Tire pressure!!!

You just met the EPA City MPG rating, great job!!!

 

Great job! :clap: Might be worth posting in the Yahoo! groups.

40/38 is what I'm planning on using, and the temps might be warmish when I get mine (~2 weeks)

Brian

 

OK, so you might get slightly better gas mileage with more pressure. In spite of what some people say I believe that you are affecting the tire contact patch in a negative way. The load across the tire contact patch is not as uniform as it could be with the "correct pressure" for the total weight of the car. So your amount of adhesion is reduced with a higher than "correct" pressure and this can be detrimental on wet roads. The somewhat increased center wear is less important to me. I know that the increased pressures started way back with the previous Prius and that was an entirely different situation in my opinion related to a much different tire that apparently responded to increased pressures well.

No tire manufacturer or tire engineer that I could find has come out and said for better performance add 6 psi, etc. They always preach against underinflation as being dangerous and detrimental to fuel economy. I really think Toyota would have recommeneded higher pressures if there was so much gas mileage to be gained. I have not perceived that much increase in ride harshness or road noise (relatively soft supension might be the reason, not sure) at increased pressures so in my mind if the higher pressures are indeed better overall, why doesn't Toyota recommend them? The tires can take it so it's just not logical. Here's a short statement from AAA on this subject. You draw your own conclusions. Is the extra 1 or 2 mpg worth the possibility of reduced adhesion when you might need it most, I'm just not sure. But I have noticed that the Prius is more skittish at 42/40 than 35/33 on the freeways around here so I'm going back to 35/33 and will see how that goes. I really have not seen anthing that affects my gas mileage "overall" by 5 mpg except maybe traffic and hills around here. So here's the AAA statement:

Overinflation: An overinflated tire has been operated at higher pressure than recommended by the tire maker or the vehicle manufacturer. High pressure causes the tire to expand more, and wear will be greater in the center of the tread than toward the sides. The inside and outside edges wear less because they contact the road with less force than does the center of the tread. Overinflation shortens tire life and can make vehicle handling seem "light" or "skittish," particularly on wet roads.

 

I'm going to continue to look for meaningful data about tire overinflation. There's lots of info out there about underinflation being a bad thing but not much on overinflation. And I guess overinflation is a relative term. It's a matter of if you believe that the correct inflation is what Toyota recommends. Maybe we need somone to do some rolling resistance test on a long stretch of deserted road. Here's another statement on the subject from a Truck site quoting Bridgestone data. I'm going to look around for something more technical from some of the tire manufacturers. I've heard that they tire manuf. are going to publish rolling resistance data soon by law. Does anyone know anything about that? Thanks.

Bridgestone engineers have tested the effect of inflation pressure over a 40 PSI range, from 20 PSI below to 20 PSI above standard. They found a two percent variation in fuel consumption over that range.

Inflation pressure has a definite effect on fuel economy and is something you can begin monitoring and maintaining immediately—regardless of the type of tires you use.

In addition, proper inflation tends to minimize irregular wear for longer tire life.

Here's the Bridgstone partial statement:

Overinflation is neither effective nor recommended as a fuel economy method. Tires on different axles make different contributions to tire fuel economy. Axle weight distribution does not accurately predict position contributions to fuel economy. Trailer tires make a bigger contribution to fuel economy than either their number or loads would lead you to believe.

Bridgestone also tested the effects of overinflation. While it did not prove to be an effective way to save fuel.

 

Here's a report on how my 17 day old car is doing with 35.5 psi front and 33 psi rear (just checked them with my super digital tire pressure gauge) after driving for 743 miles.

Driving elevation has ranged from 5000 to 7000 foot elevation ranging from mountain foothills (where I live) to 79 mph freeway driving to city street driving in neighboring Albuquerque. Driving temperatures have ranged from ~35 degrees to ~75 degrees.

The displayed average mileage on this second tank has regularly shown 48 to 49 mpg over this past 374 miles (note: there are 4 of 10 bars still showing on the odometer).

In other words, I agree. I'm perfectly happy having a car, that's still in its break-in phase making nearly 49 miles per gallon using regular gas and properly inflated tires.

 

<div class='quotetop'>QUOTE(\"jasond\")</div>

61.5 is very good mileage for going to work. What was your mileage coming home from work (round trip mileage)?

I ask because, for instance, where I live there's significant elevation difference between my home and the nearest town, thus the 'going' mileage is much better than the 'coming' mileage (especially if an Easterly wind is also packaged into the equation).

Ken

 

The upside of higher tire pressure is supposed to be longer life and better gas mileage.

However, one point I haven't seen raised elsewhere: The upside of lower tire pressure is (I thought) better traction.

Could it be that the Prius is a light enough car that Toyota is recommending less-than-normal PSI for optimal traction?

 

> In spite of what some people say I believe that you are affecting the tire contact patch in a negative way.

How come there isn't any wear recent evidence to support that?

Based on my very lengthy tests using high pressure use, the wear has been even all the way across the width of the tire. That indicates contact is evenly distributed.

In the old days, tires were not as well built as they are now. So some of the old behaviors, like that with high pressure, have changed... especially with a car that is heavier than average.

 

Sorry this is so long!!!!!

Well I don't know John. I do agree that modern radial ply tires are not going to wear in the center as much. Are you tracking the tread depth with a dept gage over a lot of miles because I don't think you'd be able to really see the difference for a lot of miles, maybe not until the tire is almost worn out? The wear is really not a concern for me anyway but like you said it is a sign of uneven contact pressures. Of course there's a lot more data out there about underinflation and I've only found some about overinflation so far. Underinflation of course is the more dangerous of the twol

So it depends on if you believe the values given out by Toyota as the correct pressures for a lightly loaded vehicle. And I ask myself what kind of liability would they incur if they recommended that we "underinflate" our tires to 35/33? Especially given the issues with the tires on the previous Prius and all the Firestone/Ford tire issues? And then there's the question of why recommend a lower than optimal inflation pressure that yields inferior gas mileage especially at slower speeds where rolling resistance is a bigger percentage of the total road load? That's not to say that car manufacturers are always right.

I have no argument about the higher inflation pressures yielding better gas mileage. How much, I really don't know. Maybe I should find out (see below).

My car's poor tracking really is my main concern here. I've had it re-aligned once and checked with a 2nd source. At this point I'm at a loss as to which way to go with my car. I drove Tosh's car and it "seemed" to track better. But it wasn't like night and day with my car. My car is less "darty" at the lower recommended pressures though.

So I'll tell you what. I'm going to try to answer my own question about the effect of pressure this weekend. I'm going run a known route several times on sat and sun with the extremes of pressure...35/33 and 42/40. I'll do this in a random fashion so I don't favor one run over the other. I may have to do this on a bunch of weekends but I'll post at the end of each and see how it goes. I'll have to cool the tires down between runs of course. It's 22 miles round trip so I'll try to make sure the car is warmed up on a short trip at the start of each run and will try to control all other variables too. Before you ask, my wife is out of town till July so I don't have a life, lol.

 

One gentleman says he operates at 5000 to 7000 feet above sea level. So does the lower air pressure at those altitudes mean --fatter -- tires with the same tire pressure as at my altitude of 300 feet. A ballon certainly continues to get larger as altitude increases.
Bob

 

Tires at elevation are normally filled and tested at that same elevation. If, though, they were filled at sea level, it would seem to me that, assuming no losses, the pressures could rise a good 20% or 7 psi in our 33/35 psi tires bringing them to 40/42 psi (still below the maximum pressure stamped on our tires though).

On researching the subject of elevated tire pressures for better gas mileage, I found it to be generally not recommended. Some mentioned that if tires were inflated to the maximum pressure allowed (stamped on the tire), the results expected would be a harder ride, skittery feeling handling / steering, and more reason for concern regarding tire damage if a pothole is hit. There were, though, two sites I came across that said that if you do choose to increase tire pressure, increase no more than 4 psi above what's on the car's label (assuming this pressure is still below the maximum rating stamped on the tire).

Here's what a few of these sites had to say on the subject:

Just Tires

Proper inflation is the single most important part of tire care. The inflation pressure on the side of the tire is the MAXIMUM operating pressure. It is not necessarily the right inflation for your vehicle. Always use the inflation recommended by the vehicle manufacturer. You can find it in your owner's manual, posted on the edge of the driver's door, on a door post or on the inside of the glovebox door. Always check inflation when tires are COLD: when the vehicle has been driven less than a mile or one hour or more after driving.

Goodyear (to its dealers)

No matter what the maximum pressure indicated on the sidewall, only recommend the air pressure as highlighted on the Vehicle Tire Information Door Placard. NEVER EXCEED THE MAXIMUM INFLATION PRESSURE LISTED ON THE SIDEWALL OF THE TIRE.

Overboost.com

Traction, temperature and tire wear ratings as assigned by the DOT are done at the tire's maximum rated pressure. For performance tires, this is usually in the mid-40s PSI. However, this certainly does not mean you should run out and air up the tires to maximum pressure! On newer cars, OEMs provide a tire pressure spec and it is usually not close to the tire's maximum pressure rating. This allows engineers to fine-tune a car's ride and handling and is always a safe bet for best overall performance even if larger tires and wheels are used. Experienced drivers have been known to play with different tire pressures for best handling; if you elect to do this, note that race sanctioning bodies usually do not permit tire pressures of less than 25 psi.

Michelin

The level of air in your tires affects your vehicle's overall performance. Not even the highest quality tire will perform well if it's not inflated properly. The correct pressure varies from vehicle to vehicle and depends in part upon driver preference. Each vehicle has a recommended inflation pressure, usually found on a placard on the door section, door post, glove door, or fuel door.

New Jersey Department of Consumer Affairs

To find the proper air pressure for your tires you'll need to consult your car's owner's manual. Your automaker takes into account several important factors when determining the right inflation level for your car and tire combination. You should never let your tires' air pressure fall below the recommended level. The sidewall of your tire indicates a maximum pressure figure; however, this number does not indicate the proper inflation level for your specific car. Rather, it indicates a top pressure that you must not exceed for any reason.

AAA

With today's higher pressures, your best guide is to follow the manufacturer's recommendations on a late-model vehicle's tire data placard. If you go higher than the carmaker's specs, hold it to no more than 4 psi and do not exceed the tire's maximum pressure rating.


Personally, I'm tempted to maybe take my tire pressures up a couple of psi, if for no other reason, than as insurance that they not get too low. Low tire pressures, in all the reading I did, were the greatest concern (safety, mileage, polution, etc.).

 

Thanks Ken for the added info. Today I embarked on my first test. I'm using route to work and back that I travel a lot and so I know what my mileage has been. I'm going to conduct these tests on the same time of day and presumably same temperatures. It's an 11 mile trip to work which includes 2 miles of 65 mph freeway. There are 18 traffic lights and no stop signs! 18!!!! Lots of gradual, moderate hills and even a couple semi-steep ones and a rolling hilly section. I filled up at the start of this test and then drove home to begin the test so the car had a 2.6 mile warm-up. I plan to use the same pump each day with the slow speed setting on the shutoff. I'll record computer predicted values as well as actual. There will be that 1.3 mi trip from the gas station as well as one 1.3 mile trip at the end of 3 round trips to work each morning. No EV button use but I'll take advantage of cruise control and try to acceleration to known roadside markers as needed, always keeping the posted limit and always trying to look ahead for traffic or lights.

Today I ran 35/33 and will not drive this car again until I replicate this test tomorrow morning. I started around 6:45 AM and was finished just before 9 AM with my 3 runs. The traffic and therefor triggered lights were beginning to influence the results on the last run. So if I can I'll do the 42/40 run on saturday and the low pressure run on sunday to normalize the traffic. This route is typically not traveled much by shoppers but lots of construction workers and beach bound people use it as the morning goes on. It was sunny today, rel humidity around 60% and the temperature rose from start to finish from 55 to 63 degrees. No AC or climate control was used. The results here are given in first leg out (11 miles), leg back, and then 22 mile round trip totals. The trip out is always much better than the trip back. I've hit as high as 70.5 with warm up on a good day out (typ is 60 mpg) and as low as 38 mpg on the way back (lots of traffic) with the range of 38-44 on the trip back.

Each of these runs took 33.75-36 min round trip with the avg speed of 39.3 mph so you can see I did have to slow sometimes or wait at the occasional light and of course accelerate up to speed after. The speed limit varies from 40-65 mph. Here are the 3 sets of results:

71 out, 41.1 back, 56 overall for 22 mi
78.8 out, 40.6 back, 59.7 overall for 22 mi
77.8 out, 37 back, 57.4 overall for 22 mi
57.8 mpg overall for the 66 miles

This car continues to impress me. I was really surprised to find that my normal commute home, even with the warm-up, is usually better than I was able to achieve today. Slower but better. That 1-2 mile traffic jam coming home, mostly in ev mode, actually helps the gas mileage! I utilize the battery high SOC more. So it is the hillyness of my commute home that hurts the most. My best of 70.5 going to work is good but there's room for improvement on the 60+ mpg I normally see. That appears to be traffic related too as long as I'm careful enough at the start during warm-up. I've found I can get 50 mpg for the first 5 min and that usually results in a very high 60 mpg commute. Oh for what it's worth, I'm not using synthetic oil yet. Waiting for my 2nd oil change planned at 5k, now about 3600.

So tomorrow I fill-up and try it again with 42/40. Any questions, comments or suggestions will be appreciated. Thanks.

 

So here's a simple plot of the first day's 35/33 tire pressure data:

 

Nice write up, Fred. Keep em coming

 

Ditto!

 

I'm with Fred on this tire thing. I did add a few pounds to my tires,upping them to 38,35, but no higher. I also bought a tire compressor from Fred Meyer here in Seattle for ten bucks. It works fine, although it's slow, and fits nicely into a small space under the trunk lid.
I think Fred is right on: why would Toyota not recommend higher tire pressure if indeed it produced increased mileage with no harmful effects? They didn't because increased pressure does have harmful effects as some of the higher pressure advocates may find out when they hit rainy, slick streets and discover the loss of tire gripping ability.
My mileage is getting better, either because I'm becoming more adept at driving the car and/or it's getting broken in and "warmed up". The outside temps are getting higher too which, I understand, helps the battery work more efficiently.
Planning a long car trip to California in two weeks and hope to do some experimentation with milage and driving habits.
Anyone else noticed that the car seems to run more on stealth mode after it's been broken in with a 1000 miles or so on it?
Bob