Tire pressure!!!

jasond · 03-26-2004, 09:28 AM

I just upped my tire pressure from 35/33 to 40/38. My drive to work has been in the 53-58 mpg range previously.

Today, it was 61.5 mpg. From start to stop (including warmup), for an 18-mile trip (30-45mph, with a little traffic, a dozen traffic lights, etc.). Battery charge dropped one notch during the trip.

It was also 50 degrees, which probably didn't hurt, and I was driving as Prius-friendly as a I could. But I got an honest-to-goodness mpg rating of 61.5 for a complete trip for the first time ever. Take that, Prius Bashers!

As all Prius owners know, gas mileage = happiness. So everyone go out and inflate those tires!

Brian · 03-26-2004, 09:36 AM

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

N9IWP · 03-26-2004, 09:48 AM

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

FredWB · 03-26-2004, 12:24 PM

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.

FredWB · 03-26-2004, 12:46 PM

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.

Ken Cooper · 03-26-2004, 12:56 PM

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.

jasond · 03-26-2004, 01:06 PM

As they say, your mileage may vary.

I didn't spend nearly as much effort trying to convince people to try a higher pressure as you did trying to convince them not to, so you must believe in this much more than I do...

I'm happy with my handling, and I waited until the winter ended on purpose. It's not like I can't go back to 35/33 if it seems to cause a problem, and it's not like I'm racing along a curvy mountain road at 80mph when I drive...

Ken Cooper · 03-26-2004, 03:25 PM

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

Quote:

I just upped my tire pressure from 35/33 to 40/38. My drive to work has been in the 53-58 mpg range previously.

Today, it was 61.5 mpg. From start to stop (including warmup), for an 18-mile trip (30-45mph, with a little traffic, a dozen traffic lights, etc.). Battery charge dropped one notch during the trip.

It was also 50 degrees, which probably didn't hurt, and I was driving as Prius-friendly as a I could. But I got an honest-to-goodness mpg rating of 61.5 for a complete trip for the first time ever. Take that, Prius Bashers!

As all Prius owners know, gas mileage = happiness. So everyone go out and inflate those tires![/b]

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

jasond · 03-26-2004, 03:40 PM

Quote:

Originally posted by Ken Cooper

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

I haven't come home yet

I did the tire inflation last night at a gas station 2 minutes from my office. The rate for the slightly-shorter trip home was 56mpg, which got a boost from skipping the first 2 minutes of warmup. That's higher than normal.. the trip home is typically 50-55. Round trip was (I believe) 58.8.

My office is about 120-150 feet below my house, sea level-wise.

bookrats · 03-26-2004, 04:03 PM

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?

03-26-2004, 09:28 AM	#1
jasond Join Date: Mar 2004 Location: Boston Posts: 165 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Tire pressure!!! I just upped my tire pressure from 35/33 to 40/38. My drive to work has been in the 53-58 mpg range previously. Today, it was 61.5 mpg. From start to stop (including warmup), for an 18-mile trip (30-45mph, with a little traffic, a dozen traffic lights, etc.). Battery charge dropped one notch during the trip. It was also 50 degrees, which probably didn't hurt, and I was driving as Prius-friendly as a I could. But I got an honest-to-goodness mpg rating of 61.5 for a complete trip for the first time ever. Take that, Prius Bashers! As all Prius owners know, gas mileage = happiness. So everyone go out and inflate those tires! __________________ 2004 Salsa Red, Package #5/AI November 10, 2003

03-26-2004, 12:24 PM	#4
FredWB Join Date: Dec 2003 Location: San Diego, Colli-fornya Posts: 331 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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. __________________ 2004 Salsa Red Prius, BC

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03-26-2004, 09:36 AM	#2
Brian Join Date: Feb 2004 Location: Orange, California Posts: 479 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	You just met the EPA City MPG rating, great job!!!

03-26-2004, 09:48 AM	#3
N9IWP Join Date: Jan 2004 Location: Southeast MN Posts: 147 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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

03-26-2004, 12:46 PM	#5
FredWB Join Date: Dec 2003 Location: San Diego, Colli-fornya Posts: 331 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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.

03-26-2004, 12:56 PM	#6
Ken Cooper Join Date: Mar 2004 Posts: 338 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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.

03-26-2004, 01:06 PM	#7
jasond Join Date: Mar 2004 Location: Boston Posts: 165 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	As they say, your mileage may vary. I didn't spend nearly as much effort trying to convince people to try a higher pressure as you did trying to convince them not to, so you must believe in this much more than I do... I'm happy with my handling, and I waited until the winter ended on purpose. It's not like I can't go back to 35/33 if it seems to cause a problem, and it's not like I'm racing along a curvy mountain road at 80mph when I drive...

03-26-2004, 04:03 PM	#10
bookrats Senior Member Join Date: Mar 2004 Location: Seattle, WA Posts: 2,843 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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?