[Yves M.]

Hi,

Be carrefull with a lower PSI in front at highway speeds:

I would expect the car to behave badly in wells on the road or with a side wind. You could jump from one line to an other one on a highway without controling it.
Just like having radial tires in front and conventionnal in the back: Very dangerous (experienced many years back). The front will seem to go left and right while the back keeping steady e.g. for your example 35 front and 44 back

The reason for the lower rear pressure is for lower weight on them and to keep the contact area to the right shape. Along with comfort (no rear wheel travel)

 

[figgy]

Lower vs higher

Aren't you going to get more traction with the lower inflation, so that if you have the front lower than the back, if anything at all happens the rear might slip more than the front, which would in fact compensate a bit for the front slipping if at all during acceleration or oversteer types of situations?

Having the front lower PSI should reduce oversteer and front wheel drive issues relative to the back. Should it not?

Unless I recall wrongly, the factory specs have the front lower than the rear. The 35 / 44 would just increase the difference more than the factory difference of 35 / 38. It would be less a change than going with 50 psi on all the tires, but still give you benefit of reduced rolling resistance for the rear without losing traction in front for acceleration and steering.

I will have to go check the manual to verify the recommended differential.

 

[Yves M.]

Hi,

In the case of having a differential of almost 10 psi between the front and back, it is not slippping that I find dangerous but the fact that the sole of the tire and the metal rim can be in different planes.
The belt (tread) would stay at the same place on the road but the rim would move relative to the tire tread.

Rim moving left and right into the none moving tire in front but much less in the rear tires. The differential of looseness is the unpredictable effect

This would not happen with the higher pressure tire, not as much. And having the wheels that control direction doing that is hazardous on other that flat road and no wind.

The recommanded tire pressure is 38 in the front tires and 35 in the back tires. Hope I was clearer.

 

[figgy]

CORRECTION: I meant understeer would be reduced with the lower front pressure vs higher front pressure due to higher traction.

 

[figgy]

Rim planes??

I must be thick.

I have no idea what you are talking about wrt the rim plane and tire planes related to PSI.

Horizontal plane? Vertical plane?

:?: Huh? :?: What?

This isn't like underinflating, it is just creating a front/rear differential.

 

[Billy]

Mr. Figgy; you are correct in that on soft or slick surfaces, at low speed when you need traction, lower psi is better. For example, when the hard core Jeep guys go on sand dunes, they let most of the air out. But I think for freeway cruising, I want as MUCH psi as safety allows. More air makes the tires harder, and it ROLLS much easier. On the Insight there is about 13% more weight on the front wheels than the rears, thus the higher psi recommendation on the front. (38, 35) I purchased an expensive, accurate gauge, and check my tires often. I run 50 lbs on the front and 48 on the rear. billy......

 

[redgiki]

40 all-around

I've found that for winter driving, 40 on every corner seems adequate.

And the earlier poster assuming the rear tires are higher-inflated than the front: not correct. It's 38 front, 35 rear. IMHO, the lower rear PSI is more for minimizing noise, vibration, and harshness than anything else. The rear tires sing like a diva at higher pressures.

Regardless, I've tried it at below-recommended (32 at all corners -- smooth ride, decent traction in the snow, crappy gas mileage in the forties), fifty at each corner (too slippery on ice/snow to be of any use in the winter, though the MPG was reasonable), and finally settled on forty at every corner as a reasonable mid-range that gave me a moderate MPG increase without completely destroying my traction in the frequent snows here in Utah.

I'm satisfied there, but when it does snow, it's still a little too slippery for comfort -- it's a white-knuckle ride. Next winter, when we have a bit more money saved, it's snow tires for sure from November through March...

 

[Sparky5501]

Figgy,

With the stock tire pressure set-up of 38psi front 35psi rear the Insight exhibits slight "understeer" characteristics. Understeer is the term used when a vehicle has the tendancy to continue in a straight line while turning. Slight understeer is recommended as safe for highway driving situations.

Now, if you want to decrease the "understeer" characteristic of your vehicle there are two ways to do so with tire pressure. NOTE: When testing tire pressure to determine handeling characteristics you should always test in 2psi incriments. This makes for safe experimentation.

You can either increase the pressure in the tires that are lower or decrease the pressure in the tires that are higher.
Increasing the low will improve fuel efficiency, but handeling will become skittish. Decreasing the high will give better traction but fuel effiency will suffer. Equalizing the pressures with the mean average between the front and rear may be the way to go in your case. Remember to test in 2psi steps or you might be in for unexpected surprises, which would be very bad on public roadways.

 

[GreenHybrid]

How can one tell if PSI really makes much difference? When driving, there are so many variables that it has to be extremely hard to accurately compare.

 

[figgy]

Reproduction of test results

I have actually been surprised at how reproducible my test run is when I do it several times in a row.

I have the luck to have a stretch of road nearby which never has a car on it after the late afternoon, is many miles long, has a constant speed limit of 50, and has only gentle ups downs and curves.

I can drive the entire length in 5th and keep the same speed on different runs and come out with almost the identical mpg results if I do trips up and back a couple times within the same evening.

I have done that for cold weather mod testing such as blocking the radiator etc.

http://fungiart.com/insight

I have yet to try it for tire pressures.

Just be sure you do your tests when the temperature is constant and winds are down, and that there are not points where you need to slow down or accelerate which is where you introduce a lot of variation.

You'll be surprised at how well the numbers replicate. I sure was surprised myself.

 

[Guest]

Hi All:

___I have to wholeheartedly agree with Billy. Since the poor driving weather conditions are mostly behind us, most should compare tire pressures for what they do for us first and foremost. It increases fuel economy. With that, give me 50 + all around and I am a happy yet still inexperienced Insighter. I think most of us own Insight’s for its mileage capability and many of us accept the distinct probability of longer stopping distances at higher tire pressures. Handling capability supposedly improves with less sidewall flex at the expense of ride comfort, and tread wear life increases mightily at the higher pressures. In other words, fuel economy is still my number one reason to own an Insight and the exact reason I am now running 50 + #’s all around. The ride at any pressure will never be as palatable as that of just about any run of the mill std. or luxury 4 door sedan even running the same pressures so why fight it? Then again, I am the guy in the right lane going the limits and braking distances aren’t much of a concern given I can probably stop the Insight in < 150’ at the limits instead most of the faster drivers on the road who might achieve football field length stopping distances in the real world at 75 mph + :roll:

___GreenHybrid, you really have to drive a properly setup Insight 5-speed to know what one is all about. I didn’t know jack about its real world capabilities other then what I have read from many of the Insighter’s in this very thread. Billy’s brand new Insight and his partial cross country drive thread is the one that probably pushed me over the top. The Insight is such a capable mileage machine that you can feel and see gains and losses by doing something as siple as turning on the defroster, A/C, or headlights. Even more dramatic fuel economy changes can be seen while driving through a relatively large change in temperatures, change in wind directions, change in road conditions, slight change in elevation, change in traffic conditions, or even how much it is raining! Quantifying a mileage change in each and every condition would be a nightmare but since you have the ability to see fuel consumption in L/km or up to 150 mpg w/ 5 mpg increments, you can actually see the fuel economy changes in real time or over a mile or two distances as the trip segment FCD is showing you what is happening on that particular trip. I haven’t even considered driving with less then 50 #’s in the Bridgestone’s so I cannot tell you that the difference between 40 and 50#’s is exactly 5 mpg but just read this and many other threads about tire pressures here. The fuel economy posted is these Insighter’s real world fuel economy experiences. At varying pressures there is a difference and that is all I need to know.

___A quick example as to what can be seen in an Insight … Just tonight was my first full blown heavy rainstorm over my entire commute and even though I had a lean burn window similar to yesterdays in similar temperatures, my instantaneous was sagging by as much as 20 - 25 mpg on the various game gauges! In yesterday evenings commute home without the lights on (it was light out), I hit 99.1 mpg over 95.9 miles. This morning’s commute to work w/ the lights on showed 89.5 mpg over the same distance and just slightly cooler temperatures (5 - 8 degrees) with no wind. Tonight’s drive home in the rain, I am showing just 74.7 mpg over the same distance and I was actually traveling even slower since everyone else was given the heavier rain. There was maybe an extra 5 miles of traffic jam/stop and go driving tonight vs. last night but I hit the first traffic halt with the instantaneous showing 76.3 mpg at ~ the 40 mile mark and pulled into the driveway at ~ 74.7 mpg. It was the rain on the flats that killed it! You can feel it, you can see it, and there isn’t a darn thing you can do about it … All the while, EPA estimates were still surpassed Just not by as much as when conditions are perfect is all

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___[email:2fao7o57][email protected][/email:2fao7o57]

 

[figgy]

Right on about speed and stopping distance

You are absolutely on the money as far as safety is concerned. If you are driving for MPG, then you aren't flinging around corners and racing in the fast lane so your ability to recover from an emergency is much much greater than someone going 25 - 35 mph faster and driving near their car's limit.

It makes me think I'll go out and bump mine up to 50's. I am still a wee bid skittish, but after a while at 44, it makes it easier.

The flip side is that the safety concern that I have now is that when I am cruising to max my mpg, I have little ability to accelerate in an emergency. When I see that big rig about to run me down from behind, I'll wish I had tweeked for performance! :roll:

 

[figgy]

You are also dead on about how sensitive mpg can be when you are really stretching it for the high numbers. I can come home with a good trip mpg, and then do a 3 way turn in my sloped drive to park the car and blow my trip mileage in the process. Sometimes I wish I lived in flatland.

 

[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

 

[figgy]

Running over OEM, but differential different

This is diverging a bit, but still interesting to try out. As long as we are talking about having all the pressures over the OEM pressures now anyway, I don't think there should be any issue of too much weight for the tires or too much distortion of the tires. I can't imagine that with such a light car any pressure over 35-38 would cause a problem (given it's only about 450 lbs per wheel).

If we were going to run them well below the Honda specs, then I would worry that they would distort under load, or get hot from rolling at underinflation. Although I've seen some people run them as low as 28 in some posts, and dealers wrongly seem to put them at 32. My initial post (doh!) had the OEM for the front mistakenly at 35 not 38.

Anyway...

Once we are above the 35/38 pressures, I worry only about less contact patch with the road (danger if you need traction vs. mpg) and possible reduced puncture resistance with rough roads, ruts or curbs etc.

My original question has evolved in my mind to why it matters at all to have the rears LOWER than the front when we are talking all over 35/38 realm anyway.

:?: Thus, I wonder if it is wrong to slavishly follow the front/rear differential in pressure when we are running psi way over the 35/38 anyway and there should therefore be no issue of weight support or tire flexing. :?: 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?

It's the reverse of the Honda recommendation, but if the Honda recommendation is based upon weight handling and not having too low a pressure for weight, then it only matters that it is above 35. Is the Honda front/rear difference for other reasons than weight distribution of the car?

 

[MikeF]

Guillermo:

I use 195/55R14 Toyo T1-S summer tires on OEM rims and use 44/40 psi

Could you tell us how your mpg on these compares to the OEM? And how noisy are these tires? I'm thinking of putting nice summer tires on once the snow is behind us, and it looks like the two high-end options are the Toyos and the Dunlops in that size.

Thanks,
Mike

 

[Yves M.]

Hi Figgy,

Sorry that I had a hard time to explain. I do a better job in French which is my first language.

The simplest way I could retry to explain my previous post would be to use the extremes:
Difference between a "conventional" tire and a "radial" tire is the internal structure ...

The conventional is stiffer. When you turn the steering wheel (read turn the rim angle to the car), the tire belt under the tread will turn at the same time

The Radial tire is not as stiff. When you turn the steering wheel (therefore the rim into the wheel) the tire belt under the effort will turn (left-right) with a lag (compared to the conventional tire).
Meaning that the rim can go in a particular direction and the tire tread in not exactly the same direction

This applies similarely to having tires in the front with a big differential of pressure to the tires in the back. Your low psi front tires would make a situation like you are running radial in front and conventional in the back

Best thing like you wrote is to try it out, just be carefull.

 

[meow77]

You know what I'm really curious about is how the different tire sizes affect mileage.

So a 165 compared to a 175 185 195 and what are the fuel mileage loses on the larger tires...

I blew a 165 and it was a total pain getting a replacement!

 

[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: .