Tyre pressures

Technical Area Engine, Transmission, Exhaust etc
2

Been experimenting with tyre pressures on my NC. using a slight slope to infer rolling resistance.  Does not readily roll at 28 psi, but rolls freely at 31psi so fuel consumption might be expected to improve at higher pressure. However, onboard computer shows little change, if anything slightly worse??? Now wonder if this is due to increased tyre diameter - which would reduce wheel revs per mile, anyone else noticed this?   Intriguing,  but maybe I should get out more. 

3

By over inflation of the tyres you will have lessened the contact patch with the road as the tread will no longer have a flat profile so you won’t be doing anything for your safety or handling. I can understand that you may see this as reduced rolling resistance but it will be taking the tyre away from the designers original plan for optimum performance. The best way to get the best fuel consumption is to correctly inflate the tyres.

4

29psi is the recommended setting for an NC.

5

When you consider the mechanical resistance that the car has to overcome in all areas of the drive train, plus the intertia of moving the car itself, I can’t see that changing tyre pressures could have anything other than an entirely negligible impact on the work the engine is having to do.

6

Agreed.

However, I found that there is a noticeable difference in fuel economy (at speed) between lid up and lid hid. 

Which is one of the reasons why I now always have the lid up on the motorway.

Of course, tyres run UNDER-inflated will waste a lot of heat and power, and consequently also have a short life.

7

When I owned an Astra H, the tyre pressure chart in the handbook showed both ‘normal’ and ‘economy’ tyre pressures for some tyre sizes; the economy pressures being, generally, about 0.5 bar higher than ‘normal’.  Vauxhall clearly didn’t see any safety issues on that vehicle.  From an engineering viewpoint higher tyre inflation pressures will reduce rolling resistance and I believe running tyres at very high inflation pressures is a technique used to ‘massage’ the official fuel consumption figures, so it works.  Whether these very high pressures are safe for day to day motoring is open to question.  I’ll admit to running my Mercedes between normal and full load pressures so it never runs underinflated and hopefully it helps the fuel consumption too.  There is anecdotal evidence that ‘normal’ pressures are based on comfort (and the Astra data may support that) but I’d not recommend significant increases over the maximum recommended figures just in case.

JS 

8

Harder tyres = smaller contact patch = less grip

C’mon folks, we didn’t buy the 5 for economy :smiley:

(Can understand hood up on motorways though, they are not fun places for roof down)

9

As seriousrikk says, over-inflated tyres = less grip.  It’s a stupidly dangerous practice, especially in the wet, just for the pursuit of 1 or 2 more mpg.  A false economy anyway, IMO, as the reduced contact patch in the centre areas of the tyres will wear far more quickly than the equal wear of a correctly inflated tyre.

10

I note the range of responses, my OP received, but I feel that most of you missed the point. I am not trying to scrimp a few extra mpg by risking safety and handling - if I was that mean I would not be driving a rather thirsty 2.0l NC in the first place.  I should have phrased the question as “How sensitive is the onboard computer measure of mpg to tyre pressure?” because the results of my little experiment ran counter to what I expected.

 I also note the touching faith which many of you have in the accuracy of  tyre pressure gauges - my range from 28 -31 about a nominal 29 psi is ~10%, which pretty much falls on the +/- 5% of actual pressure which is as accurate as I would expect from any commercial gauge. 

11

I had 4 x new tyres fitted yesterday.  The fitter (local, well established tyre company which I’ve used many times before with no issues) said that the old tyres were all quite a lot more worn on both shoulders & asked me what pressure I ran them at.  I replied 2.0 BAR (29 PSI) as per Mazda’s instructions.  He said that the old tyres must have been underinflated so to make the tyres wear evenly, he’d pressurised the new ones to 2.4 BAR (35 PSI) instead & recommended that I leave them at this pressure, maybe reducing to 2.2 BAR (32 PSI) if I found the ride too hard.  However I check the pressures regularly & they’ve always been run at 2.0 BAR (29 PSI),

I last had the tracking checked only about a month ago & it was spot on; in fact it’s been checked two or three times over the last 5 years & no adjustment’s ever been necessary - I’ve had the car from new so the shoulder wear isn’t a tracking issue.  (also front & rear tyres were equally affected).  I do recall that, when they were new, the (asymmetrical) tyres which were removed yesterday did have less tread depth on the shoulders, as part of the design.  Additionally as I’ve had 26,000 miles out of the old set I’m not complaining.

Not too happy about running the new tyres at 20% above Mazda’s recommended pressure - as other posters here have said, grip / safety are much more important than fuel economy or, as seemed to be the tyre fitter’s approach, ekeing out as many miles as possible from the tyres.

Been thinking about this & I think I’ll let them all down to 2.2 BAR (32 PSI) & see how it goes?

Also a little unsure of whether the fitter knew what he was doing because the inside of all 4 wheel rims, visible through the gaps between the spokes, have all been very badly scratched / gouged when the old balance weights were taken off.  A lot of the old glue was left there too.  Only spotted this when I washed the car today.  Anyway I’ve removed the glue & touched in the damage to stop the alloys corroding, but obviously it’s quite noticeable though so not very happy!

 

12

I fully understand where the OP is coming from regarding his question about the registered fuel economy readings.
Increasing the wheel diameter will fool the computer (and the odometer) because it will appear that the car has travelled a shorter distance than it actually has, simply because it will be counting wheel revolutions rather than actual road miles travelled.

I have another car, an old Suzuki Swift 1.0, which uses 70 profile summer tyres. In the winter I run it on different wheels, fitted with 80 profile tyres. According to my satnav, with 70 profile tyres the speedometer reads 73 mph at an actual 70. With the 80 profile tyres, there is no margin, 70 mph indicated is a true 70 mph (no margin for error with the M1 speed cameras on my daily commute)!

When I check the mpg the car does by comparing miles travelled “on the clock” since the last tankful, there is no noticeable difference between the tyres. But the car will actually have travelled further than the odometer has recorded, so there will be a slight increase in mpg in reality.

I have to say that some people seem paranoid about tyre pressures. However, unless large adjustments are made to them, there will be negligible effect to the footprint and the alleged safety of the tyre. Better a few more psi than too little. The latter can cause overheating and tyre failure.

13

I reckon garages/dealers including some of the tyre fitting places all have gauges that automatically inflate to 35psi and above. Everytime I take my other car for servicing it comes back with 35+psi in the tyres. Two weeks ago I collected it and it had 35 and 38psi F/R. Sure they can’t be a—d to check so inflate to a set pressure.

To tell you to over inflate is wrong. Mine are 28psi on the Mk3 sport, tyres wearing evenly.

14

Hi I am new to the club and have been following this thread. I am running my tyres on my NC at 29psi, but I was wondering if any one knew what the original temperature rating of the tyre should be. My tyres are the same size, make and model, but two are V rated and two are W rated.

Thanks
Steve

15

Hi having read through the page in this link http://www.mazdone.com/minfo-1209.html  There is a section here 

NOTE:
- Perform tire pressure adjustment when the tires are cold. Tire pressure will vary according to the tire temperature, therefore let the vehicle stand for 1 hour or only drive it 1.6 km (1 mile) or less before adjusting the tire pressures. When pressure is adjusted on hot tires to the cold inflation pressure, 

if I’m understanding the text correctly,  it is asking mx5 users to drive their cars, from cold, for approximately 1 mile, before adjusting the tyre pressure. 

16

It is saying the tyres must be cold when you adjust the pressures.  Let the vehicle stand for an hour after being driven or drive it no more than 1 mile before making the adjustment.  Driving it any more than a mile will heat the tyres up too much and give false readings.  It is not saying that you should drive it for a mile before checking the pressures.

I hope this clarifies things.

17

The V and W codes are the speed ratings of the tyres.  V is a max speed of 149mph and W is a max speed of 168 mph.  Both fine for the NC.

You say that the tyres are the same just different speed ratings.  It is important on the MX5 not to mix tyres of different types on the same axle, and ideally have all 4 tyres the same type.  When I bought my NC it had one odd tyre on the rear and that corner frequently lost grip.  Pretty soon replaced all 4 to the same.

 

 

18

 

  I love motorway driving with the hood down - The Who blasting out of the Bose - marvellous. ( or am I just weird ? )

  Always try to have tyres at 29 psi - checking them every now again and trying to get bob on 29 is quite enough for me, let alone altering the setting.

  The little computer thing seems to be always right regarding MPG, so I leave it on temp setting - it’s good at that too.

  Finally - will we go back to Imperial Gallons at petrol stations post EU ?  Never got the hang of my 4.546 times table 

 

19

I know that we’ve gone a bit off piste with the ‘hood down on motorways’ question, but I don’t understand why people write that they don’t like taking an MX-5 on the motorway with the lid off - I find it exhilarating - wind in hair etc, but each to his own I guess.

Back to tyre pressures though, I can’t imagine that a couple of pounds either way is going to make much difference to the tyre wear or fuel economy.  However, with the low-profile tyres fitted to MX-5, I wouldn’t fancy riding on 35 psi for long, it would be like travelling on solid rubber ! 

20

I agree that riding on low-profile tyres such as the 17# ones fitted to the Sport if pumped up to 35psi would be like riding on cobblestones…but any low-profile tyre (e.g. less than about 50% section) will have less compliance in the sidewalls anyway. A lot of the better ride quality on the non-Sport models with 16# wheels and deeper section tyres comes from the more compliant tyre sidewalls.

One thing to bear in mind is that the wear pattern on the tread will reflect the dynamic profile of the tread patch, and if the pressure is changed, it too will change…BUT NOT IMMEDIATELY.
In other words, it will take time for the newly-presented contact patch to wear in to the correct profile. You will see this particularly if the pressure is reduced, from, say, 32psi to 27 psi, as advocated by some folk here. The outer shoulders will suddenly be in much more frequent contact with the road, and the immediate impression will be of improved grip. That will soon wear off, however, as the shoulders wear away and the normal - slightly convex - pattern of wear becomes established.
The converse is true when raising the pressure.
A very good check of your alignment is to check the evenness of wear across the width of tyres, and the unifomity of that pattern across both wheels on that axle. A bit more wear on the front shoulders is normal, just due to lateral steering forces. Toe-in and camber errors will be readily apparent in differential wear.

Sorry if I teach granny to suck eggs…

Aldi