75ZT Community

[Chartermark]

I am due for a tyre change soon, am and currently on 225/45/17's, as I have Meteors and want to keep them.

I understand that 16" rims give a 'softer' ride due to their elevated profile. Bearing this in mind I'm considering going up to 225/55/17's.

Does this mean the tyre height is therefore 10mm higher, than those I presently have and should run a little softer?

Any knowledge much appreciated.

[takestock]

225: 45: 17

225 is the width in mm
45 is 45% of the width and that becomes the height of the sidewall
17 is the rim size

[Chartermark]

Thanks Dave,

Simple when you know how, as the banker said to Gordon.

At nearly an inch taller I would think the 55's are going to be the new chappies. Price variance and reviews are extreme in both cases.

I got 22,000 on the front and the backs are still very good at 30, 400 - the make 'Sunew', cost £36 each!

Probably from the land of Kim ll-sung, but they have proven to be a good wearer.

[takestock]

No problem Martin, if i remember correctly 17" should be 55 profile anyway. You will notice the difference

[Chartermark]

Dave,

You may remember I bought a set of Meteors from Alistair, which he kindly delivered to the last Nano Meet for me by rowing boat!

They've now been completely refurbed, so I'm going to get a set of 55's and keep the other set as spares. They are originals and two are now flaking round the pillars, so I'll get them re-done too. I don't think I'll even bother with the tyres that are on them.

They'll no doubt look better as well, as they'll fill the wheel arches a little more.

Also, I can't help thinking sometimes when I approach the car, that it looks like it's got 'slippers' on, 45's are low.

[Duncan]

You have to be careful when changing the profile, but keeping width and rim the same, as it increases the rolling radius, which affects the speedometer.

No idea what combination is right / wrong, but changing from 45 to 55 will make a difference.

Apparently some tyre websites can help calculate the changes.

[Chartermark]

Thanks Duncan, very good point ...

I'll have a look at that. I suppose I could also cross reference to the Sat Nav. For instance I notice the latter invariably bleeps when I exceed a limit, well after the speedo has already indicated an excess?

Also, when I was recovered on Thursday, the driver of the trailer told me that Sat Navs and Tachometers are usually more accurate than speedo's. I do notice the way trucks seemingly breeze past me on 'average speed checks' by a good 4-6 mph, which may validate his claim somewhat?

Interestingly a 16" rim with a high profile, probably has a similar rolling profile to my low contour 17's?

Also Rover offered 17" Meteors with, as Dave says '55' profile tyres. I can't see them forking out for differentially calibrated speedometers?

A good debate here somewhere?

PS - just checked here,

http://www.club80-90syncro.co.uk/Syncro ... ulator.htm

Quite a difference, but easily rectifiable mentally I would imagine ,,,

[James.uk]

This may help..

http://www.miata.net/garage/tirecalc.html
...

[Duncan]

A couple of points.

Yes the speed indication from GPS will be more accurate. The speedo has to be calibrated between -0 and +10% plus (I think) 3kmh for Europe. Other markets vary, which usually ends up with a plain -0 +10% satisfying all markets.

So with production tolerances, tyre wear allwoances and so on, they tend to sit in the middle of that band, around +5%. There also needs to be some allowance if different tyres can be fitted as standard. For example on the older cars, you can have 195s or 205s on the car with the same profile.

Regarding tyres. If I understand how it works, and I'm no tyre expert, you could change from a 45 to 55 profile, but maintain the rolling radius. You would have to get tyres with a narrower width. Now these figures are only to show the principal, and the Rolling radius isn't the same as the actual radius, but: For example, a 205/45 tyre has a sidewall of 92.5mm. A 175/55 tyre would have a similar sidewall at around 96.25mm where a 205/55 would be 112.75mm.

[Chartermark]

This may help..

http://www.miata.net/garage/tirecalc.html
...

James,

What a superb site many thanks. I spent about an hour in between it and eBay, getting an idea on prices with different profiles etc.

For example a set of 4 x 225/55/15 costs £225 delivered (Sunew), I've had them on the backs with over 30,000 miles and still VERY Legal. I got 24,000 out of the fronts, so although they're cheap, they work for me.

Go up to 255/55/17 and the price increases for 4 to £332.

I expect though that my mpg would decrease as the 'footprint' of the tyre enlarges creating more friction. Alternatively though, the enhanced road holding would be excellent I imagine?

Then we get into the insurance malarkey of them not being standard - blah, blah, that'll be another 80 quid on your premium thank you.

Interestingly, and not in the least connected I had a cig lighter phone charger delivered today. Upon opening it, the first thing the instructions advise is 'DO NOT EAT THIS PRODUCT'. I told my mate who's coming to stay next weekend that we'll have a few pints, and then come back here for a nosh, as I've got a couple of phone chargers and a box of DVD's in the fridge!

I blame it all on the Belgians!