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  #1  
Old 01-10-2005, 02:07 PM
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Real Name: Pete
Location: Dunwoody Ga
Hybrids: 2005 Honda Civic Hybrid
Posts: 22
Default Tire pressure does effect MPG - How much is too much ?

I'm at about 2000 miles on an '05 Auto HCH. My MPG has remained around 40-41. I do mostly to/from work (13 miles) stop and go plus short spurts on the hi-way and have several hills to deal with which makes it difficult to increase my MPG. Before the last tank I increased the tire pressure from 30 to 35 psi. The mileage on this tank (180 or so miles) is hovering between 47 and 48 MPG (per the guage, which I believe to be a bit overstated). I hesitate to go much higher being concerned with tire wear and traction - mostly traction. I hear of folks at 40 psi and above and wonder how tire wear and traction are effected ??
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  #2  
Old 01-10-2005, 02:25 PM
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Real Name: Larry S. Singleton
Location: Phoenix, AZ
Hybrids: 2007 TCH and Loving It !
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That is a raging debate (has been forever) and I think there is no solid answer. Everyone has their opinions, here are mine:

1. Honda engineers have a reason for recommending 30 PSI. I do not know for sure what that is, but it's probably the best "compromise" for merging traction, tire longevity, and safety.
2. Higher PSI will affect traction to a degree, and will affect tire wear to a degree. Personally, living in Phoenix AZ (home of 320+ days of Sun) I am not concerned with traction problems. I do not drive my HCH like a sports car, and wet weather rarely affects us at all. Tire wear is not that much of a problem if I rotate the tires enough and these tires are pretty cheap to replace (at $65 each) if the wear is uneven enough to make me replace the tire after 30 to 40 thousand miles.
3. The main thing I have to worry about in overfilling tires is HEAT. It gets HOT in Phoenix, and if I over-inflate to say 40 PSI on a tire rated at 44 PSI max, the heat expansion will take that PSI up over 50 on the hottest days if I have driven on the highway for a few miles - I saw that last year in August. So I need to be careful in Summer to keep it about 35-38. I have braved 40 PSI this Winter and have seen no problems.
4. I inflated down to 32 PSI on my recent trip to Texas, where I knew I had a loaded car and would be driving in snow/rain/ice on occasion. I sacrificed max MPG for max safety.

So each person should consider their own circumstances and base their PSI on how their life is. I can generally over inflate a little in the Winter and I have to be more careful in the Summer.
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  #3  
Old 01-10-2005, 06:16 PM
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Real Name: Wayne Gerdes
Location: Northern Illinois
Posts: 2,567
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Hi RoyalF:

___ Start here … Tire Pressures

___Always fill cold (whatever the ambient is) and expect a 1 # increase for every 10 degree increase above ambient or straight tire heating due to the drive. Burst pressures of today’s tires are well over 100 #’s so unless you are really crazy, you won’t blow them out. In fact, over inflated tires produce far less heat then the same tire when under inflated. Longevity will improve tremendously. The 30,000 mile Bridgestone RE92’s when inflated per the Insight manual/door sticker last just about that amount of time. When inflated to 50 +, there are those that still have not replaced them and are well south of 100,000 miles now. The Corolla’s GY Integrity’s lost 5-6/32 over her first 30,000 miles at < 40 #’s and just 3-4/32 over last 40,000 miles at 50 – 52 #’s if that helps?

___To much? When I drove for the record distance on a single tank last June and repeated it back and forth to work last September, I jacked them up to 60 #’s and haven’t ever looked back.

___Increasing tire pressures improve FE, tire longevity, and turn in. What higher pressures hinder is stopping distances and ride comfort.

___An HCH’s minimum would be 50 #’s if you are looking to maximize your FE. If it were mine, I would be running 52 - 55 #’s myself.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

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  #4  
Old 01-11-2005, 07:25 AM
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Real Name: Larry S. Singleton
Location: Phoenix, AZ
Hybrids: 2007 TCH and Loving It !
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Default Do not overinflate HCH tires due to safety

Quote:
___An HCH’s minimum would be 50 #’s if you are looking to maximize your FE. If it were mine, I would be running 52 - 55 #’s myself.
Wayne, the Max PSI on the HCH OEM Bridgestones is 44 PSI. Running above that in the hot summer is dangerous business, risking a blowout due to overinflation.

Any HCH owner who overinflates their tires that much is risking safety for a couple of extra MPG - I would say that's not a good tradeoff.

I can safely run mine at 44 PSI in the winter, but in the summer, the heat drives the inflation pressure up like a hot air balloon. Like I said, I checked them after a short highway drive and they were up over 50 PSI.

Last edited by Jason; 01-11-2005 at 01:43 PM. Reason: Please enclose quotes in [ quote ][ /quote ] tags. (no spaces)
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  #5  
Old 01-11-2005, 05:30 PM
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Real Name: Wayne Gerdes
Location: Northern Illinois
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Hi Larsb:

___You are making statements that do not hold true in the real world in the least. Burst pressures as stated above are in the neighborhood of 100 - 125 #’s. You will never reach that unless your car was on fire. Guess what the Insight’ers Bridgestone RE92’s and HCH’s Bridgestone B381’s (many HCH’s include these as OEM stock) MAX sidewall state? 44 #’s as most any other tires do. Do you know how many millions of miles have been driven in Insight’s with the RE92’s and HCH’s B381’s at 50 + #’s? Just to let you know, there are quite a few Insight’s and HCH’s in the desert southwest besides your own.

___You can run whatever pressures you want but you will never reach the FE that your automobile is capable of by running at the std. as listed inside your front door or by running max sidewall of 44 #’s. Which you decide to choose is up to you but bringing up the safety card when you are tooling down the road with 2 children at 70 + mph is far more dangerous imho.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

.



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  #6  
Old 01-12-2005, 08:45 AM
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Real Name: Larry S. Singleton
Location: Phoenix, AZ
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Quote:
Originally Posted by xcel
..... but bringing up the safety card when you are tooling down the road with 2 children at 70 + mph is far more dangerous imho.___Good Luck ___Wayne R. Gerdes ___Hunt Club Farms Landscaping Ltd.___waynegerdes@earthlink.net
Are you implying that driving 70 + mph in a modern car is unsafe? Or just with overinflated tires? I would never put my kids in danger unnecessarily. But when you are driving on a freeway trip of 14 hours plus and you are accomplishing it in one day and the speed limits are 75 mph on the Interstate, you do what you have to do.

I had a blowout in my Chevy Avalanche pulling a U-Haul at 70 mph and it was not a lot of fun. When I was a kid, my crazy stepdad had a front blowout going 70+ and almost killed us by slamming on the brakes.

So I am justifiably a little paranoid about blowouts.

Because of your "guru status" Wayne, I will indeed investigate the possibility of running at higher pressures, but I admit when I checked my pressures on a not so hot August day and saw an extra 10 PSI in my tires, I got a little concerned. I want neither a blowout nor a tire which needs replacement early because of undue wear on the middle section, which CAN and DOES happen in long periods of overinflation - I found that out with my Avalanche tires.
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  #7  
Old 01-12-2005, 03:34 PM
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Real Name: Steve
Location: Atlanta, Ga
Hybrids: 2004 Civic CVT Hybrid
Posts: 1,796
Default Max sidewall cold pressure rating

In my own experience, all my cars since I began driving have been inflated to the max cold pressure rating of the tire. My father runs his the same as well.

I've never had a blow out which I would describe as a complete & sudden tire failure, but have picked up my share of nails & screws and one left a hole large enough to flatten it in less than 30 seconds.
I've never had to replace a tire prematurely for running them on max pressure as long as the other tire maintenance points are not ignored..like rotation, balancing and alignment.

The only downside in my experience in Max rated sidewall pressure is a harder ride. I've dropped my pressure to the suggested 32PSI a couple of times but it feels like I'm slogging through wet sand and I drop 7-10MPG.

Here in Atlanta we don't usually get very much wintery mix, but in the case of a rare ice or snow storm I'd likely drop it off of the Max setting a little for that paticular trip.

.

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT

Last edited by Hot_Georgia_2004; 01-12-2005 at 03:41 PM.
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  #8  
Old 01-12-2005, 04:47 PM
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Real Name: Wayne Gerdes
Location: Northern Illinois
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Hi Lars-ss:

Quote:
Originally Posted by lars-ss
Are you implying that driving 70 + mph in a modern car is unsafe? Or just with overinflated tires? I would never put my kids in danger unnecessarily. But when you are driving on a freeway trip of 14 hours plus and you are accomplishing it in one day and the speed limits are 75 mph on the Interstate, you do what you have to do.
___I am not saying over-inflated tires are un-safe at all.

___The proof of higher speeds being less safe can be proven with simple physics … Besides the increased emissions, increased fuel consumption, higher wear and tear by traveling at higher speeds, there is the safety issue.

Reaction Distance = Reaction Time * Speed

Speed is ~ constant during this period because you haven’t touched the brakes yet but you might have slightly pulled your foot from the accelerator? Perception + Reaction time = ~ 1.5 seconds for most.

Braking Distance = [(Initial Speed (ft/s))**2]/(2* Deceleration)

Deceleration is ~ constant during the severe deceleration phase at ~ 30 ft/Sec**2 using C&D’s 70 - 0 mph braking distance of 181 ft for our Little Beauty’s. This was from C&D’s January 2000 Insight Road Test.

88 ft/sec = 60 mi/hr.

Reaction distance + Braking distance = Total distance traveled before stopping.

At 50 mph: Total braking distance = 110’ + 90’ = 200’
At 75 mph: Total braking distance = 165’ + 202’ = 365’

___That is > ½ a football field longer to stop from 75 mph!

___So two things … Would you rather hit something at 50 mph or less (or possibly not at all!) given you might actually have time for some braking or accident avoidance steering correction in any distance > 110’ before the collision or at 75 mph where you have probably already collided with whatever (165’ or less) before you had the chance to touch the brakes or make a correction? Remember that your perception + reaction + actual braking time at 75 is almost ½ a football field longer then at 50 mph. I am not even including the damaging impact energy that has to be dissipated! How many accidents could be avoided with that extra ½ football field of distance anyway?

___Someone else can check my math from the following:

http://www.sci-ed-ga.org/modules/dri...stigation2.pdf

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

.



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  #9  
Old 01-12-2005, 04:48 PM
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Real Name: Jason Siegel
Hybrids: 2004 Toyota Prius
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Now, now, boys. Lets not start a flame war! =)
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  #10  
Old 01-12-2005, 06:17 PM
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Real Name: Wayne Gerdes
Location: Northern Illinois
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Hi Lars-ss:
Quote:
Originally Posted by lars-ss
... I admit when I checked my pressures on a not so hot August day and saw an extra 10 PSI in my tires, I got a little concerned. I want neither a blowout nor a tire which needs replacement early because of undue wear on the middle section, which CAN and DOES happen in long periods of overinflation - I found that out with my Avalanche tires.
___In terms of wear, the Corolla LE w/ Auto had almost 30,000 on her GY Integrity’s with 35 #’s all around (the Prius II is OEM’ed with those) and was showing edge wear as well as total wear of 5 - 6/32 of 11/32nd’s when new. Within 3 - 5,000 miles of that discovery, I had them up to 50 #’s. Edge wear was stopped dead in its tracks and the family and I drove her for almost another 40,000 miles afterwards. I replaced them just 2 months ago at the 68,000 mile mark w/ 2 - 3/32nd’s left and even tread wear all the way across.

___To sum up the Integrity’s wear both before and after driving them to pressures beyond max sidewall, I had uneven edge wear and had lost ~ 5/32’nds of tread at the ~ 30,000 mile mark while running 35 #’s. Over the next 38,000 miles and running 50 #’s, I lost just 3/32nd’s tread wear, edge wear was halted, even tire wear across the entire face, and of course the much enjoyed increase in FE.

___The Insight’s Bridgestone RE92’s are only down 2-3/32nd’s of 9/32’nds after 36,000 miles from new and they are only a 30,000 - 35,000 mile tire to begin with! I just took the RE92’s back up to 55 #’s last night in fact due to the natural bleed off and temperature drops so make sure you check them often …

___Finally, although only a wish right now, I pushed the Continental’s on the Ranger P/U back up to 50 #’s last night from the ~ 44/44.5 #’s I found them at after almost 3 months of not paying attention. With our fluke 52 - 61 degree temps for my drive home tonight, I dropped under ½ tank (per the Fuel Gauge) at the 368 mile mark. Although it is not entirely linear given the extra ~ 1 - 1.5 gallon available at top off, I might actually hit 40 mpg’s on this tank? That is if the head winds are not to strong in the morning and afternoon temperatures do not sink as fast as forecasted. The only way this type of FE is possible is with the tires inflated to 6 #’s above Max Sidewall and the rest of the techniques and setups we all use. Our temperature fluke sure helped out tonight as well

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

.



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Old 01-12-2005, 06:17 PM
 
 
 
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