Tyre pressures question

No Pants

I have a mkI Mazda 3 MPS. Came with 215/45/R18 tyres on it. Right now, that's the size of the tyres on the rear. The fronts are 225/40/R18. Eventually the rear tyres will be this size also, but they're seemingly not wearing.

The tyre pressures listed on the sticker inside the door are as follows:


________________Up to 3 people___________Full load
Front________________33___________________35
Rear_________________32___________________45


Sorry for the ****ty formatting, but it's best that I can do on this level of effort.

Right, so it's mostly me in the car, with the missus allowed in every now and again. So 33/32 is fine for me.

However, in August I plan to be carrying four adults in the car for the first time. My questions are as follows:

1. Is 35/45 the way to go?
2. I know the tyre can probably take 50psi, but still, 45psi seems high for driving on. Am I worrying about nothing?
3. Should I amend the front tyre pressure (at all times) to account for the difference in tyre size?

Stheno

In terms of the setting of 45, my car has front tyres set at 40 as per recommendation and it's fine.

Aint Eazy Being Cheezy

I usually just inflate them as much as they'll go.

nc19

45 for the rear on that car is too much. 38 max is what id be doing.

full load means 5 adults, luggage and prob roof rack

also, 225/40 is not correct for that car

No Pants

Duracell Bunny wrote: »

I usually just inflate them as much as they'll go.

I don't want to be bouncing around the road, I want the car to handle as normal, but with more people on board.

nc19

Duracell Bunny wrote: »

I usually just inflate them as much as they'll go.

Seriously?

tossy

I've pretty much always had 225/40/18's i've always ran 32psi and never bothered changing that when carrying a full car, never noticed any adverse wear or drop off in performance.

As for inflating them as far as the will go - don't do that.

Jesus.

That question wouldn't even enter my head tbh.

You'll be grand as she is bro.

No Pants

nc19 wrote: »

also, 225/40 is not correct for that car

The difference in price was €40 a corner. The tyre dealer recommended them, I had researched changing the tyre size on MPS owner forums, they've passed the NCT, turning arc is not affected and I cannot fault their performance.

What exactly is the problem with using this tyre size?

40now

Duracell Bunny wrote: »

I usually just inflate them as much as they'll go.

Are you a blonde by any chance mrs bunny, only joking but that's a seriously bad idea.

corkgsxr

Id go for 38 rear. Covers all fine.

and I wouldn't worry about how much they can take. Its well over 200psi before they pop.

my van takes 65 psi

Guy:Incognito

corkgsxr wrote: »

and I wouldn't worry about how much they can take. Its well over 200psi before they pop.

Yeah but if someone has them pumped up like rocks the cars probably only drive on about an inch wide strip of rubber in the middle of the tyre. It'll be off the road and in a ditch the first time they try take a corner while the car is moving.

nc19

No Pants wrote: »

The difference in price was €40 a corner. The tyre dealer recommended them, I had researched changing the tyre size on MPS owner forums, they've passed the NCT, turning arc is not affected and I cannot fault their performance.

What exactly is the problem with using this tyre size?

too wide, will put extra strain on balljoints, bushes etc.

also, wrong rolling radius may affect speedo and wheel speed sensors

challengemaster

nc19 wrote: »

too wide, will put extra strain on balljoints, bushes etc.

It's 10mm difference, it won't matter a flying fúck.

nc19

challengemaster wrote: »

It's 10mm difference, it won't matter a flying fúck.

No bother. You keep on doing it and ill keep on fixing the cars.

im sure you know better than the experts in Mazda that spent a lot money testing and testing and testing until deciding what the correct tyre size should be

Hector Mildew

nc19 wrote: »

No bother. You keep on doing it and ill keep on fixing the cars.

im sure you know better than the experts in Mazda that spent a lot money testing and testing and testing until deciding what the correct tyre size should be

Is it always a bad idea to fit a tire size different to the oem, including narrower widths? (I started a thread recently hoping to get a definitive answer to this http://www.boards.ie/vbulletin/showthread.php?t=2057224243)

corkgsxr

Guy:Incognito wrote: »

Yeah but if someone has them pumped up like rocks the cars probably only drive on about an inch wide strip of rubber in the middle of the tyre. It'll be off the road and in a ditch the first time they try take a corner while the car is moving.

My point isnt to pump them up to 200. Its dont be afraid to go over 33

No Pants

nc19 wrote: »

too wide, will put extra strain on balljoints, bushes etc.

also, wrong rolling radius may affect speedo and wheel speed sensors

I'm not too concerned about the speed sensors, but does the 10mm extra width and the 6/7mm lower profile really cause all that? Genuine question. I don't put ditchfinders on it and if it's going to cause problems, I'll consider handing over the extra cash for the unpopular size next time round. Will probably be Uniroyal Rainsport 3s regardless.

Wheelnut

The difference in the rolling radius of the two tyre sizes is about 2% which is small enough to be ignored. I don't think you will come to any harm if you are not pushing the car to the limits, however it is more conventional to have the wider tyres on the rear.

The extra 10mm width will not put any significant loading on the suspension parts but you should check that the tyres clear the bodywork when at the limits of movement, ie: full bounce or full lock.

Other advice here is not wrong but is a counsel of perfection.

Here is a link where you can compare tyre sizes, there is a calculator about 3/4 way down the page: http://www.carbibles.com/tyre_bible_pg4.html

No Pants

Wheelnut wrote: »

The difference in the rolling radius of the two tyre sizes is about 2% which is small enough to be ignored. I don't think you will come to any harm if you are not pushing the car to the limits, however it is more conventional to have the wider tyres on the rear.

The extra 10mm width will not put any significant loading on the suspension parts but you should check that the tyres clear the bodywork when at the limits of movement, ie: full bounce or full lock.

Other advice here is not wrong but is a counsel of perfection.

Here is a link where you can compare tyre sizes, there is a calculator about 3/4 way down the page: http://www.carbibles.com/tyre_bible_pg4.html

Thanks for that. I had already checked the diameters, circumference, etc. on another site (a Miata site if I recall correctly), but there's some interesting stuff in there on the change to the odometer readings. However, it's comparably to the change when using a brand new tyre vs. an old and ready to be replaced one.

I've never had any problems with wheel clearance. The wider tyres would have gone on the back if they'd needed replacing first, but they didn't. The Toyos on the front were just over 3mm, so I pulled the trigger on those first. The idea was then to rotate the replacement tyres to the back when they were ready to change and then put new tyres on the front again. However, a year has passed and I can't see any change in the two rear tyres at all. They're just not wearing, whereas the fronts are progressing nicely and will probably need replacing again before the end of the year.