Tires and pressure

Alot being said lately about saving fuel by increasing your vehicles' tire pressure...
Probably true, if you have let your tires go (have you checked your tire pressures this month?), it may be time to check them. It's hard to tell if you have 35psi, or 20psi, just by glancing at a tire. Truckers are known to knock thier tires with a hammer at truck stops; the tires rings at a certain frequency if ok, a lower-pitched "thud" if one is going flat.
Get a good tire guage...
By keeping your tires properly inflated, you are probably saving not only gasoline, but maybe your life!
A flat tire builds up heat, starts smoking, then slowly thrashes itself into pieces. This failure is a function of heat, which is proportional to vehicle speed.
Lets say your tire goes flat overnight. You get in your car, drive off, and take relatively slow speeds through the subs (less than 45MPH) on your way to the highway. At this point, you are still clueless to any tire issues.
Now you enter the freeway, and speed up to 65MPH. You begin to settle in, but behind you, your fellow motorists are starting to see light smoke coming from your car!
It takes a few miles before you start to sense a problem, usually a strange noise, followed by increasingly thicker smoke, as seen in your rear-view mirror.
In just another mile or so, your tire lets go completely, just when you and your vehicle are at high speed, and surrounded by other motorists!
It's preferable to loose a front tire at speed. You can still steer the vehicle, because the rear of the car remains stable.
If you loose a rear tire, the rear end becomes unstable, and you will be sawing the wheel to keep control. This is a hard concept to get across to customers at the tire shop ("Why should I put my new tires on the back? I steer with the front!") but it's the correct procedure.
How can you tell if your tire is going flat?
If it's a front tire, it will pull the steering wheel in the direction of the low pressure tire (RF tire loosing air, steering wheel will tend to turn right) because the right tire has higher road drag ( theres' that MPG thing again) compared to the other tire.
A low tire on the rear is harder to feel on the road. If your LR tire is low, you may not feel it until you hit a hard right turn. The bad tire has very little sidewall strength, therefore it slides easily! (We used to pump our tires to 50PSI at the Autocross to strengthen the sidewall, therefore giving better grip. Joey Chitwood would inflate his stuntcars' standard roadcar tires to 100psi for his two-wheeled stunts!)
A few months back, I was preparing a 2004 Porsche Carerra S (rear engine, 380HP, 3400lbs) for our used car lot, and I ran it up to 95MPH on PCH on a clear stretch. I hit a bump, and the rear end started slowly fishtailing, first left, then right, in one second intervalls (left right, tick-tock-tick-tock). I couldn't catch it with the steering, so I finally hit the brakes, which moved the weight forward and stopped the swaying!
When I got back to the shop,I checked the tire pressures, 38psi, 38psi, 40psi, 20psi! The left rear tire was low, and when I hit the bump, the tire became a spring and started bouncing and occilating. The right rear tire did bounce, but it didn't occilate because it's sidewall was stiff enough to transfer the "bump" into the suspension, where the shock absorber could do it's job.
I vowed never to test drive a single car without checking the tires first!
I also lost pressure on the rear tire of my Kawasaki once.
I was booking down the highway at 80MPH, and as I went into a long left hand turn, I realised that I was steering the bike to the right to go left!
Again, the sidewall was in a weakened state, rolled under the rim slightly, and adhesion was lost!
I dared not touch the brakes, so I just eased off the throttle, and over a two mile stretch, I drifted to a stop.
I found a large roofing nail in my rear tire sidewall, my neighbor was a contractor, and his truck would leave a trail of nails and tacks where ever it would go!
Proper tire inflation is also critical on a wet road.
At 60MPH, a tire at 35psi has acceptable traction because the round-ness of the tire gives the water on the road a short path (through the tires' grooves) as it passes into the front of the tire, and out the back.
The same tire at 30psi is a little less round, the contact area goes up, and the waters' path becomes longer through the grooves. There is a higher volume of water in the grooves of your tires, making hydroplaning possible at a lower speed.
At 20psi, the tire is no longer round, and the contact patch is pushed into a concave shape by the road friction. The water on the road now enters the front grooves of the tire, pools inside the contact patch, and lifts the tire right off the road!
So ultimately, to avoid big trouble, and possibly save a few gallons of gas, get a good tire guage (not a $3.00 special, spend some money!) and keep your tires inflated correctly, for all the right reasons...