From the Tirerack Tech Info:
Disadvantages of Overinflation

An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear. Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher and transmit more noise into its interior. However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures.

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I have a fair amount of tire experience and it has been my experience that auto manufaturers set the tire pressure to what they determine to be the safest as far as handling is concerned. Tire manufacturers set the max psi at the safest they can be driven on without popping.

Obviously, one could go a pound or two in either direction and prob not experience any problems. But I have precious cargo and am willing to sacrifice that .6 mpg for the added insurance.

But thanks anyway for your "Wrong and perception" comments.

 

I appreciate you taking the time to share your method. FWIW, it wasn't overly complicated for me.

 

In the case of my stock tire combo on the '03 HCH, handling most definitely did improve, very significantly, at 42 psi instead of 30 (above that I saw diminishing returns). I also saw a large drop in rolling resistance (confirmed by faster coasting speeds on commonly driven hills) however I believe that is an unusual case. I observed that the higher pressure level for handling created a lot more noise, so Honda set the reccomended pressure lower to create a cushier ride, rather than optimizing the pressure for handling/performance. In the owner's manual it actually suggests setting pressure at 36psi for sustsined higher-speed driving--if higher pressure were inherently worse for handling, I doubt they would reccomend that.

I believe that this pressure was in effect the same as underinflating the tires -- stock pressure felt like driving on flat tires in other vehicles. They felt mushy, held poorly around corners, and had terrible steering response at stock pressure The reccomendation should have been higher (or a tire not designed for such high pressure should have been used).

On other vehicles I have experimented with changing the pressure, I have seen no improvement whatsoever in handling, and observable improvement in coasting speed, just increased noise/harsness levels. So for most vehicles I would agree. I don't know if the same benefits of inflation are possible on the '06 model year.

 

Sigh... I think people misinterpret our recommendations about raising tire pressure to mean "jack it way up there" to the 60+ range. That is *NOT* what we're on about. Auto manufacturers choose their tire pressure based on their perception of safety+comfort+stupid driver fudge factor. In other words, safety dictates that they don't seriously under-inflate. Therefore, they can recommend no less than ~30psi. On comfort, some tires *do* get exceptionally stiff at high pressure, while other suspensions just transmit too much road noise, so anything greater than about ~40psi is out as well. Finally, we come to the stupid driver fudge factor. How many cars, trucks, SUVs, whatevers have you driven that didn't have 32-35 as the recommended tire pressure? If you asked a ten-year old what tires should be pumped to, what answer will they give? Probably whatever's on their bike, which (at least when I had them) was.... you guessed it, 35psi. Thus, if Joe Schmuck doesn't read his user's manual (and what *average* driver does), odds are he'll go and pump to 35psi.

Now, wrt that Tirerack Tech note, they're right. The contact patch is reduced whenever the pressure goes up, TO A CERTAIN EXTENT. It's a diminishing returns problem, because no matter how much you pump that tire, the car weighs a substantial amount more, and you'll never overcome that. Likewise, despite the reduced contact patch, the added stiffness reduces contact deformation, which in turn enhances the tire's ability to channel water through the tread and maintain positive grip on the road. I believe their primary concern is folks driving at pressures exceeding sidewall spec. Tire manufacturers put a significant safety factor (a previous thread on GH suggested as much as 3x) on whatever the "Max pressure ##psi" stamp is, and I don't think anybody here would suggest you exceed that. Get too high, and yes, potholes become more of a problem, and you will get over-wearing of the center portion of the tread. Stay within spec, though, and modern steel-belt radials should remain constant across the entire tread.

Finally, a note on perception vs. actual handling. DT (Mike) has already commented on his observations in his '03, and my experience with the '06 is similar (one turn on my daily commute in particular used to feel sloppy even at 35mph, at higher pressure it's clean at 45). But perhaps a far stronger vote for going to sidewall spec instead of manufacturer spec comes from the law enforcement community. Just about every department will run at sidewall for the improved handling and fuel efficiency, and it's not uncommon for them to have their training cars at pressures *exceeding* sidewall as rookies learn how to maneuver their cars at high speed.

 

I have one turn on my commute where I try to keep the speeds up for momentum at the start of a hill. When I first got the car, tires were at 32 psi, and I squealed through the corner with sloppy handling. Frankly, it caught me off guard and I was alarmed at the poor handling.

After filling to 40 psi, I now make that corner with no squealing rubber and much better handling. Yes... same speeds... same angle of turn. I'm sold on the handling improvement.

As for the other characteristics? Well, I don't really know if it improved mileage. My car was breaking in and I was improving my driving skills while I was adjusting pressure, so I don't know which of the three contributed the most to my present mileage numbers.

On road noise, that was a definite disadvantage. It's loud enough that I can no longer reliably use the voice recognition feature for the NAVI.

So, I have one advantage (handling), one question mark (MPG), and one disadvantage. I believe my sidewall limit is 44, so I feel very safe with the 40 I have in there now. I'll stick with that!!!

 

I keep my tires at sidewall max. I slowly increased the pressure by a few lbs at a time, and each time my mpg went up another couple of miles. I barely noticed the change wrt noise. The pavement you run on makes a big difference with regard to noise. Also, I just finished a 150 mile trip with 2 bikes strapped on the back, luggage and all of the gear for myself and passenger in the trunk, going 68-73 mph with the cruise control, A/C set on 76, and on very hilly interstates. My mpg was 40.7 at the end of the 150 miles. I knew that the bikes were acting like giant parachutes on the back of the car, and with the CC and A/C going I knew my mpg would take a big hit (I've been averaging much higher).
So, try inflating the tires and be patient as your car is still new and your mpg should only get better with time and practice.

 

Okay, you guys convinced me to try it. I put the tires up to 41psi and reset the meter on the interstate this morning -- no a/c, ~60mph, ~10 miles. When I exited the freeway I had 60.7mpg at the stop light.

I know it won't last when I throw some city driving in there, but it's cool to see 60!

The week before I was averaging 40, but overall for the cars total mileage ~326 I have 36mpg. Should I put this tank in the database?

 

Yes, put it in the database. It is real information and you will be able to chart your progress. This tank is part of the whole experience. Congratulations on your 60mpg run!

 

Just a few add-ins;

1. I inflated my tires to 36 PSI and it makes a noticable consistent improvement in FE. Also, the HCH 2 is a very well designed car for road handling, so if there is some sort of handling loss at some PSI level, it isnt very noticeable, at least at 36 PSI. I cant tell a difference but I am not Johnny Rocket either.

2. I also agree that a quick accel and generous use of the IMA/electric motor up to cruising speed makes the most sense after 10 MPH is reached from a stop.

3. AS a sidenote, that tire pressure will fluctuate with temperature big time, at least here in Houston. I check and reinflate as needed each tank. Usually it overall drops 2 PSI per tank or thereabouts at most.

4. Kenny- you obviously have quite favorable driving conditions to achive your mileage and keep techniques to a simple minimum, but your experience seems to differe from the vast majority of posters here on the forum.

 

Last night on my way home from a Labor Day outing I had two bars left on the tank. No worries -- could be good for 200 miles. Within 10 minutes of driving I had no bars! With no gas station in sight I tried to keep it 55mph on the interstate -- trucks zooming buy... I made it about 5 miles to a gas station -- put in a hair under 11 gallons. So, I know I had plenty more room to go, but still a slightly harrowing experience.

Why do I bring this up? Because I am done with my first tank. It averaged 37.6 mpg. I am thinking for the first 200 miles I got 35 mpg and for the last 200 I got 40. This past week, I did up the tire pressure to 41 psi and, of course, my driving skills have improved. Since filling up last night and my morning commute I am averaging 44mpg.

I know it will only go up -- though I' still be thrilled to avg this overall.

Thanks all for your advice!

noflash