Car Tyres - Aspect Ratio / Profile

davemie

I know very little about car tyres, but I know enought not to go back to the guys I last visited, 275e for one tyre, ouch! OK, it was 255/35/18, but still.

Anyway, I've looked around and found Camskill and am planning to get two tyres off them. The two work out less than what I paid for one, that just got punchered, last Jan.

My question is about the tyre's Aspect Ratio/Profile. I understand that the 35 is a % of the height of the sidewall compared to the width of the tyre (http://www.carbibles.com/tyre_bible.html#aspectratio)

Would getting a 255/45/18 or 255/50/18 make much of a difference? Apart from looks, would they protect my alloys better, last any longer, better fuel economy, etc?

I haven't had any problems with my 35s, I don't drive bumpy roads or anything like that.


The only reason I got the 255/35/18 is because that's what came with the car. The front tyres are different, 225/40/18.

RobAMerc

the 50's should defo make a difference to ride quality and will protect your wheels better.
But they may also throw out your speedo a bit, and will defo look quite different.

You should check the online price and ring your local guy and ask can he match the price - the guys in Sandyford tyres did for me and I didn't have the trouble of getting delivery organised etc.

mullingar

According to Carbible with those sizes, your wheels will be 26mm taller. MAKE SURE you have at least 13mm clearance all round (radius) when the car is fully loaded.

Your speedo will be 4% under reading - this means your speedo will now reads correctly as most speedo's are 5% under reading.

This also means your MPG will improve, 0-60 time will reduce.

GigaByte

Use this calculator to help you.

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

davemie

Cheers guys, that helps a lot.

As a side note, I assume the speedo difference and 0-60 time would only change if you are changing the tyres on the drive wheels, or the wheel that the speedo is connected to.

e.g.
if you change the back tyres and your car is a front wheel drive, then there would be no change to the speedo and 0-60 time.

But if you change the back tyres and your car is rear wheel drive, there is a change.

Would that be correct?

I got GPS in the car which outputs meters per second, so I can calculate what the speedo different is. At least I can use the excuse, sorry Garda, I just changed the profile of my tyres and didn't know I was speeding :-)

Steady Zeddy

I wouldn't recommend going more than 5% on the profile - e.g you shouldn't go beyond 255/40 18 on your current rears and 225/45 18 on the fronts -any greater and you risk rubbing your tyres on the arches (particularly when the suspension compresses on bumps etc).

Also your acceleration will be slower with increased diameter.

If you are looking for more rim protection then buy a tyre that offers this.
If you have enough width on your rims then perhaps you could consider going with something like a 265/40 18 on the rear and a 235/45 18 on the front. This would give you slightly more on the profile as well and would grip and certainly look a lot better than a higher profile.

Anan1

Nobody has yet asked the obvious question - what wheel/tyre size did the car have when new?

davemie

It has one of the original tyres that the car came with on Michelin Pilot Sport 2 - 255/35/18 and the other is a Bridgestone - 255/35/18 that I replaced last Jan, but they are too expensive for me! Especially as I only got about ten months out of the Bridgestone.

I had a look and there is loads of space in the wheel well, but not a lot of space on the outside, so I guess I will stick with the size that is currently on.

Thanks for your help guys!