Over Inflated Tires, Does it Save More in Fuel Cost than Tire Wear?

That may not cost you any greater wear on the center of the tread, and certainly won't reduce traction in rain on pavement. The actual drawbacks are greater vulnerability of the tire to certain kinds of damage, reduced traction on rough surfaces like gravel, and of course a rougher ride.

 

I'd been running my 215/45R17's at close to 40 psi, but they're low profile, and the ride was rough. A few tankfuls back I dropped them to 34 psi all 'round (spec is 332/32 f/r) and: mpg doesn't seem to have gone down. At all.

 

What extra wear on the center? With modern steel belted radials, most passenger car drivers are seeing none, in fact seeing more even wear as the shoulders wear more evenly.

The center wear was much more of an issue on old era bias ply tires.

Higher pressure should help prevent hydroplaning. But there seems to be a lack of consensus about regular wet traction without hydroplaning.

 

If the Prius weighed what all other cars weigh, the center of the tread might get more friction from properly inflating your tires (the best roll attainable) but it's a lite weight and if you do inflate properly will find no undue wear of the center of the tire.
Might I add, the dealer inflates to 35psi, and all other cars inflate to 32, so IMHO, inflating a Prius to 32 will wear the sidewalks

 

I ran a set of Michelin Primacy MXV4 at 40/38 psi and had even tire wear for 62,000 miles with 60,000 mile warranty.

 

It doesn't? That is news to me.

 

So I ran an experiment several years ago with tire pressure. I fully inflated the tires and drove ~10 miles on the highway during the early afternoon when temperatures plateau. Then I pulled off and quickly measured the tread temperature and let some of the air out:

After the initial measurements, the tires quickly cool off in a couple of minutes. I was using an IR thermometer. A better tool would be a penetrating temperature probe but these are not common (i.e., expensive.) These results suggest:

• Higher pressure -> lower temperature (lower wear)
• Lower pressure -> greater temperature differences (uneven wear)
• Wheel misalignment is evident in the temperature profile
• Higher pressure -> reduces misalignment effects (evens the wear)

My subjective impression is higher pressure in the tires makes them more responsive and improves steering accuracy.

Bob Wilson

ps. Some other metrics:

 

I agree. If we could replace the structs and shocks with 'softer' ones, it would have the same effect yet our tires could run fully inflated.

Bob Wilson

 

Thanks for sharing your research Bob!

 

Mostly I am hoping others will take a clue and figure out how to do their own experiments. Read the literature including track car articles on tuning suspensions and tires. Then walk out to your car and ask yourself,'What questions do I have that I could test myself?'

Let me give a suggestion . . . roll-down test. Many of us have access to a straight, fairly flat, road in the area. Fully inflate your tires to the maximum side wall and then reach a fixed speed, say 15 mph, and shift into "N". Measure where the car stops in both directions. Then take out "n psi" from the tires and repeat. Continue until you reach the minimum tire pressure, say the door jam pressure -5 psi. Then plot the results.

For a statistically defensible result, get at least three samples on each direction, a total of six. Better, take 5 samples each way, a total of 10, and toss out the lowest and highest and average the middle three. Be sure and record the temperature and winds. Use either very early, pre-dawn or early afternoon when temperatures will be as constant as possible for all the runs.

Where am I headed?

Let us say we develop a well defined, roll-down methodology. We can use that with new tires to plot our own rolling resistance charts and data. No longer subject to 'marketing nonsense,' we can share actual metrics for the different makes and models of tires.

Bob Wilson

 

I glaze over just READING a lot of your stuff, never mind carrying out similar, but yeah: there's so many questions here, along the lines of "what will happen if...". And often they're simple, easily tried and easily reversed mod's.

 

A liitel off topic, but where did you get this test table?

 

I think Bob was slipping a bit there, some of those tires look dated, ie: it's an old report. Looks to be Consumer Reports.

 

Correct! The Sumitomo was my tire of choice but they discontinued them when I needed tires for our 2010 Prius.

So I went with Yokohamas slightly over-sized as measured by revolutions/mile. This reduced the indicated and pump-measured MPG error to less than 1%. It also means my speedometer and GPS speeds match within less than 1 mph.

Now I'm running the European style of tire rotation on the 2010 and so far, I'm please. The European style is the fronts stay and the rears stay. But when the fronts wear down, the new tires go on the rear and the rear tires go on the front. So I don't have to buy four new tires but only pairs at a time.

Bob Wilson

 

I'm European and I'm first hearing something like that, everyone I know buys all four new and every one I know drives dedicated set of winter and summer tyres - meaning one set get rotated once a year and are driven only half a year.

 

The format absolutely screams 'Consumer Reports'.

 

All I'm going to say, is that when I first got my Prius I experimented with higher tire pressure.

But ultimately, In my personal opinion, The Prius has a unique braking system and a interactive traction control system that makes choices based on feedback from the tires and road, and this can be influenced by how your tire is inflated.

So anyway after a few months of experimenting with higher inflation rates, and suffering from a rougher ride, and I think the "bounces" causing the traction control to kick in when it otherwise might not, I just adjusted everything to the recommended PSI in the door jamb.

I think the ride IS smoother, any MPG loss is nearly undefinable, and I even think the Prius is "safer", since my tires aren't "over- inflated".

Do what you want to do. But I don't think any benefits of supposed MPG gain are worth the rough and I think potentially less safe ride.

I'm careful to check my PSI, and try to keep it as close to recommendation through seasonal changes. But I don't bother with keeping the PSI higher than recommended. I'm thinking Toyota built the Prius, and probably a lot of thinking and testing went into the best compromise between economy, safety, with their PSI recommendation. I'm not going to throw that out the window for a chimerical and questionable MPG improvement.

 

Just do Bob's coast-down test, divide the speed change (in ft/sec or m/sec) by the time, and you'll have deceleration rate (in ft/sec² or m/sec²). Divide that by g (=32 ft/sec² = 9.8 m/sec²), and you'll have your tires' rolling resistance coefficient (albeit slightly contaminated by wind and by miscellaneous drive-train frictional losses). Neat stuff, huh?!

However, I can't think of a patch of road anywhere near here that's level for a sufficient distance, and where I could coast to a stop that many times without getting run over or arrested.