Anybody got measurables on the PSI-MPG relationship?

Probably needs to be broken down into wheel/tire size and of course the type of tire. On the OE 16" tires, I'd think that 40-ish is as good a PSI as any. Above that and you'll get diminishing returns.

 

I keep mine at 42/40 (Prius V, five, 2012) and am getting 45.6 mpg.

 

Bumped into an article yesterday saying something like: LRR tires rolling resistance drops about 30% the first 20-30 minutes of driving, reflecting the dual effects of higher tire temp and higher pressure as the tire heats up (p.4). Also rolling resistance much higher in winter.

HANDBOOK for ...ROLLING RESISTANCE (UMich)
http://deepblue.lib.umich.edu/bitstream/handle/2027.42/4274/bac0913.0001.001.pdf

 

^^^
That would explain some of the lower MPG people report in cold weather. Maybe we need tire heaters?

 

Here is what I got using the equations in the article for one tire.
I assumed MPG change is 20% of rolling resistance change (TireRack.com says its really 10-20%). This is for warmed up "equilibiurm" tire. Assuming 35 psi is base case, note that going below 35 psi is more rapid fall off.
Going to check my pressure right now: 32,34,34,35 lets add some air!

 

That doesn't seem like much of an increase to justify intentionally going to a higher psi. I would imagine the car would have a rougher ride, who knows on the wear of tires.

 

I think the shape of the curve is correct, so falling below 35 psig (especially right now as temps plummet) could start to shave off MPG. So the main thing keep them a little over 35 psig so they don't fall below 35 psig. Perhaps the shape of the curve is correct but the effect is bigger for modern LRR tires? (that article was 1980-ish)

 

from epa.gov

 

^^^...seems reasonable...the tire handbook that I referenced above was by "Clark" in hlunde's post.
Clark and Dodge make the point that the pressure effect is 1/P (reciprocal pressure) so the effect should be bigger on the low side.

 

42/40, getting 53.6 MPG. Keep speed @ 55/60 MPH

D

 

"Diminishing returns" is a remarkably useless metric given the number of things it is used for. Most things which affect gas mileage are ALWAYS diminishing returns. That tells you nothing about where the optimal point is regarding that change.

A simple (non-car) example: The best heat loss reduction comes from the first (say) inch of insulation. Every subsequent inch of insulation provides less heat loss; 'diminishing returns'. Does that mean that the optimal level of insulation is 1"? NO. Optimal is around 16" somewhere. Because the point is reduce heat loss until the cost of insulating would be higher than the cost of the fuel saved.

... end rant. Sorry.

 

I agree. The best part about running increased tire pressure is that its free, so cost isn't even a factor. Tire life comes into question, but most of the info I've read about running increased pressures points to the tires lasting at least as long as if they'd been run at a lower pressure.

I personally just set them at the sidewall max of 51PSI and leave it at that. I've never thought that my Prius rides rough.

 

I don't exceed the pressure recommendation of the tire, I roll with 35-40 psi. IF I have the time I will put 40 but if not I just get them the same psi in front then same in the back front might have higher pressure than back. Economy has a lot to do with the temp of the ENGINE not the tires. ICE has to reach optimal operating temperatures and prius will use electric motor for the first few mpg then the ICE will kick in when all the hard torque work is done. Most others ICE are doing 100% of that work and they are heating up much quicker.

 

Really? Please explain. What's your background?

 

I note no background information in profiles of either skwcrj or cdltpx ...

 

Of course, wjtracy was just pointing out that the tires heat up as well. A warm ICE vs a cold ICE is a much bigger difference than a warm tire vs a cold tire. But there is still an advantage (according to the article he read) when the tires are warmed up.

 

I agree. A warmed up engine along with a full (60% SOC) battery will yield better fuel efficiency than starting with a cold engine. LRR tires also makes a huge difference on your lifetime mpg.

I was questioning cdltpx's explanation because it goes against everything that has been written on numerous posts on these forums. He may just be another "internet expert" or an engineer with a certain specialty that pertains to our cars. Either way, I'm all ears.

I'm not expert about the HSD or the Prius but I have a great quest for knowledge. This desire to learn more about the Prius along with chasing the mpg game takes me to the Gen-III forum frequently. There is a wealth of information under the Gen-III forum written by guys way smarter than me about these cars. I recommend everyone visits the Gen-III forums frequently. You'll be amazed!