Author Topic: Tyre pressure V Fuel consumption.  (Read 5358 times)

Expatman

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Re: Tyre pressure V Fuel consumption.
« Reply #15 on: February 18, 2023, 02:51:18 PM »
Being risk averse I always stick to the manufacturer recommended pressures.

Yes, me to... only which one?
On my tire pressure sticker I see this:
~ 160 km/h/100 mph : 2.4 bar/35 psi - 2.3 bar/33 psi
160 km/h/100 mph ~ : 2.7 bar/39 psi - 2.6 bar/38 psi

If 39 - 38 is safe above 100 mph then who am I to think that pressure is not safe below 100 mph as well?
I even dare to add 1 or 2 psi extra!
I have been driving various cars for at least 10 years with tire pressures about 4 or 5 psi higher than recommended.
Handling and comfort still fine and the effect? See my (measured) fuel consumption... See also Kenneve's opening post.
So you know better than qualified experienced engineers at Honda development? They will have conducted extensive trials to quote the optimum tyre pressures for safety, comfort, longevity, roadholding and MPG so it surely is sensible to follow their recommendations.

Jazzik

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Re: Tyre pressure V Fuel consumption.
« Reply #16 on: February 18, 2023, 03:32:47 PM »
I could be wrong but I thought the higher pressures were for heavier loads ?

Must check the sticker ....... tomorrow

I did check it this morning (just to be even more sure... :D) The pressures I mentioned are for higher speed
For heavier loads the pressures are even higher.
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Jazzik

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Re: Tyre pressure V Fuel consumption.
« Reply #17 on: February 18, 2023, 03:47:11 PM »
So you know better than qualified experienced engineers at Honda development? They will have conducted extensive trials to quote the optimum tyre pressures for safety, comfort, longevity, roadholding and MPG so it surely is sensible to follow their recommendations.

No, I don't know better and I follow their recommendations. But they give two recommendations, of which I choose the one with the highest pressure. If that (a bit) higher pressure is fine above 100 mph can someone explain to me why it wouldn't be below 100 mph...?
I have not noticed any special or abnormal tire wear for at least 10 years. The handling and comfort are fine and: MORE ECONOMICAL.
If nothing goes right, go left!

Pine

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Re: Tyre pressure V Fuel consumption.
« Reply #18 on: February 18, 2023, 05:01:47 PM »
Pressures are increased at higher speeds to combat the centrifugal forces pushing the centre of the tyre tread outwards and also to increase the stiffness of the sidewalls to reduce flexing in the tyre.
In both my Mk1 and Mk2 Jazz I had uneven tyre wear of the front tyres which was much improved by putting in an extra 2psi.

Marco1979

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Re: Tyre pressure V Fuel consumption.
« Reply #19 on: February 18, 2023, 06:47:36 PM »
A tire tends to change shape (wider, sides pressed sideways a bit) at the bottom side. Less pressure is more deformation.

At higher speeds, tires rotate faster and deformation and returning to the original shape occurs many times per second. So: lower pressure -> more deformation -> more heat gain and rolling resistance -> more increase in pressure.

Pressure is measured at standstill with cool tires. When driving at high speeds, a low pressure tire might heat up much more and have a higher pressure when compared to a tire with higher pressure at standstill. Sounds counter intuitive…

Jazzik

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Re: Tyre pressure V Fuel consumption.
« Reply #20 on: February 18, 2023, 07:01:17 PM »
I wonder why people would be afraid to increase the tire pressure slightly.
For the Jazz I read a standard pressure plus a (slightly) increased for heavy loads and in addition a little more increased for high speeds.
Toyota gives a standard pressure plus a so-called "ECO tire pressure" for many models. You guessed it: the ECO pressure is about 3 to 4 psi higher than the standard pressure.
This way they endanger their customers in an ecological way... ;D

Edit: I found in the Mk3 forum this reference to recommended tyre pressure:
Most manufacturers do not state a minimum tyre pressure, per se. The recommended value is the amount of air that a tyre needs for a car to handle properly and safely, and anything under that value is not recommended,...

Source: https://www.pirelli.com/tyres/en-gb/car/driving-and-tyre-tips/how-to-read/recommended-tyre-pressure

My conclusion: the recommended tyre pressure is not necessarily the optimum tyre pressure...
« Last Edit: February 19, 2023, 11:09:23 AM by Jazzik »
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Lord Voltermore

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Re: Tyre pressure V Fuel consumption.
« Reply #21 on: February 19, 2023, 11:19:14 AM »
The doorjamb figures appear to incorporate 'comfort' and 'eco pressures  without actually saying so.
  Less than 160 kph  will include rough roads, so a compromise is made for this.   You shouldnt lower pressure even more. . 
Over  160kph will tend to be  be on relatively smooth motorways etc where high pressures have big advantages with little affect on ride.     If you are happy with the result I see no reason why you cannot always  run higher pressures if you want to.  The specifications allow for quite big increases. 
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Pine

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Re: Tyre pressure V Fuel consumption.
« Reply #22 on: February 19, 2023, 12:13:09 PM »
160kph is 100mph.  I won't be exceeding that speed in a Jazz or any car.  I will keep to my normal habit of using the manufacturers settings plus 2psi.  Never had a problem with this although on one car with low profile tyres it noticeably increased the harshness of the ride. 

Jazzik

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Re: Tyre pressure V Fuel consumption.
« Reply #23 on: February 19, 2023, 12:15:50 PM »
....where high pressures have big advantages with little affect on ride.     If you are happy with the result I see no reason why you cannot always  run higher pressures if you want to.  The specifications allow for quite big increases.

I must confess that our previous car, Yaris Hybrid on 195/50R16, had a recommended tire pressure of 32 psi in the front and 29 psi in the rear.
We drove with 40.5 psi front and 38 psi rear for four and a half years. Advised by the dealer.
Wear pattern completely even, no extra wear in the middle of the tyres. Handling and comfort
If nothing goes right, go left!

embee

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Re: Tyre pressure V Fuel consumption.
« Reply #24 on: February 19, 2023, 01:11:01 PM »
Tyre pressures are not a specifically defined value, you can vary it to suit your preferences within a reasonable range. It's one of those things where manufacturers have to be fairly specific simply to satisfy the consumer, not many vehicle owners are particularly well versed in the subtleties of auto engineering. To keep it simple they usually give one or two values for different scenarios, lightly laden or fully loaded for example.

It's usually given as a compromise for the things discussed, rolling resistance (= heat build up and fuel economy), comfort, refinement, handling etc.

It's probably advisable not to run significantly lower pressures than the low recommended value, but you can try the higher value and if it suits you then fine. The max permissible pressures printed on the tyre sidewall are usually a structural consideration, but are usually well above anything you'd use in practice.

10% above the minimum recommended values will generally be fine, using 36 instead of 33 for example. If the plate says 38 for fully loaded then try that if you wish.

Of course it is your responsibility to ensure the vehicle is roadworthy and not use unsuitable pressures.

Jocko

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Re: Tyre pressure V Fuel consumption.
« Reply #25 on: February 19, 2023, 04:13:35 PM »
The data plate for my Fabia shows a 20% increase in front tyre pressures and a 50% increase in rear tyre pressures between standard and fully laden. 205/45 R16 tyres.


Bazzzer

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Re: Tyre pressure V Fuel consumption.
« Reply #26 on: February 20, 2023, 03:49:27 PM »
It's strange to see over 20 posts on the subject - presumably trying to squeeze out an extra mpg or two - but no mention of the effect on braking distance.

It's not good -

Kenneve

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Re: Tyre pressure V Fuel consumption.
« Reply #27 on: February 20, 2023, 04:08:47 PM »
It's strange to see over 20 posts on the subject - presumably trying to squeeze out an extra mpg or two - but no mention of the effect on braking distance.

It's not good -

I accept what you say, but that chart cannot possibly relate to the Jazz.
1.5 bar is equal to 21.7 psi and nobody in their right mind would run with such a low pressure, the handling of the the car would be terrible.
No, as others have said, stick with the Honda recommended pressures, which I suggest their engineers have developed over a number of years, for optimum fuel economy & braking etc,  which in effect is what I said in my original post.

embee

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Re: Tyre pressure V Fuel consumption.
« Reply #28 on: February 20, 2023, 04:53:06 PM »
It's strange to see over 20 posts on the subject - presumably trying to squeeze out an extra mpg or two - but no mention of the effect on braking distance.

It's not good -
While there seems to be some general consensus on the principle that reducing pressure gives shorter stopping distances in some situations, generally dry road (I have read through the full paper where that graph comes from), it is not an entirely clear conclusion, as per https://www.dekra-roadsafety.com/en/correct-tyre-pressure-is-crucial/
Quote - “Moreover, braking behavior is reversed on wet roads. There, a high pressure yields the shortest braking distances,” said Koch.

In summary, probably best not to depart too far from the recommended values.

(Probably a case of "search the interweb and ye shall find whatever answer ye doth want")

Jazzik

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Re: Tyre pressure V Fuel consumption.
« Reply #29 on: February 20, 2023, 05:20:54 PM »
I accept what you say, but that chart cannot possibly relate to the Jazz.
1.5 bar is equal to 21.7 psi and nobody in their right mind would run with such a low pressure, the handling of the the car would be terrible.

That chart probably relates to my fathers first car, a Morris Minor 1000 series III (1956) on 5.20-14 crossply tyres.
If nothing goes right, go left!

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