratings:

Here is a sample (not my test):

PSI (before/after): 32/43
MPG (before/after): 42.9/45.6
Make/Model/Year: Honda/CRX/1991
Manufacturer and Type of Tire: Sumitomo All-Season
Size of Tire: 175/70/R13
Tire Ratings: A B 800

So there ya go, piece of cake. Just post up when you start so we'll know who is running it and then again when you're done and have results. Best way to submit the results is just to cut and paste the first little form thing and plug stuff in. Thanks.

PS: I start next fill up.

If at all possible your base MPG should be based on 3 tanks of gas as well. It should go like this: 3 tanks of gas to get base MPG, then inflate tires, and then another 3 tanks of gas to get new MPG. This way we can measure even the smallest difference, and we don't have to rely on, "I get around 34ish miles per gallon."

exact is always better.

We need to set up guide lines for checking air pressure. I suggest having a look at this link from a tire store as to how to check the pressure.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1&currentpage=29

Use the same tire pressure gauge for all testing. These things can varry + or - 1 psi.

The pressure sould be checked twice a week for these tests. Some tires loose air faster than others. Plus if there is a hot or cold weather spell the pressure will change.

Use the same gas pump and nozzle for all gas fills. Meters on pumps can varry up to a tenth of a gallon on a 5 gallon fill and still be legal.

If your car manual says the pressure front and back should be the same maybe try a third test with more air just in the front tires just to compare. Most FWD cars have 60% of the weight on the front wheels.

Definately good suggestions, I think some of those things like same pumps are in Matt's general testing thing, but I dunno about the gauges, that is prolly a specific to this thing, so I will add it to the instructions up top, thanks.

I totally forgot about gauges. Remember, the best way to determine if something acually affects mileage across cars is to keep as much the same as possible. It would be nice if we could all use the same meters, gas stations as each other. That would really solve a lot of error. Unfortunately we all live in different states.

I think a good digital pressure gauge would be wonderful for this experiment. if my car is up and running soon I'll be able to participate in this study.

Okay, everyone who is interested now is the time to pipe up or ask questions in this thread of by messaging me on this site. I am about to start my official testing of tire inflation ripe and early monday morning! I will take befor and after pictures of my tires as well as stating before and after statistics to try and give a better idea of what is going on with the tire.

We will be testing this principle to see the truth there. I'm hoping I can squeeze a few mpgs out of it at least, but that's what the test is for.

I would love to participate in this study, but right now my car has mismatch tires on it. the rabbit has matching tires, but it needs a transmission and suspension, etc. so, do you think i should do it now, or wait till i have the rabbit together?

Mismatched tires shouldn't make that much of a difference really I dun think. Just make sure to give the different ratings on the tires when you're done and then do it again when the rabbit is together. If the data with the mismatched tires doesn't fit a curve properly then it can always be thrown out, though I don't think it will create such a variation, however, in the interest of testing purity just report both types of tires and blah blah blah and we shall see if it works out crappily, though I doubt it, so go for it I think.

Checking my tires tonight. Gasing up in the morning on the way to work. Set the pressure at 44/40 a few days ago. Will do the stock pressure after the over inflated test. Waiting on gas prices to go down to fill up and start the test. Good thing. Gas has gone down 20 cents in the last 4 days here.

So the testing begins.

That's good to hear. I think I waited out at least 50 cents around my parts. Good to know we'll have at least three testers now, hope we can grab some more.

I'll be able to do this very soon, as soon as the wind stops and I can paint my car. this will be my first test for the group. After that I'll attempt to get the PCV catch can going, and then perhaps a few other tests like the K&N air filter and other small things.

Be sure to test tire pressure often, as air creeps out of the tires at a rather quick rate.

What happened to the 'tone? I thought it was gonna be your first testo?

You're right... I totally forgot. I havn't had my coffee yet today.

First pictures of my tires for reference.

Before (35 psi):
<img src=http://www.filefarmer.com/cruisur/DSC02520.jpg>
After (45 psi):
<img src=http://www.filefarmer.com/cruisur/DSC02521.jpg>

It doesn't and didn't look like much of a difference. And the handling didn't seem to suffer. They are old busted *** tires anyway, I'm gonna order new ones soon and get them on in a few weeks. I went from 35 to 45 psi. The control mpg I am using is 41.5, cuz when I filled up this morning I got a miracle 45 mpg, haha, I am happy about that unless something got screwed up, but I think it is possible that I actually got that. Anyway, here starts the test. Three tanks. Everyone else who is working on this chime in!

Still on my first tank. Noticed the car coasts better with the higher pressure.
The ride is a bit harsh. Feel small bumps and holes more. I can live with it.

I've done my first highway test with 40psi all around. Here are the results:

<table border="5" cellpadding="5">
<tr>
<th colspan="2">
<b>9/21/05 - Tire Pressure Trial #1</b>
</th>
</tr>
<tr>
<td>Fuel economy:</td>
<td>35.0 mpg</td>
</tr>
<tr>
<td>Previous fuel economy:</td>
<td>34.7 mpg</td>
</tr>
<tr>
<td>Net gain:</td>
<td>0.3 mpg</td>
</tr>
<tr>
<td>Trip:</td>
<td>79.45 miles</td>
</tr>
<tr>
<td>Fuel used:</td>
<td>2.272 gallons</td>
</tr>
<tr>
<td>Fuel grade:</td>
<td>Shell V-Power 93 octane</td>
</tr>
<tr>
<td>Driving type:</td>
<td>Highway</td>
</tr>
<tr>
<td>Target speed:</td>
<td>65 mph</td>
</tr>
</table>

Comments: I warmed up the tires for about 15 to 20 minutes before filling up with air. Tire pressures before and after listed below. The test went fine but when I went back to fill up, I had the early cut-off problem. I finally got the pump to stay on, but I can't be entirely certain it cut-off at the right time. It is in the ballpark of my previous test so that gives me some comfort in this method of measurement. I will conduct a second verification trial to try and confirm these results. So far it doesn't look like tire pressure plays a big role in my car. Even though the pressures were low, the tires were not saggy looking and didn't lift up much when filled up. Small bumps in the road respond sharper than before, but I also have an aftermarket suspension that makes the ride a little tighter anyway.

Another variable that should be held constant for all tests is fuel used during the test. Filling up on Shell one trial and Diamond Shamrock the next time could affect the results. If you absolutely have to switch, you should perform two more trials on the new gas to recalibrate your base mpg.

After one more trial of tire pressure I'm moving onto acetone. So far I've made two changes (PCV canister and tire pressure) and haven't seen any results. I'm eager to get some more mpg. I just hope my hydrogen generator doesn't flop, because right now that's what all my mpg dreams are built on.

<table border="5" cellpadding="5">
<tr>
<th>Tires</th>
<th>Before filling up</th>
<th>After filling up</th>
<th>After test</th>
</tr>
<tr>
<td>Front left:</td>
<td>27 psi</td>
<td>40 psi</td>
<td>40 psi</td>
</tr>
<tr>
<td>Front right:</td>
<td>26 psi</td>
<td>40 psi</td>
<td>40 psi</td>
</tr>
<tr>
<td>Rear left:</td>
<td>28 psi</td>
<td>40 psi</td>
<td>40 psi</td>
</tr>
<tr>
<td>Rear right:</td>
<td>28 psi</td>
<td>40 psi</td>
<td>40 psi</td>
</tr>
</table>

There is at least +/- 1psi of error in these measurements. I used the standard pencil gauge for checking pressure. As the results show, no rise/drop in pressure was noticed after test. I am led to believe the tires were sufficiently warmed up at the time I filled them up.

Addendum:
FYI for those participating in this experiment. I checked my tire pressure the other day. At night with the tires cool I measured 37 all around. The following day after driving around for a while with the outside temp 102, tire pressure was 41 all around. Temperature plays a big role in pressure, so drive around for a little and double check your pressure, then begin the trial.

__________________
Scouring the country for an excellent condition Civic VX

Here are the results of Tire Pressure Trial #2. Not exactly what I was expecting. There was a small drop in mpg measured. Not sure if there was error in the top-off method or other intangibles.

Tire pressure was checked before and after the trial. I had to add about 2 psi to each tire to get up to 40 psi. There was no change in pressure after the trial.

<table border="5" cellpadding="5">
<tr>
<th colspan="2">
<b>9/30/05 - Tire Pressure Trial #2</b>
</th>
</tr>
<tr>
<td>Fuel economy:</td>
<td>33.8 mpg</td>
</tr>
<tr>
<td>Previous fuel economy:</td>
<td>34.7 mpg</td>
</tr>
<tr>
<td>Percent difference:</td>
<td>-2.59%</td>
</tr>
<tr>
<td>Trip:</td>
<td>79.40 miles</td>
</tr>
<tr>
<td>Fuel used:</td>
<td>2.349 gallons</td>
</tr>
<tr>
<td>Fuel grade:</td>
<td>Shell V-Power 93 octane</td>
</tr>
<tr>
<td>Driving type:</td>
<td>Highway</td>
</tr>
<tr>
<td>Target speed:</td>
<td>65 mph</td>
</tr><tr>
<td>Temperature:</td>
<td>78 degrees</td>
</tr>
</table>

Comments: I'm not sure how to explain the drop in mpg after inflating the tires nearly 13psi. All I noticed tonight was that the winds picked up during the drive back. I could feel my car being blown around a little. It was a little colder since the last trial. That could have some effect.

As for the top-off method, I still think it's the most accurate way to measure mpg at this time (except for OBD2 + ScanGauge). I am referencing <a href="http://pesn.com/2005/03/25/6900071_Acura_Acetone/">Sterling D. Allen</a> at the PESWiki site and his results with this method. He got very consistent results which somewhat confirms the accuracy when topping off. He also noticed a slight change in mpg with a change in temperature.

I only planned on doing two trials for this experiment but I think I'm going to do one more just to see what happens before moving on to acetone. Hopefully it will be done in the next few days.

__________________
Scouring the country for an excellent condition Civic VX

So I have to admit, today's trial was really testing the spread in fuel economy at different temperatures.

<table border="5" cellpadding="5">
<tr>
<th colspan="2">
<b>10/1/05 - Tire Pressure Trial #3</b>
</th>
</tr>
<tr>
<td>Fuel economy:</td>
<td>37.0 mpg</td>
</tr>
<tr>
<td>Previous fuel economy:</td>
<td>34.7 mpg</td>
</tr>
<tr>
<td>Percent difference:</td>
<td>+6.63%</td>
</tr>
<tr>
<td>Trip:</td>
<td>85.10 miles</td>
</tr>
<tr>
<td>Fuel used:</td>
<td>2.302 gallons</td>
</tr>
<tr>
<td>Fuel grade:</td>
<td>Shell V-Power 93 octane</td>
</tr>
<tr>
<td>Driving type:</td>
<td>Highway</td>
</tr>
<tr>
<td>Target speed:</td>
<td>65 mph</td>
</tr><tr>
<td>Temperature:</td>
<td>94 degrees</td>
</tr>
</table>

Comments: That's a 3.2 mpg increase from last night's test. I was expecting to see some difference and that's why I decided to do this test today in the heat. Temperature was 16 degrees hotter than last night's test.

I hit a lot of red lights getting to the highway, which is about 3 miles from my gas station. It doesn't look like it screwed up things since my fuel economy went up.

I still had a full tank from topping off last night, so I drove around for about 10 miles to kill some gas. If gas volume does change with temperature, I wanted to eliminate that as a variable. Fuel economy for those 10 miles was calculated at about 26 mpg, about what it should be for city driving.

I don't plan on doing another test in the daytime because I was dripping sweat during this trial. I had to keep everything else constant, which means no A/C and windows up. It's all in the name of science though, so I'm ok with that. My other trials were all done around midnight so heat is not an issue. Outside temperature is more constant at this time anyway so that should partially eliminate it as a variable. At least today I could enjoy the drive through the beautiful Texas hill country though.

I missed my usual turn-around exit, so this trial went a littler further than the others.

One thing I haven't mentioned is that I always have my stereo system on pretty loud during the trial. It helps the time go by faster. As long as I do this for every test it shouldn't introduce error. I don't think eletrical load would play a huge role in fuel economy anyway. I overheated my amp today and in trial #1. It's rated for 300W which is a little too small for the rest of my system.

I'll be travelling about 130 miles tomorrow, almost all highway and 65 mph except for a small stretch of 55 mph. I'll use this as my last and final trial for tire pressure. I'll leave in the morning and return in the evening, so hopefully temperature will be about the same both ways. After tomorrow, it's all acetone.

__________________
Scouring the country for an excellent condition Civic VX

Here are some pros and cons of different tire pressure settings.

Low

+ better handling
+ less chances of getting a puncture
- outer parts of tire wear out more easily

High

+ less rolling resistance
+ better fuel economy
- middle part of tire wears out more easily

Keep in mind that we also spend money on tires, not just fuel. It's important to make them last as long as possible in order to save.

These are all things I've thought about recently. I was quoted $95 per tire for good LRR tires at Big-O. I might try somewhere else. Quite honestly, if I end up paying $95 per tire, there is no way I'll over-inflate them. I do'nt need the center wearing out any faster than normal. I'd rather just check every week and keep them at max PSI.

95 bucks seems like a lot. <a href=http://www.greenseal.org/recommendations/CGR_tire_rollingresistance.pdf target=_blank>This PDF<a/> <font color=black>says they should be a lot less. I looked for some on the place I bought my tires online (free shipping) and I didn't see any though, but I didn't look anywhere else cuz I should be working.
You're running the hx 14s with the, right?

Yeah, $95 seems like a lot to me too. I might check around (discount tire,etc.)

buying from Big-O makes sense though. They offer free rotation and balancing for the warranty of the tire (these ones are 60,000 miles). Getting your tires rotated and balances usually costs around $50 each time. Big-O also offers a free replacement. You don't get prorated like other tire places, you get a whole new tire for free if it is damaged, etc.

Anyway, I'm not sure the guy actually was showing me a LRR tire though, so i need to do some more research.

I am using HX 14 inch rims (or I WILL be using them). This means that I'll have to get 185x60R14 so my speedo doesn't get screwed up.

I just bought some 195 60 15 tires for my saturn for $40 each. I know you can find civic tires for a similar price.

__________________
2008 EPA adjusted:
http://www.gassavers.org/gaslog/sig.php?id=249
http://www.gassavers.org/gaslog/sig.php?id=414
Distance traveled by bicycle in 2007= 1,830ish miles
Average commute speed=25mph (yes, that's in a car)

This is the final trial I am doing for tire pressure. Sorry it took me a while to post my results.

<table border="5" cellpadding="5">
<tr>
<th colspan="2">
<b>10/2/05 - Tire Pressure Trial #4</b>
</th>
</tr>
<tr>
<td>Fuel economy:</td>
<td>34.0 mpg</td>
</tr>
<tr>
<td>Previous fuel economy:</td>
<td>34.7 mpg</td>
</tr>
<tr>
<td>Percent difference:</td>
<td>-2.02%</td>
</tr>
<tr>
<td>Trip:</td>
<td>132.1 miles</td>
</tr>
<tr>
<td>Fuel used:</td>
<td>3.885 gallons</td>
</tr>
<tr>
<td>Fuel grade:</td>
<td>Shell V-Power 93 octane</td>
</tr>
<tr>
<td>Driving type:</td>
<td>Highway</td>
</tr>
<tr>
<td>Target speed:</td>
<td>65 mph</td>
</tr><tr>
<td>Temperature:</td>
<td>80/78 degrees</td>
</tr>
</table>

Comments: The two temperatures are for the trip there and the trip back respectively. Fuel economy was right where I expected given the temperature I conducted the trial.

It does not appear that there is any substantial gain for my car by over inflating the tires. It is possible that the base MPG was off because I didn't have values over a range of temperatures. That is one thing I gain from all these trials for future tests. I know precisely how my car will run during different times in the day.

I have not fit my values to a curve yet, but I'm wondering if there should be some scaling factor to normalize fuel economy at a constant temperature. It's not possible for everybody to do their trials at the same temperature, but maybe we could come up with a method to calculate fuel economy at a standard temperature, like 72 degrees for example. That way our results will be easier to compare to each other and to our other trials if they were done at a different temperature.

Here's a summary of my trials for this experiment. Notice the strong correlation of temperature and fuel economy:
<table border="5" cellpadding="5">
<tr>
<th>Tire Pressure Experiment</th>
<th>Trial #1</th>
<th>Trial #2</th>
<th>Trial #3</th>
<th>Trial #4</th>
</tr>
<tr>
<td><b>Fuel Economy</b></td>
<td>35.0 mpg</td>
<td>33.8 mpg</td>
<td>37.0 mpg</td>
<td>34.0 mpg</td>
</tr>
<tr>
<td><b>Temperature</b></td>
<td>81 degrees</td>
<td>78 degrees</td>
<td>94 degrees</td>
<td>80/78 degrees</td>
</tr>
</table>

I obviously learned more about the effects of temperature than tire pressure, but that is even more valuable to know because it will effect every test we do. Ultimately tire pressure played very little into my results but I'll probably keep them around 40 psi anyway. I don't think there's any reason to go overboard with tire pressure in attempts to maximize fuel economy. Just keep them filled up to the recommended pressure and check them about every other tank.

__________________
Scouring the country for an excellent condition Civic VX

i just messed up my experiment, so i'll start over. I changed the oil, and it did have an effect on my gas mileage. darn.

What kind of oil to did change to and from? Was it a positive effect on the gas mileage?

My test results. My tire pressure before was 32F and 30R. I got 23.8-24.5mpg

I filled up the tires 37F and 35R and got 28.3-28.7mpg. The 27.4mpg is when I had 2 other people in the car and I was going 70mph.

__________________
http://www.gassavers.org/gaslog/sig.php?id=6
http://www.gassavers.org/gaslog/sig.php?id=115