[Guillermo]

165/65R14 OEM tires hold very little volume of air so they require 38 PSI at the front and 35 at the rear because of the 57/43 weight distribution of the Insight. Reducing the front tire pressures will overheat the front tires and reduce the load carrying capacity to below OEM specs.
A softer front tire will improve straight line traction for accelerating and braking. But the tire sideways will flex too much so handling performance will suffer. The lateral forces will flex the tire so much that part of the sidewall with scrape the road and much of the tread will be lifted off the ground.
Go to an auto slalom race and talk to the guys there.
Chalk your sideways and do some skid pad work, that's the best way to optimise the tire pressures.
If you want much better grip and handling and don't mind using a little more fuel get some high performance summer tires.
I use 195/55R14 Toyo T1-S summer tires on OEM rims and use 44/40 psi

 

[Guillermo]

Last night I got back from a vacation visiting cousins in Miami, now I can answer some questions.

figgy wrote:

Why not run the rears at 50 psi for mpg and keep the fronts in the 40s for the extra traction for acceleration and steering?

The tires are an important part of the suspension design.
Changing the tire pressures changes some of the suspension characteristics of the car. All production cars are designed to UNDERSTEER because it's much easier to correct this situation, and there is less chance of injury and death if you crash straight into something then hit it sideways!

Using 50psi in the rear and 40 psi in the front will increase the chances that the car will OVERSTEER. This is not safe and should be avoided.

In a fast corner or an emergency collision avoidance situation there are 2 main reasons oversteer can happen when using much higher pressures in the rear tires compared to the fronts:
1) If both the front and rear tires hit even small bumps the rear end will bounce more then the front because of the higher tire pressures.
2) The rear contact patch would be much smaller then the front because of the higher pressures so the rears may have less grip then the front tires.

Tire review and tire width/fuel consumption:

I upgraded to 195/55R14 Toyo T1S ultra high performance summer only tires on my OEM rims. These tires are the exact same diameter as the OEM tires. These tires have about the same road noise at the OEM tires.
But the Acceleration, braking and cornering grip and comfort level is much improved over the OEM tires. These tires improve the feel of the car at all speeds and doesn't track the road like the OEM tires. I'm not exagerating when I say that just changing these tires makes it feel like i'm driving a different car! Feels like i'm driving a larger car!
Driving over deep puddles with the OEM tires can be scary as the car jerks to that side. With these tires under the same conditions provides a very different experience, it just channels the water and resists hydroplanning in a way that it eliminates the jerking feeling. I feel so much safer using these tires.

Fuel consumption is usually about 10% more with these tires.
But that just an oversimplified answer.
Fact is the fuel consumption increase seems to be less then 5% at low speeds and 15+% at high speeds.
These tires have a higher rolling resistance (stickier rubber compound) AND they increase aerodynamic drag (because they are wider).
These factors decrease the maximum speed at which lean burn can occur. This means the most noticeable increase in fuel consumption (almost 20%) occurs if you are comparing fuel consumptions near the lean burn threshold.
Driving fast or very fast (rarely in lean burn) fuel consumption seems to be 10% to 15% more.
At slow speeds (at a constant speed so increase tire weight is not a factor) the fuel consumption increase seems to be minumal maybe 5-10%.

Overall my average is about 10% increase in fuel consumption with these tires, and I still get better fuel efficiency then CVT insights :wink: .

 

[Guillermo]

See new post about tires where we will continue this discussion that has gone off topic...