Wayne Gerdes, founder of hypermiling, started with a Honda Accord

[Pew]

High tire pressures resulting in better handling / braking seems counterintuitive. Tires are the first part of your suspension and allowing them to deflect around corners and during braking is what I presume the engineers accounted for when designing the car to handle safely.

I'm not sure why Wayne would put that down. The only benefit to high tire pressures is better rolling resistance and turn response. Otherwise higher tire pressures = less grip = less performance.

I personally do not like to put too much air in my tires. But to be very specific, the current tires on my car now has maximum psi on the sidewall as 51psi. So, I am sure that means the manufacturer is saying if that tire is filled up to 51psi "cold",it is ok to drive with it.

And remember when we drive the tire heats up and that 51 can quickly get to 55 or more. I am sure the tire manufacturer is aware of that.

It's not so much the blowout pressure; the problem lies in significantly reduced performance and a large NVH increase. All that extra NVH in overinflated tires is transferred directly to the suspension components. Any small imperfection that was soaked up by the tires at door sticker PSI is now significantly increased and transferred directly to the suspension as well as tramlining and bump steer are now more noticeable.

 

[RedSpider]

Wayne Gerdes' advice on psi for serious hypermiling; "MAX sidewall is what I would recommend for most as it is well within the safety limits of your car and tire and allows better Fuel Economy than the pressure listed in the driver’s side door."

I don't know what you guys think about this.

I think its pretty ignorant. Max sidewall pressure is the acceptable maximum pressure for that tire for its load rating, but not necessarily what is acceptable or safe for *any specific vehicle*. Those specific limits are in that cars owner's manual or the door jamb placard, and those numbers aren't just there for occupant comfort, the way some of these hypermile people act like on the forums when you run into them. That particular tire may fit a multitude of very different cars that may have significantly different weights. A tire that fits a 3800 lb car may work fine at 40psi, but that same tire at 40psi on a 2900 lb car would be grossly overinflated. Not a problem for that tire, its a problem for that CAR and how its going handle during braking and around curves with rock hard tires with a greatly reduced contact patch.

Grossly overinflating a tire can dramatically reduce the amount of footprint the tire has, which can create very unsafe braking and handling. Great gas mileage sure, but at what cost to safety. Overinflating a tire is defined by the vehicle manufacturer, NOT the tire maker's maximum sidewall pressure.

Do what you want, but you go sliding through a wet intersection in your Prius with your tires pumped up to 58psi and T-bone mom and pop killing them both, in this day and age don't go around thinking your grossly overinflated tires wont be used against you.....

 

[Dave9]

It doesn't have to be slower over the whole journey. You can coast towards a stop light and get green before you are there vs maintaining speed, braking and sitting there stopped for 30 seconds. In both cases you cross the intersection at the same time.

If there is traffic, that does often slow down the traffic. The coasting, makes the vehicle behind, cover less ground, and the vehicle behind that one, and so on, with many of them losing speed by applying brakes because they didn't expect to be coasting so long. The result in denser-city traffic is then that more vehicles don't make it past the prior stop light too, further backing up traffic.

 

[PetitFrereArmada]

I changed my oil this morning with 0W16 and a Fram TG 7317. I then went and got my tires balanced and rotated at Discount Tire and psi set to 45psi.

Oil change, Balance and Rotate tires and psi set to 45psi! Time to experiment more hypermiling...

Let me try a bit of hypermiling and see how that goes. I normally hypermile in city driving with 40psi (instead of oem 33psi), but this is my first time asking them to put 45psi in my tires. Let me try this for a month and see what I think.

I left Discount Tires to Costco to fill up my gas tank. I then went ahead drove around this morning running some errands. No freeway driving. All city driving with top posted speed limit of 50mph. This is my mpg so far.

 

[aquariuscsm]

But what do you consider overinflated?

Anything over what the door jamb says.

 

[Dave9]

Overinflation depends on the max load rating of the tire vs the load per axle of the vehicle. The higher the load rating of the tire vs the axle weight, the lower the point where a tire becomes overinflated.

I usually like to run about 10% higher than the door jamb sticker for stock size tires (closer to door jam for larger than stock tires), but never higher than the tire sidewall max PSI spec.

With modern vehicles, manufacturers are trying to eek out a little higher fuel economy and will spec closer to a good compromise (leaning towards fuel economy), but older vehicles sometimes even had too low a door jamb spec. For example '90s Ford Explorer with 26PSI, was way too low for a vehicle with a ~5000lb tow and (~ 800? lb) cargo rating. Those tow and cargo rating #'s may be off a bit but still, not off enough that 26PSI is going to cut it.

 

[Jetronic]

If there is traffic, that does often slow down the traffic. The coasting, makes the vehicle behind, cover less ground, and the vehicle behind that one, and so on, with many of them losing speed by applying brakes because they didn't expect to be coasting so long. The result in denser-city traffic is then that more vehicles don't make it past the prior stop light too, further backing up traffic.

Only if the coasting car can avoid the red light completely, which isn't a given. AND it starts from a speed greater than 0 if the light does turn green again in time.

 

[Slow Car Sport Mode]

It doesn't have to be slower over the whole journey. You can coast towards a stop light and get green before you are there vs maintaining speed, braking and sitting there stopped for 30 seconds. In both cases you cross the intersection at the same time.

Leave enough gap someone will jump in and make you have to hit brakes anyway.

Unrelated to the above post: Honda Fits have really thin, narrow tires.