Tyre Pressures

[JazzandJag]

My neighbour has recently bought a new Jazz SE CVT and is concerned about  the tyre pressure recommendations on the plate on the drivers "B" pillar. These state that the same pressures of 33F and 32R are to be used for all loadings between driver only and five people plus luggage. For towing the rear pressure is to be increased to 38 psi. His concern is that in his long driving experience he has never before owned a car where the rear tyre pressures do not need to be increased for fully laden driving. The dealer was unable to explain the situation and referred the matter to Honda UK who also were unable to explain. They have referred the matter to Honda Japan and a response is awaited.

Does anyone here have any thoughts on the matter? For my part I have been happy to accept the recommended pressures at face value and have not adjusted the pressures when fully laden without any noticeable adverse effects on handling. I can't remember what the plate on my 2013 Mk2 Jazz recommended.

[mikebore]

I have noticed this and thought the same thoughts. Will be interested to see any replies.

The label is quite unambiguous (unlike some I have seen on some cars).

Very convenient.



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[culzean]

The tyre information plate is just that,  tyre sizes and pressures recommended by car manufacturers.

The weight in a car is spread over all four wheels,  and if extra rear weight is within the wheelbase of the car some of it will naturally transfer to front wheels,  and you could argue that if you brake with 4 or 5 people in the car you transfer more weight to front wheels, as you do going downhill,  so why not have more pressure in front as well,  the reason for increasing rear pressure only when a trailer is attached is that the weight is on the towbar, which is outside and to the rear of where the rear wheels touch the ground, so that weight is putting more load on rear tyres but actually taking weight off the front wheels because car is being pivoted around rear axle.  Like yourself I have never changed tyre pressure on any car just because I am carrying passengers or not,  what is the point of keep raising and lowering the pressure.   How many times do you have 5 people and luggage in a Jazz anyway compared to just the driver and no luggage ?  Would you drop the pressure when just a driver in the car ?  I think the extra or less weight falls within tyre makers 'tolerance' any way.

Tell your neighbour there are more important things in life than worrying about tyre pressure to that degree - trust the engineers.

[John Ratsey]

I assume that the reason for adjusting the tyre pressures according to the load is for two reasons: (i) to get even tyre wear across the tread and (ii) to have the headlights pointing in the right direction. The range of tyre pressure adjustment is unlikely to significantly affect the handling. The Jazz has adjustable headlights which avoids the need for (ii) but (i) is still a risk if the vehicle is consistently used at one end of the load range.

However, the TPMS adds a new dimension to the equation: The driver needs to remember to redo the TPMS calibration after each adjustment of pressures. Otherwise there will be false alarms. I therefore suspect this is why Honda have not indicated a pressure range on the Mk 3. Most likely the recommended pressure is appropriate for an intermediate loading condition in order to balance out the tread wear if the car experiences a range of loading conditions.

[VicW]

I therefore suspect this is why Honda have not indicated a pressure range on the Mk 3.

There is no pressure range on my 2011 Mk 2 either.

Vic.

[culzean]

(ii) to have the headlights pointing in the right direction. The range of tyre pressure adjustment is unlikely to significantly affect the handling. The Jazz has adjustable headlights which avoids the need for (ii) but (i) is still a risk if the vehicle is consistently used at one end of the load range.

I would guess it is the extra load on rear suspension caused by passengers and luggage in the boot that makes the headlights  go up, rather than the small amount the tyres would deform.  It makes sense for a car maker to choose an average pressure for the tyres to cover a wide range of loading conditions.  As I say, I have never even thought about adjusting pressures for passengers or not,  but do make sure before a long journey that pressures are as stated on tyre plate. I find with Michelin tyres the pressure does not drop much over long periods as they seem pretty airtight.

[jazzaro]

The values reported on the label on the pillar are usually what manufacturer write for the high load condition. I have the same values for my Renault, but I also have some values for standard running and so  lower pressures.
Manufacturers use to write  low pressure settings to achieve more comfortable running, but this settings are not good for high loads together with high speeds. So they write a  set of safer pressures, aiming to avoid tyre overheating.
@John Ratsey: adjustable headlights at 99% are due for the rear springs compression caused by rear passengers and luggages, not for tyres. Rear level can vary about 2-3 cm depending from load, and only some mm came from tyres.

[John A]

Looking for information on towing using my 2106 CVT Jazz, and this seems to imply that I can, but I'm sure that I read that you shouldn't with a CVT gearbox. An on-line copy of the manual even mentions using the "S" position when in hilly terrain when towing, so it's saying that it is ok. Any thoughts?

[plasma]

The tyre information plate is just that,  tyre sizes and pressures recommended by car manufacturers.

The weight in a car is spread over all four wheels,  and if extra rear weight is within the wheelbase of the car some of it will naturally transfer to front wheels,  and you could argue that if you brake with 4 or 5 people in the car you transfer more weight to front wheels, as you do going downhill,  so why not have more pressure in front as well,  the reason for increasing rear pressure only when a trailer is attached is that the weight is on the towbar, which is outside and to the rear of where the rear wheels touch the ground, so that weight is putting more load on rear tyres but actually taking weight off the front wheels because car is being pivoted around rear axle.  Like yourself I have never changed tyre pressure on any car just because I am carrying passengers or not,  what is the point of keep raising and lowering the pressure.   How many times do you have 5 people and luggage in a Jazz anyway compared to just the driver and no luggage ?  Would you drop the pressure when just a driver in the car ?  I think the extra or less weight falls within tyre makers 'tolerance' any way.

Tell your neighbour there are more important things in life than worrying about tyre pressure to that degree - trust the engineers.

+1, good post.

Plasma.

[culzean]

Here is tyre information plate for my wifes MK2 Si ( attached PDF )

Only rear pressure raised when a trailer attached, otherwise front higher than rear ( I suppose because weight of engine and petrol tank towards front ).  Same pressures for 175/65R15 and 185/55R16 as well.