[Jay 968]

My car currently has the OEM 225/50 16" on the back. The winter tires I really want (Continental ContiWinterContact TS810 Sport) doesn't come in that size, but it does come in 225/55 16".

I'm thinking that a 5% deviation in aspect ratio won't cause any problems. Am I correct, or asking for trouble?

Many thanks in advance.

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[flash]

it will have some good and some bad effects

good - improved highway fuel economy due to the increase in final gear ratio

bad - gearing change could make it harder to hold traction from a stop in the snow

bad - mismatching sizes front to rear, more specifically outside diameter can cause the ABS to become inoperative - there is a limit there - this one can be VERY bad

bad - a 55 series tire will generally be narrower than the same size in a 50 series tire - this means less tire on the ground which can mean less traction - i know it sounds odd, but it's true - the width measurement is relative - check the specs on the tirerack site

if you are going to do it, i would definitely do all 4

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[Tom030]

<!--quoteo(post=82117:date=Oct 20 2009, 10:25 AM:name=Jay 968)-->QUOTE (Jay 968 @ Oct 20 2009, 10:25 AM) <{POST_SNAPBACK}><!--quotec-->My car currently has the OEM 225/50 16" on the back. The winter tires I really want (Continental ContiWinterContact TS810 Sport) doesn't come in that size, but it does come in 225/55 16".

I'm thinking that a 5% deviation in aspect ratio won't cause any problems. Am I correct, or asking for trouble?

Many thanks in advance.<!--QuoteEnd--><!--QuoteEEnd-->

Sounds like you're planning on snows in the back only. Not a good idea for a number of reasons.
The combination of a higher aspect ratio, lower speed rating and snow type tread on the rear only is a recipe for serious oversteer.
You would be better off going to 205/55R16's all the way around for the best combination of safety and handling. For snow application the rule of thumb is the narrower the better. The 205 cross section is approved by Porsche for the 968.
~tom

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[Anchorman]

When I was running on winter tires, I had 16" wheels 7" wide in front with 205/55, 8"wide in back with 225/50. I don't use these anymore, so if anyone is interested...the tires are Bridgestone Blizzaks with several seasons of tread remaining.

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[Jay 968]

I plan to do all four tires. It's just that the rears aren't available in the OEM size for this particular tire.

I'm now thinking to put 205/55/16 on all four corners. That will preserve the OEM tire circumference and give me better snow traction with the narrower width. So the question now is, is that too narrow a tire for my OEM 16" rear wheel?

Thanks everyone for the input.

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[Kim]

seems odd to me re the width thing. I have been lead to believe that a 225 means 225 mm wide at the widest part of sidewall. the 50 means that the height is 50% of the width. so a 225/55 should result in a marginally taller tire with the same side sidewall width as a 225/50.

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[flash]

almost - the sizes vary a bit as they go along the profile change - a 225 is not necessarily 225mm - in fact, actual sizes vary from manufacturer to manufacturer by as much as 20mm - you really have to look at the specs of the specific tire to see what the measurements are - tire rack has much of this information on their site

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[stewhatch]

I was taught that when fitting winter tires to a 2WD vehicle, the rears should preferably be wider then the fronts - even if you are driving a FWD car. The reason given is that in a straight line you don't want the rear tires riding in the front tire's tracks. I've always gone this route and can report this setup works. The only exception would be an AWD or 4WD vehicle which would require all four tires to be exactly the same size. After seeing Flash's last post, I'd have to think a 4 x 4 would also require all four tires be the exactly the same brand / model as well.

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[jlm968]

I can advise, from experience, that you will hate the 205's on the rear when not in snow. I tried it one winter according to the recommendation in the owner's manual, and those skinny wheels in back just felt squirrely the majority of the time driving on non snow- or ice-covered roads. 225's on the back are just fine.

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[Jay 968]

Thanks all. I guess I will have to choose a different tire in order to preserve the OEM sizes.

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[mattipuh]

I'm using 17'' 205 front and 225 rear, feels s*** to drive but you get used to it after 2-3weeks. [img]style_emoticons/<#EMO_DIR#>/smile.gif[/img]
profile was 50 rear and maybe 55 front.. cannot remember exactly. does it's job quite ok during winter. thank god it's springtime soon, and MPP's are under soon again..

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