A serious question on pressure.

If you drop back the pressure to say 34/36 the ride will be a little softer and the MPG will be slightly reduced. In my case when I measured it the reduction was about 1 to 1.5 MPG (in 55 MPG)

 

There are too many variables for you to determine this effect in one month. I run MPG tests by averaging over a 12 month period in an attempt to deal with changing temperature's effect on MPG. MPG regularly varies more than 10% from summer (near 50 in my 2004 Prius) to winter (low 40's). Your difference in MPGs that you attribute to PSI is most likely due to the increased average ambient temperature this month over the preceding period.

JeffD

 

this is probably due to higher temperatures and shift to summer blend.

I read somewhere results of tire pressure on rolling resistance and the conclusion was that after some "optimal" value increasing pressure hardly had any impact on MPG. Which value exactly depended on tire/size/car weight.

 

If you are convinced that lowering your tire pressure helped your fuel economy. Do an A-B-A test on a set course, and record all the variables and post it here.

 

You said your mileage went from 45 mpg to 46.4. That is an increase, if no other factors are involved. if other factors are involved, such as the switch to summer blend and increased temperatures, I would expect a larger increase, so what you are seeing would actually be a decrease. A test with other factors controlled would resolve the issue for you.

Many people here at PriusChat have done exhaustive testing of mileage under as controlled conditions as they can manage. The consensus seems to be that higher pressure improves mileage (to a point, of course). A single anecdote doesn't really throw any doubt on that conclusion.

 

You need to do more robust testing as noted above. There are way to many variables that you have not taken account of. This is especially true during a pivotal time for fuel economy changes (spring and fall).

I'd be willing to wager money that you lost some mpg but the effects of wind, fuel blend, temperatures, commute/trips, road surface condition etc.. have masked that loss and instead showed an increase.

Please don't think we are trying to flame you. It's just that you have not shown us a solid methodology for testing your hypothesis and since it flies in the face of more robust testing, we are all skeptical. In the end though, if you like the ride better than with higher pressures the mpg difference is moot. Set the pressure where you will enjoy it most.

 

As a contrary example, back in the winter I went from doorjamb recommendations at purchase to 41/39. My mileage got better.

Both our cases involved other variables, so we can't say just from our experiences. That being said, peanut butter and honey taste good together, the sky is blue, and 42/40 will give you better mileage than 35/33 ;-)

 

*disguises self as Corwyn*

Yes!

Corwyn out.

 

Some people have done testing, you can see at: Snapshot: effect of tire pressure on rolling resistance - MetroMPG.com

 

you realize that Metro was equipped with 145SR12 tires, and it weights 2.3 times less then Prius?

 

I am saying that you don't know that. You just don't have enough data to reach that conclusion. Those who do have enough data, say it. I am just believing them.

Or perform controlled test for yourself.

 

Read all the other replies in the thread, people explained very thoroughly why that's not true.

I'm not being a jerk, you just have to realize you're contesting something that just is. It's one of those "fact" things

42/40 will give you better mileage than 35/33 in an exactly-the-same scenario. There's no question about that. Remember, you didn't test for the same scenario. You tested for what looked like the same ride, while I can guarantee that none of the following were precisely constant: outside temps, wind speed, wind direction relative to the car, coolant temp at startup, coolant temp throughout the ride, battery state of charge, time spent getting through stage 2, were you in stage 3 or stage 4, traffic, etc., etc., etc., ad nauseum.

 

Mike actually there is a question about it. While per conventional wisdom the statement above is true, it is hard to be assertive without supportive experiments and numeric results.

For example what if the gain in MPG is 0.1MPG, and the variation due to conditions (wind, temperature, traffic, lights, etc) is 2MPG? Gains will not be detectable in real world, and that is what he states?

 

It's probably better to say that, in the real world with a mix of influences at work, higher pressures make it easier to get higher MPGs. It's not magic: the effect of increased tire pressures on fuel economy will not be apparent if you simultaneously move to Alaska, drive like a maniac or start towing a U-Haul. Dude who saw no effect was simply not looking very hard.

 

For some people the earth remains flat and dinosaurs were co-habiting the planet with humans about 6000 years ago.