C5 - Variation In Tyre Pressures

[Randombloke]

In this post in this topic about alignment, Ronin raises the issue of the tyre pressures posted on the drivers side door frame being inaccurate, because they may be out of date or not for the tyres now on the car.

 

I'm interested to see how much experience we have of other tyre pressures.

 

I usually use 2,5/3.0 bar front/rear, on an 02 2.2 HDi Estate, H3+. I will edit this post later to put the psi values in brackets. The tyres on the car are Dunlop SP2000. The values are taken from the door frame, and are for fully loaded.

 

I have noticed that compared to 2.3/2.5 or 2.5/2.5 this gives better fuel economy, and slightly sharper handling but at the expense of ride quality.

 

Can any other members comment? Has anyone else tried the maximum value - 10% values suggested in the link above?

[Ronin]

I can shed some light on this subject a bit more.

Many years ago I bought a brand new Vx Astra GTE 16v but went for a tyre/wheel upgrade from 195/55/ vr15 to 215/40/ zr 17 from new.

They looked good but the ride was always a bit harsh and the car suffered from the dreaded torque steer a lot.

When the car went in for serverice at the main dealers it came out and felt a lot better. then a week later I checked the tyre pressures (according to the door frame it was rated at 34psi) but my actual pressure was 28psi. So needless to say I pumped them up to the reccomended settings again.

Over time I noticed the scrabbling effect returned along with the harsh rideand the tyres were scrubbing off on the outer edge.

 

Second service came along and the same thing, low psi after the service so I called back into the main dealer and spoke in person to the tech who had worked on my car and asked him why?

 

He told me about the - 10% rule that he used and why it was relevant. So I was a bit sceptical about this but he assured me that it was actually correct for my car and tyres. He said that a lot of vauxhall's suffer from the torque steer effect and customers often blame it on the gearbox, lsd suspension geometry or even the "too much power going through the front wheels excuse" when in actual fact it was all down to the tyre wall not having enough give due to the wheel beading and unusual offset they have... Reducing the pressure in accordance to the max psi rating gives the tyre just enough flex in the side wall but still gives the stability in straight line driving..... A miracle for vauxhall's I thought.

 

So I reduced the pressures once again back to - 10% and low and behold the handling, comfort and fuel economy improved.... Still a bit sceptical at this point and expecting some unusual tyre wear on the outside edges and leaving me with a hefty bill. I kept with the advice and I was very surprised indeed to see it was even wear and I got 24,000 miles out of the front wheels (an achievment in itself on a GTE and especially the way I drove it. BTW I was getting about 2mpg more!

 

I sold that car and since then I have noticed with other new cars that I have owned that the dealer technicians seem to vary up or down the tyre pressures but rarely stick to the reccomendations and it always seems strangely to be around the -10% mark. My C3 was a prime example although I did actually find that -12% of max psi suited my driving style.

 

Now to my C5... My tyre max psi rating is 44 so - 4.4 @-10% =39 or 40 psi on 195/65/hr15 a full 10psi above the 2.1 bar (29psi) ... So now you will be thinking i'm crazy!

Well, think about this for a moment...... We all know how good the the ride is in a C5 so comfort isn't an issue but bear in mind that the sidewall @65 is still quiet tall and will flex a lot less than set at the recommended psi = better cornering and grip.... Then there is less friction on the road surface = better mpg (so far about 2- 3) but braking should increase (not that I have noticed and I have made a few rapid stops) and so far no uneven tyre wear and I have had the car 4 month now.

 

Should you all try it?

Well thats totally up to you but think about the fact that all handling, economy issues have to start somewhere and can come from a simple matter as tyre pressures! I figure that I have saved in fuel alone with my old Xantia (God rest it's soul) about £180 after 12,000 so in effect I got the price of my tyres back in the space of a year!

I leave it to you guys to ponder over the pro's and cons.

[72dudes]

Interesting topic. I tend to run my tyres at just below the recommended pressures. In my C5 2.2 HDI hatch, I use 32 psi on the front and 29-30 on the back. I rarely have any people or weight in the back, and my thinking is that as probably 65% of the car's weight is at the front, it's unnecessary to have as much pressure in the rear tyres.

 

Now, I'm the guy who gets 44.5 MPG average for this car (brim to brim over 15k miles) so I don't think economy is suffering, and tyre wear seems normal too. As long as the pressures are not ridiculously different to that recommended, it's all down to personal choice.

 

I used to own a classic sports car, a 1968 MGC GT (3 litre straight six iron lump) and these cars were lambasted for poor handling when new. However, it was all down to the wrong tyre pressures being recommended on the Press cars which were road tested! As long as you had 28 psi in the front and 22 psi on the back (very low by today's standard) and not vice-versa, the car handled fine!

 

Back to the C5, I find that the 33-34 recommended makes the car too thumpy around town, although fine on the motorway. In fact if I know I'm in for a long motorway journey, I will tweak the pressures up a bit. Like I say, personal taste and common sense.....

[DaveHerns]

Interesting topic. I tend to run my tyres at just below the recommended pressures. In my C5 2.2 HDI hatch, I use 32 psi on the front and 29-30 on the back. I rarely have any people or weight in the back, and my thinking is that as probably 65% of the car's weight is at the front, it's unnecessary to have as much pressure in the rear tyres.

 

Now, I'm the guy who gets 44.5 MPG average for this car (brim to brim over 15k miles) so I don't think economy is suffering, and tyre wear seems normal too. As long as the pressures are not ridiculously different to that recommended, it's all down to personal choice.

 

I used to own a classic sports car, a 1968 MGC GT (3 litre straight six iron lump) and these cars were lambasted for poor handling when new. However, it was all down to the wrong tyre pressures being recommended on the Press cars which were road tested! As long as you had 28 psi in the front and 22 psi on the back (very low by today's standard) and not vice-versa, the car handled fine!

 

Back to the C5, I find that the 33-34 recommended makes the car too thumpy around town, although fine on the motorway. In fact if I know I'm in for a long motorway journey, I will tweak the pressures up a bit. Like I say, personal taste and common sense.....

 

 

Interesting topic

Are we talking about variations from the car manufacturers recommendations or the max pressure details on the tyre sidewall ?

 

Assuming it's the former and finding my 2006 C5 2.0 Hdi Estate a bit harsh at low speeds , I tried lowering the tyre pressures from the recommended 2.5 bar - about 35 psi . 31 psi in the rears makes it much smoother over potholes, lowering the fronts didn't do much except make the steering less sharp

 

Why do modern cars run at such high pressures ? Is it the lower the tyre profile , the higher the pressure ?

[72dudes]

Interesting topic

Are we talking about variations from the car manufacturers recommendations or the max pressure details on the tyre sidewall ?

 

I'm talking about variations from the recommended, but Ronin is talking about 10% less than the tyrewall max.

 

 

Assuming it's the former and finding my 2006 C5 2.0 Hdi Estate a bit harsh at low speeds , I tried lowering the tyre pressures from the recommended 2.5 bar - about 35 psi . 31 psi in the rears makes it much smoother over potholes, lowering the fronts didn't do much except make the steering less sharp

 

Agreed, although you'd probably want to increase back up to 35+ if you were carrying a heavy load or 2 people in the back.

 

 

 

Why do modern cars run at such high pressures ? Is it the lower the tyre profile , the higher the pressure ?

 

Yes, I think so, and a bit of fashion! Today everyone is more concerned about sharp handling and grip. I remember my dad having a Renault 16 in the early 70's and the recommended front tyre pressure was 24psi! Lots of sqealing on roundabouts, but a very smooth ride. The backs lasted for 50k miles and my dad wrote to Michelin complimenting them!

[Randombloke]

Now to my C5... My tyre max psi rating is 44 so - 4.4 @-10% =39 or 40 psi on 195/65/hr15 a full 10psi above the 2.1 bar (29psi) ... So now you will be thinking i'm crazy!

Well, think about this for a moment...... We all know how good the the ride is in a C5 so comfort isn't an issue but bear in mind that the sidewall @65 is still quiet tall and will flex a lot less than set at the recommended psi = better cornering and grip.... Then there is less friction on the road surface = better mpg (so far about 2- 3) but braking should increase (not that I have noticed and I have made a few rapid stops) and so far no uneven tyre wear and I have had the car 4 month now.

 

To be honest I was not convinced about this theory. So it made sense to prove or disprove it. However it comes from someone who can be seen to practically knowledgeable so it would be silly not to try. After all how long does it take to let the air out of the tyres if not?

 

Set front and rear to 2.7/2.8, roughly 39/41.

 

Travelled with hang glider today, best is usually 38-40.5 mpg. Reset for this trip, got 41.6mpg.

 

So, it worked. Definite vibe of less rolling resistance.Slightly more connected feel on the road compared to 2.5 bar

 

Downsides:

 

Harsher ride, but not much worse than with 2.5 bar.

 

More road noise, move than with 2.5 bar and noticeably so.

 

Should you all try it?

Well thats totally up to you but think about the fact that all handling, economy issues have to start somewhere and can come from a simple matter as tyre pressures! I figure that I have saved in fuel alone with my old Xantia (God rest it's soul) about £180 after 12,000 so in effect I got the price of my tyres back in the space of a year!

I leave it to you guys to ponder over the pro's and cons.

 

I will be going with the -10% tyre wall max style suggested above, i.e. 44psi or 3 bar -10% for motorway trips and hang glider travel of any sort. I will resort back to 2.5 for general use round town.

 

Thanks for suggestion.

[Randombloke]

Had a result with fuel consumption on the French motorways - best ever economy with 2 hang gliders ever.

 

However, now I'm driving around generally in the UK (mixed driving) the pressure had to go back or the ride is way too harsh.

 

I have also invested in a gauge as the place I usually top up is not proved to be unreliable.

 

Thanks for the advice. Top up to -10% of max for long motorway journeys, stick to recommended for mixed driving.