Over-Inflating Tires

The moose is right, what you would gain on the swings with higher tyre pressure you would lose on increased tyre wear. How sad would you feel seeing tyres thrown away with plenty of good rubber around the edges and none in the middle. Not to mention the potential to sail off into the wilderness on a wet bend with a bit of a ripple in it.

Sure, solid rubber tires give even less rolling resistance - you know, the kind they used before inflatable tires were invented. So long as speed is kept to minimum traction loss is of little concern. Especially in moving heavy loads the less give in the tire there is the more economical it is to roll the load. That's why they use solid steel wheels on trains.

yes.. extremely rigid tires might be nice, but i think they would be like driving a train off the rails.. not so good in the corners and rainy conditions

I just bought a set of Yokohama ENVigor tires that are rated for low rolling resistance (as well as very high dry, wet, snow and ice traction) that are rated for 51 PSIG cold fill pressure. I am running them at 45 PSIG and have NO choppiness in ride (they are original equipment size on a Camry Hybrid) and have experienced both very high accelerating traction (both the engine AND the electric drives are active with almost 300 BHP being applied the front wheels that USED to trigger the traction control with some tire chirping) and tight turning traction- turns at over 10 MPH faster than before with no squeal).

I recently took a trip of over 1100 miles- 1000 on expressways- and got over 2 MPG better than with the original equipment tires previously mounted. Based on these results, I intend to take pressure up to 50 PSIG for next level ride checkout and mileage check.

IF the tires are designed for high pressure, use it.

While these tires are in fact designed for high inflation pressure, remember that the pressure you use must balance against the actual weight of the vehicle they are mounted on. If your inflation pressure is greater than needed for the weight of the automobile, the tires will still wear to the center.

By all means, try using the higher inflation pressure, but check the wear across the tread frequently, and reduce the pressure if you see increased wear to the center. Marginally better fuel economy is not worth ruining an expensive set of tires.

To clarify, I am not advocating over-inflating the tires. I wanted to confirm my intuition that over-inflation will indeed improve gas mileage.

Based on the comments above, I believe its right. Increasing the hardness by over-inflating does indeed help with the gas mileage.

Now to continue the thread, can I fill the tires partially with water, properly inflate the tires (to save on the tread) and still gain gas mileage (now I am ignoring some of the added weight for the purposes of discussion).

Partially filling the tires with water will not do you a bit of good, as the tires will still have the flexibility imparted by the compressed air above the water. All you will succeed in doing will be do increase the un-sprung weight of the vehicle, with a subsequent loss of performance.

It is worth pointing out that it used to be a common practice to partially fill farm tractor tires with water, the added weight giving an increase in traction on the uncertain surfaces which they so often used on.

Now, you might consider filling the tires completely with water, pressurized to the appropriate level. However, not only will you add considerable weight, because water in incompressible, you will also drastically degrade handling performance, and increase the chance of punctures. It will also probably shorten the tread wear life.

This is called "Hydroinflation" and is common (as stated by DrMoose) in agricultural equipment, as well as heavy construction equipment. Calcium Chloride (CaCl₂) mixed with water is the usual fill.

Your mileage will suffer, but traction will be improved.

I drive a Volkswagen Passat turbo diesel, and I always inflate the tyres to the maximum allowed by the tyre manufacturer - 44 psi - rather than to the setting suggested by the vehicle manufacturer - 32 psi.

This saves me approximately 3 - 5 miles per gallon.

Yes there is a slight extra noise transmission on cats eyes and other bumps in the road, but well worth it for the free rolling and the fuel saving.

I have not noticed any loss or cornering capability at legal speeds.

There is a remarkably large difference in rolling resistance of tires when inflated to their designed pressures. The range in production tires for cars is from .006 to .014 -- more than a 1:2 difference.

Tires are highly engineered products, and even with tires inflated to the same pressure, rolling resistance can vary dramatically. The rubber compound has as much effect as inflation pressure (within normal inflation pressure range), because it it the rubber that has hysteresis, absorbing the energy of flexing. In a "perfect" idealized tire, the energy required to flex the tire at the front of the contact patch would be returned to help turn the tire at the rear of the contact patch. In practice this does not happen, with the wasted energy showing up as heat.

Other aspects of construction, such as tread thickness and design, incorporation of various additional components (such as "layer") sidewall thinness and flexibility, also effect rolling resistance.

For any tire installed on a given car with a given load, there is an optimum inflation pressure that results in even wear across the tread. There are many tires that when inflated to 40 psi have greater rolling resistance than another type inflated to 35 psi. So, probably best to buy low rr tires, and inflate them to something close to the manufacturer's recommendation. Unfortunately, many people have complained of low lifetimes of low rr tires: even 20,000 or 30,000 miles when you would expect 50,000 or 60,000.

Adding water to a tire will increase rolling resistance, because of the frictional drag of endlessly stirring the water, in addition to carrying around excess weight. It would also make suspension control more difficult and costly. Only on very low speed vehicles (i.e., tractors) does water added for weight make any sense.

Your post brings a question re: production tires.

How about a special purpose tire, such as passenger aircraft? Are they produced, and matched into sets, or is manufacturing more precise/strict? Is it enough of an issue that it is considered?

And to a lesser degree, high performance auto tires (e.g., NASCAR). Is rolling resistance a large enough issue to match their characteristics as a set? Maybe match port/starboard side tires as a set?

My daily driver is a 2007 Ford Focus wagon. The manual boasted 35 MPG highway but I never achieved better than 32 MPG even with modification of my driving habits.

I commute 136 miles round trip per day and really wanted to get closer to the advertised 35 MPG.

I always kept my tires inflated to 32 PSI but noted that the small print on the sidewall of the tire rated max pressure at 42 PSI.

Realizing that inflating the tires to 42 PSI would probably give a poor quality ride, I compromised on 38 PSI.

I now consistently get 34 MPG and the ride has not been effected to any large degree.

DISCLAIMER - I do deflate the tires to 32 PSI during the winter months due to the snow pack on most of our roads out here and occasional slick spots everywhere.

Driving in the rain or on wet roads at 38 PSI requires modification of your driving habits as well.

As MoronicBumble noted, tires are engineered products.

I'll add that there are tires designed for a softer ride and tires designed for a stiffer more responsive ride. There are also tires designed for extra traction. Each of these will have its own inflation recommendations.

One thing to note is that the rims often force one towards a specific tire type. Many sports cars have larger rims that take lower profile tires. This reduces the flexing of the tire giving better grip to the road. It also stiffens the ride making it bumpier and rougher.

Luxury sedans, mini vans and the like usually have smaller rims with larger tire profiles. This cushions the ride and the passengers will not feel road roughness as they would with a the low profile tires.

When you buy the car you should know what type of ride you want and how the rims and tires will affect the ride.

Dr Moose and MoronicBumble have also mentioned that other parts of a tire affect the handling and ride as well, wall thickness and flexibility, etc.

The low profile is also an attempt to lower the weight of the tire. In the suspension system of a car, the wheel, the axle, brake system, ball joints and other suspension parts all move in relation to the car. In order to absorb bumps and dips in the road, the wheel needs to move quickly and responsively. A heavy wheel assembly cannot respond quickly to road imperfections, a light wheel assembly can. This is especially important in sports car suspensions. Every attempt is made to lighten the wheel assembly. This includes using magnesium or aluminum rims and minimizing the size of spindles, brake rotors, ball joints and linkage. If you add water to a tire you are adding extra weight, thus nullifying the efforts the car manufacturer made in the first place!

Heed Dr Moose's warning about matching the tire pressure to the weight (including payload) of the car. If you carry a heavy load, you may need to increase tire pressure. For short trips this may be more effort than it's worth. If you add a camper/cap to a pickup, you will probably want to increase the rear tire pressure. The best thing to do is to monitor the tread wear every 1000 miles or so. If you notice wear in the middle, lower the pressure. Wear to the outside means you need to increase pressure.

There is "over inflation" and "over inflation"......

If your car manufacturer recommends says 32PSI with max driver and passenger and say 44PSI with the family and luggage, then 44PSI is NOT over inflating.

I have always used this higher recommended pressure on my cars for probably 30 years or more...

I have never worn a tyre in the middle.

I always have good MPG.

I have always had remarkable tyre life with 100,000 miles normal on many different models of car.....not all though, but always far more than others achieve....

I have had only two punctures in all those years.

I pull heavy trailers very often.

I check the tyres temperature when loaded, if any one tyre goes above my body temp I increase the pressure slightly. (check with the back of your hand against the tyre!) till the tyre runs cooler.

Overheated tyres burst.....better a bit too much pressure than hot liquid rubber....

I am not a slow driver when its safe and legal to drive fast.....in Germany we have no blanket limits.