Spare Tyre Pressure

catgic · Jul 28, 2014

nsfbr said: ↑

catgic said: ↑

NSFbr - "Up-Pressuring" is a term I coined, and use. It is not the EPA's term. I extracted the "0.3-To-0.4...Rule-Of-Thumb" from the Official EPA site, over seven-years ago. I cannot comment on how this original EPA "0.3-To-0.4...Rule-Of-Thumb" blurb may or may not have been tweaked, changed, adjusted, or reworded and reworked since then.

I am just sharing with the gathered eyeballs what I do, and reporting the results I obtain from doing them. Readers are free to take my opinion, and implement my practices and techniques, or ignore them. If you or others do not believe the EPA "0.3-To-0.4...Rule-Of-Thumb," then ignore it, and run the OEM P215/50R17s tires on the Prius v (vee) pressured at Toyota's recommended 33 PSI Front/32 PSI Rear shown on the door post placard. As for me, I shall continue chasing every easy to get incremental MPG I can get. I shall be continuing to run my OEM Toyo Proxes A20 P215/50R17s tires at 51 PSI Front/50 PSI Rear.

FWIW FYI: On my old, iconic GEN II/2G, "Up-Pressuring" the tires rewarded me with an incremental extra 3-ish MPG Per Tank-Full, over what I was logging running my 2G Petrol Warfighter on the 2G's OEM Goodyear LRR tires pressured at the "Door Post" PSIs. As I reported above, "Up-Pressuring" my OEM Toyo Proxes A20 P215/50R17s tires has delivered me an incremental extra 2+ MPGs.
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Sorry for the long reply. I'm wordy. TL;DR summary: Diminishing, and eventually negative returns from increasing tire pressure too much. Door jam numbers are absurdly low. Best numbers somewhat lower than you seem to be running at, but much higher than the door jam.

Okay, so here is where I think - my opinion, but based on some very real experience and access to test data (albeit ~15 years old) from a major tire manufacturer from Japan - you go wrong. The EPA's rule of thumb speaks to underinflation, and very specifically does not speak to inflating tires above the recommended level. This really matters because rolling resistance is comprised of two main elements - sidewall flex and tread flex. There are other factors, but those are the 95% constituents.

Underinflating your tires greatly increases losses due to sidewall flex. Greatly. To the point where you can ruin the tires if you do it long enough. That is what you are fixing when you pump up the tires. But, after a certain point, there is no more to gain from doing so. Yes, you can start to reduce the already small loss of the contact patch flex itself, but that is small anyway. (This ignores the benefit of overinflating tires if you have alignment issues. Much better to get the car aligned though.) So the EPA is talking about the increased loss from having a tire with excessive sidewall flex. Get rid of that excessive flex, get rid of the increased loss.

Now here is the flip side - as you pump your tires up more and more, you actually begin to increase the losses due to tread flex. That is, the squirming around of the treads into the crevices in the road and the open gaps in the tire. Keep in mind that tire rubber is incompressible, so when the tires are pumped up more, the parts of the tire that actually contact the road must deform more. Higher pressure, more deformation. more deformation, more wear as compared to proper inflation when traveling in a straight light. By the way, this is the reason that slicks, or worn tires for that matter, have lower rolling resistance. Less squirming.

Slightly counter intuitive, I know. But this is also why you can get very different mileage depending on the road surface. Nice smooth black top with minimal surface roughness - go ahead pump up to 50 psi. It is probably still as good as you can get, or at worst, not any worse than down at say 40 psi. But when you are on the more common slightly open asphalt used to improve porousity to avoid standing water, I promise, 50 psi is not your friend.

The last big negative about overinflating your tires is that it increases the losses due to the unsprung mass of the car. Again, that is road dependent, so on a perfectly flat, smooth highway it makes little or no difference. On a less than perfect road, it can matter quite a bit.

If I still have you let me clarify that by the terms underinflation and overinflation I refer to pressures on either side of optimal inflation. And that bears no relation to what is on the door jam of the car. That value is absurdly low (a point of agreement I expect!) My best guess as to what is optimal is that it is in the low 40s for the front and about 40 for the rear on the v. The purely empirical reason for saying that is that I saw over a series of drive tests, both on the v and on my previous gen II, that the increase in mpg stopped being detectable about 40/38, and my knowledge about how the curves should behave tells me that there is still a little bit to gain above that point (just smaller than I can measure over the course of a controlled drive test.) So, I pump to 43/41, and top up when I see something below 42/40 in any location.

So yes, I have no intention of ever running at what is indicated on the door jam. There are many reasons for those numbers. Maximizing mpg isn't one of them.
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NSFbr - Bless you, my son. Go in peace and sin no more.

Spare Tyre Pressure

catgic Mastr & Commandr Hybrid Guru

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Spare Tyre Pressure

catgic Mastr & Commandr Hybrid Guru