where do the gains and losses start and stop with tire pressure? - Fuelly Forums

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Old 12-13-2009, 01:55 PM   #1
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where do the gains and losses start and stop with tire pressure?

ok so I went from 30psi all around today(due to wintery conditions, not expected again for a few weeks) to
32 front psi
36 back psi

the difference was from sorta sluggish, crunchy shifting, not really, super good grip, ........to this car is floating around and my gearbox is barely struggling to shift gears...

the car right away gets to a higher mph due to the lower rolling resistance, i can tell right away, my problem is when I approach 34psi front 38back, I loose handling on the highway bigtime

there for this qualifies as a mod that the cow and other would claim as increase speed of car and mpg(how did you word it)

my problem is that any higher on the psi (34/38) makes me have to slow down, downshift, ect around corners, here and there and everywhere, so I think ive found the sweet spot on this car... with these tires

and my one other thing that im wondering about is an increase to say 44psi+ 51 in the case of one of the other saturns on this board... actually beneficial to less rolling resistance in that wouldnt a wider, but hard tire gripping the surface induce this more so than a super hard(from excess psi) thinner tire gripping the surface, has anyone graphed this out.... I would think that finding a sweet spot and being able to handle better, slow down less, and downshift less sound outweigh the benfites of slowing down constantly and a horrid bumpy ride on top of that...

please I want to see this graphed... I think maybe the tires im running toyo spectrum,(which are low rolling resistance tires) may be better at this than other tires due to their wider profile regardless of the tires' size 195/60/15=7.75in wide on a toyo... vs 195/60/15 falkens which would be about 7.25 or less wide

and also the harder rubber in the toyo tires(made to last 80k+ miles) may create the false/psuedo/included result of higher rolling resistance even more so than a softer tire of the same width(probably only measurable by .4 of a mpg.. lol)/if all variables are the same

just something to think about for daily driving/safety of hypermilersss... I guess

I wonder if Car Company Engineers read this stuff?
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Old 12-13-2009, 02:10 PM   #2
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Quote:
Originally Posted by spotaneagle View Post
I wonder if Car Company Engineers read this stuff?
The engineers aren't involved in specifying the tire pressure, except to calculate the minimum pressure that won't result in liability lawsuits. Your car specifies 30psi?

If your handling has suffered, you definitely need to back off the pressure. More pressure has always made my vehicles more stable and given me far better handling, but the lightest vehicle I've ever had was a 2800 pound 1997 Pontiac Grand Am on 205/55-16s...and I didn't try increased pressure on that car until I had excessively soft tires that forced me to. I don't know how stiffer tires would have done with that pressure on that car.

Those excessively soft tires were Bridgestone Turanza T were made for long treadwear. Mine lasted 80,000 miles and still had plenty of tread left when the sidewalls got pretty cracked. They had a treadwear warranty that covered them for up to 6 years regardless of how many miles you put on them (note that nothing like that is sold anymore). I inflated them to the max 44psi and they went from sloppy handling to great handling, and never cracked as long as I had the car.

Personally, I wouldn't even tolerate compromised ride quality, let alone handling.
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Old 12-13-2009, 03:34 PM   #3
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its funny cause it depends what tires are on what car, AND THERE IS NO GUIDE TO THAT!, more like tire guy opinion then real world fact
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Old 12-13-2009, 03:41 PM   #4
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im saying the harder a tire is, the higher the rolling resistance is if all other variables are the same, fact or fiction?
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Old 12-13-2009, 04:05 PM   #5
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You may have to define "harder" better. Harder because its carcass is stiffer? Harder rubber compound in the tread and/or sidewall? Harder from being inflated more?
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Old 12-13-2009, 04:56 PM   #6
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harder as in the tread itself being harder a harder tire also including side wall is what I mean
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Old 12-13-2009, 05:57 PM   #7
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Quote:
Originally Posted by spotaneagle View Post
harder as in the tread itself being harder a harder tire also including side wall is what I mean
That's a great question. I haven't read anything that would indicate it either way, nor can I decide based on what I do know.

Harder tread: Less traction= less friction; less tread squirm.
Softer tread: More traction = less energy wasted sliding; easier tread squirm.
Which one wastes less energy, though?

Stiffer sidewall: Possibly less sidewall deformation
Softer sidewall: Sidewall deformation uses less energy. I think this one is clearer than tread compound. You're going to have the same sidewall deformation either way, might as well not waste more energy doing it.

You mentioned that it depends what tires are on what car. There's also size, pressure, road conditions, and far too many other variables that just can't be researched, measured, or predicted. It is for that reason that I really don't like the idea of trying too hard to choose tires for efficiency...instead, choose for variables that you CAN know, like treadlife, traction, price, and (if your car responds well to high pressure) maximum pressure rating.
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Old 12-13-2009, 10:17 PM   #8
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I've noticed that unbalanced tires will be smoother at low pressure, and bounce more at high pressure.
Vehicles with poor alignment will not track straight, and high tire pressure exaggerates this tendency.
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Old 12-14-2009, 12:21 AM   #9
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The Solstice with its 18X10 tires gets into the 30's with 28 psi, boost it to 44 psi and you get 28, take it down to 20 for snow covered roads and its back to 28 mpg.

The Focus with 16X8's is like driving a car with a sldge hammer under the seat when driven at 44 psi, its super harsh, take it down to 36 psi and its creamy. So far I have run the tire psi all over the map with the Focus and still got the same mpg, the Solstice is very sensitive to tire psi though.
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Old 12-14-2009, 04:38 AM   #10
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The lightest vehicle I've ever owned is my 81 Buick which weighs in at 3,283 pounds. I've never owned a FWD vehicle either. That being said, with nearly 20 years experience owning that car I have learned a thing or two about tires for it. It operates on P195/75 R14 tires. The tires say "max 35 PSI" on them. Inside the door jamb, it says the recommended pressures are 35 front, 35 rear. My experience on that car is that it has its best handling and even tread wear at 40 PSI. For many years my main consideration was not anything other than looks when I bought tires. I bought Goodyear Invicta GA(L) tires because they were availible with the wide double whitewall. I ran that car with those tires on it until about 2 years ago when my last set wore out, and I could no longer get them. When I did buy new tires I just bought some midrange priced name brand tires (I forget the brand right now) because the car doesn't go but maybe 2,000 miles/year, and if I ever restored the car I would want to put wide whitewalls on it again.

On my 74 and 86 Chevy pickups I always ran my tires at 40 PSI. On my 98 GMC Sierra I'm still working out what I need. I bought tires 2 load ranges higher than specified for that vehicle. The new tires are max 80 PSI. I have been running them at 70 PSI front/rear. Handling is still good but the jury is still out on the ride quality. I may end up going down to 65 PSI because sometimes I still think the ride is just a bit too harsh.

It all depends on your vehicle, the tires you have, and your personal tolerances for handling and ride quality. My recommendation is to start at the pressures specified in the door jamb, and work from there. In any event you should not be running a pressure that causes abnormal tire wear.
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