Optimal Tire Pressure

Harden 'em up too much and you will damage them.

A few pounds over the recommended pressure is probably best.

I don't really buy this 'maximum pressure' guideline. Tires are part of the suspension of your car. By filling them up to a point where they can't compress any more, you're letting lots of harshness through to the bushings and springs. Less comfort - which you may be ok with - and more wear on the bushings. And less roadholding and longer braking distances.

Mine came from the dealer with huge overpressure, and I let it off gradually till it became driveable in my eyes. I think I even save gas driving it that way, because cornering speeds are higher, so I don't have to brake as much ;-)

cu .\\arc

The studies I've followed, "Transportation Research Board Special Report

286, Tires and Passenger Vehicle Fuel Economy" indicates the more a tire is inflated, the lower the hysterisys losses and deformation during driving. This leads to crisper steering and handling.

Another report is DOT HS 809428 "Preliminary Findings of the Effect of Tire Inflation Pressure on the Peak and Side Coefficients of Friction." Although maximum cold pressure was not in their data, it gives a good indication of what to avoid.

So far, I have had no problems running 51/49 on my Sumatomo T4s and have gotten very good wear reports from the tire rotations. It looks like I'll get all of the project miles and perhaps a few more.

Bob Wilson

No, it doesn't.

That's ridiculous.

So if I inflate my tires to 100psi, by your take I should have the handling and steering of an F1 car?

It certainly would. After inflating to 100psi, you'd have the handling and steering of any car with exploded tires.

Within limits that's correct. More pressure results in stiffer sidewalls, giving crisper handling. I've found the 'sweet spot' on mine (standard Michelin tires) at around 2.6-2.8 bar.

OTOH, my real world driving has many less than perfect road surfaces, and especially smaller ripples are no fun with this tire pressure. Light braking may even activate the anti lock system, and cornering makes the car wander to the outside. By lowering pressure to around 2.4 that tendency is completely gone, the car feels considerably safer at comparable speeds. I have around 2.2 at the rear tires, because it make the car somewhat less front heavy. This is with light load, usually only driver or driver plus one passenger.

If your driving is mostly smooth roads and highways, higher pressure may work.

cu .\\arc

I've found that running at 42/40 psi (front/rear) gives me the best trade-off between ride, handling and MPG. Using the factory pressure of

35/33 causes a significant drop in fuel economy, without (IMHO) an offsetting increase in drivability.

Whatever pressure you decide to use, you should have the fr>

Isn't 2.4/2.2 just at (or a little below) the factory recommendation?

Thanks to Google books, I picked up the following graphs from:

Gyenes, L. and Mitchell, C.G.B., "The Effect of Vehicle-Road Interaction on Fuel Consumption," Vehicle-Road Interaction, ASTM STP 1225, B.T. Kulakawski, Ed., American Society for Testing and Materials, Philadelphia, 1994, pp. 225-239.

inflation lookup table:kN/m**2 - psi

200 - 29 225 - 33 250 - 36 275 - 40 300 - 44 325 - 47 350 - 51 375 - 53

lookup table: km/h - miles per hour

80 - 50 90 - 56 100 - 62 110 - 68 120 - 75

Tire rolling resistance is not a linear function with speed and inflation pressure and these charts give a clue about tire drag. For those who prefer a softer ride, go for it. For me, local speeds and roads are such that maximum sidewall pressure up front and -2 psi in the rear works quite nicely.

Bob Wilson

You know, Mythbusters showed that the closer you tailgate a big rig truck, the better your gas mileage.

Even down to the last inch, there were significant gains.

So pump those babies up, and plant your front bumper half an inch from a big rig as you go down the highway.

Exploding truck tires, can be a hazard; breathing air from a tire underwater doesn't work; and driving in reverse on ice is ineffective tire traction improver. However, Gyenes and Mitchell have shown:

Bob Wilson

It actually took me a minute or two to figure out the units - and even then I'm not quite sure.

The kN/m^2 are bar. 1 bar000 Pa = more or less atmospheric pressure

Keep in mind though, that when referring to bars, one usually refers to relative pressure. If my tire has 2 bars, it means it has 2 bars overpressure vs the atmosphere. Therefore I'm somewhat unsure about the graph. And I guess that's why scientists love SI units like Pa ;-)

No real relevance - I wouldn't expect Prii to come with anything but radial tires. I have a 40 year old classic that came with radials back then :-)

Keep in mind though that the higher the speed goes, the less important is tire drag relatively to air resistance. Latest at quick country road pace (100 km/h, about 60 mph) I wouldn't care so much about tire drag and more about keeping windows closed and not having any added air resistance. But then, a riced Prius with spoilers and the like is rather seldom.

cu .\\arc

You didn't quote me all the way.

Mythbusters showed that tailgating a big rig truck definitely saves on fuel.

So in addition to pumping those tires up to 1000psi to get crisper steering and handling, in addition to the fuel economy benefits, you should also be tailgating big rig trucks. As in, an inch or less off your front bumper.

I mean, if your goal is fuel economy, do it up.

I found this converter page:

So at 1 bar, your running ~14.5 psi relative to the outside air? bar - psi

1 - 14.5 2 - 29 3 - 43.5 4 - 58

Use just the radial tire coefficient of drag.

There are non-linear effects that occur between 65-75 mph that can run my NH11 Prius mileage from a high of 53 MPG at 65 mph down to 39 MPG at

75 mph. Apparently tires also make a contribution along with a control law. This fall off in mileage does not follow the V**2, aerodynamic drag profile.

The practical effect is to identify another performance limitation along with the control law that limits MG1 to 6,500 rpm. Larger diameter tires and wheels, say 10% larger, would still be subject to radial tire drag at higher speeds. However, they would shift the MG1 speed management laws up 10%. If this brings 50 mph through 75 mph, the results would be most excellent.

Today, I'm using Sumitomo P175/65R14, T4, 919 rev/mi. It may be possible to fit P205/70R14, 831 rev/mi. and cut the vehicle rpm by 10% at any given speed. Thus an indicated 65 mph would be an actual 71 mph. This could defer the MG1 rpm control law but still be subject to the increased tire drag.

Bob Wilson

I stayed on topic, "Optimal Tire Pressure" and ignored the rest. Perhaps you might start a thread on tailgating.

Posted on the sidewalls is the maximum pressure. For my Sumitomo T4s that pressure is 51 psi. BTW, there is an essay by the Tire Rack:

"Disadvantages of Overinflation

An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear. Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher and transmit more noise into its interior. . . . "

This is something folks who live with bad roads should consider. Fortunately, North Alabama is blessed with good roads that seldom have potholes. With good roads and a morning commute that seldom exceeds 55 mph, I run my tires at their maximum pressure rating, 51 psi front and

49 psi rear.

". . . However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures."

The reduced rolling resistance pays at the pump but the steering and cornering stability is especially nice. I regularly take turns and curves at +5 to +10 mph over what other vehicles can handle. For example, one of my favorite curves is at:

34 42' 31.00" -86 40' 11.05" (use Google maps)

This curve is rated at 45 mph for most cars. But I enter it at 55-60 mph on cruise control and don't blink an eye. Tailgaters often wind up backing off quite quickly because they have to.

Bob Wilson

Why? You're all about performance and mileage, and are happy to tell people to PUMP 'ER UP! to get better mileage.

If you're all about getting better mileage, then let's get better mileage. Pump those tires WAY up (and enjoy crisper handling and steering as a bonus!)--but why stop there? Go ahead and tailgate those trucks to within half an inch.

I mean, if you want to get good gas mileage and all.

In fact, pumping the tires up is small potatoes in the gas mileage game compared to tailgating trucks on the freeway.

On highly overinflated tires?

My fear is that people will actually believe you and try this crap that you're trolling.

Not needed. I like to follow trucks at a safe distance, +200 ft., using cruise control. The other traffic will smoothly pass me and the truck without a problem.

I drive an NHW11, 2003 Prius and have a recorded average of 52.6 MPG for

40,000 miles:

You may want to visit CleanMPG.com to discuss this technique.

No, just the maximum side wall pressure.

Actually, I prefer folks to give it a try. You can always go up to the maximum sidewall pressure and take a little test drive. If you don't like it, pull over and let the air out. Belief has nothing to do with it.

Bob Wilson

But you could get better.

But far less risky than tailgating trucks at a few feet or inches.

Those familiar with "Solo 2" autocross (timed runs through pylons), will recognize that increasing street car tire inflation pressure is one of the very first things (after cleaning out the trunk) done to improve handling in order to reduce times.

They also recognize that such changes are for the track, and they change the inflation pressure back before driving home and around town.