Alternative tyre sizes

Hi all.

Question regarding tyre sizes on a Fiat Cinquiccento(sp?).

My brother has a battered N reg example of a base spec (steel wheels). The Haynes Big-Book-Of-Lies says 135 85 13,145 75 13 and 155 65 13 (or something along those lines) can be fitted. (This is odd, because the ratios he told me give a range of 14% on the rolling circumference)

The original ones on the back were 135 _55_!

He now needs two new front ones (the ones he's got aren't good in the wet), and wants to put 165 55's on there. He's already got 165 55s on the back.

1) Will this be dodgy for insurance purposes? 2) Will they fit? 3) Will they be safe?

I've never bothered changing a tyre size by this much (I put 155R13 on my

1st car which normally came with 145R13).

While I'm at it, what's the other number associated with car wheels/tyres, something like 5.5J or 6J. Is it the separation of the rim or something?

TIA,

Pete.

Reply to
Pete Smith
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according to the FIAT brochure (I kept one from when I was looking for and subsequently bought my Cinq' Sporting), standard tyre sizes for the Cinq were 145/70R13 for the S, 155/65R13 for the SX/Soleil and 165/55R13 for the Sporting. Note that the first two are on standard steel rims (don't know what size) and the Sporting had 5.5Jx13 alloys.

Whether 165/55R13 is a good idea depends on the width of the steel rims. they obviously fit OK on 5.5" rims, and also on the 5" rims of the optional Abarth alloys. I'd be surprised if they fitted OK on 4.5" rims which is what I suspect the standard Cinq to have.

Another point to consider is the cost of the tyres: 165/55x13 is a size unique to the Cinq' and Sei', there are only AFAIK 2 tyres that come in this size: Pirelli P7000 and a Goodyear, which raises costs and lowers availability (Kwikfit and ATS wanted something in the region of £75 per tyre last time I asked, and that was a few years ago as I've been running on 175x50/13s for quite a while). 155/65 (say) should be much cheaper and easier to come by.

Regarding insurance, when I first fitted the slightly different profile tyres I checked with my insurers first and, once they were sure I wasn't changing my wheels as well, were quite happy for me to do this with. The only problem I had was that the first tyre place I tried refused to fit a tyre that wasn't standard to the car. (Subsequently I've found local places that are happy to fit anything that's safe, and much cheaper too

- £53 per tyre inclusive on Pirelli 175/50x13 P5000 Dragos)

HTH

Reply to
Tim Vincent

wheels/tyres,

something?

It refers to the spacing between the wheel rims. It's directly related to the width of tyre that may be fitted. Mike.

Reply to
Mike G

or

Should have mentioned the numbers are inches. Presumably the 'J' refers to the type of rim.

Reply to
Mike G

Does anyone know exactly how it relates? I have seen 205-40-17s fitted to a 7x17 wheel. By my calculations 7" = 177.8mm, which is clearly less than the width of the tyre.

Presumably you can go a certain percentage over the width of the rim when choosing a tyre width, because of the roundness of the tyre, but how much can you go over?

Al

Reply to
Al Reynolds

1) No. 2)Ah, well you see this is the thing. It all depends on the width of the rim. 3) If 2) is satisfied then yes.
Reply to
Conor Turton

This was in some ways the primary concern.

We know a little place. They'd not fit dodgy tyres. They even constantly (when I still went there) check their little book for the correct speed rating, so they don't fit under-rated tyres.

Pete.

Reply to
Pete Smith

car

or

fitted.

I think your calculation is a little out. :-) The wheel is 177.8mm wide, but the tyre is 205mm. Or an 8.07" wide tyre is on a 7" wide rim.

Difficult to give a precise figure, as the safe o/s of tyre, also depends on the ratio of the tyre width to it's hight, or profile. The lower the profile the original tyre has, the less choice you have when it comes to fitting an o/s width tyre. Too wide, or indeed too narrow, and the tyre bead will not seat correctly on the rim. That is assuming a wish to maintain the original overall diameter or circumference, so the speedo and mileometer readings remain correct. Best to check on the internet for a safe alternative tyre for a specific wheel size. Mike.

Reply to
Mike G

If your tyres are below the speed rating recommended by the manufacturer, your insurers generally won't cover you.

Reply to
Doki

Bollox.

Load rating yes, speed rating is irrelevant for driving in the UK.

Reply to
Tim S Kemp

That's precisely my point. On my Accord I have 195-60-15s on a 7" rim, which is a 7.7" tyre. So in the standard setup, the tyre is 10% wider than the rim. This is how the car ships.

In Halfords, almost all their 17x7 alloys are fitted with

205-40-17s, so the tyre is 15% wider than the rim.

Therefore, there must be some guidance somewhere about what widths of tyres can fit on a particular width of rim. I agree with you that it would depend on the profile of the tyres.

The site below says that the width of the tyre should always be more than the width of the rim:

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it gives no guidelines as to how much more. On this site:
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suggests that the standard advice is to make the rim width75% of the tyre width, but that people have gone as high as90% of the tyre width to gain better performance. Put theother way around, the standard guidance would have tyrewidth as much as 33% higher than the rim width for a softerride but worse handling, dropping to 10% higher than the rimwidth for better performance and a harder ride. I have noidea whether this makes sense. On rec.autos, I found this formula:

average tire width (mm) = 27.86 * rim width (inches) + 22.8

So for a 7" wide wheel, this would give an average tyre width of 215mm (nearest to 217.8mm). This would make the tyres about 22% wider than the rim width, which fits with the previous site's suggestion of between 10% and 33% wider. I have seen suggested tyre widths for 7" wide wheels range from 195 to 245, which is between 10% and 38% wider than the rim width.

One other thing, apparently most wheels are wider than their nominal rim width by 0.5-1 inch. This is apparently because the nominal rim width is measured as the gap between the inner and outer walls of the rim, which can be quite thick.

--------------------------------------------------------------- So, my conclusion so far is as follows...

For a particular nominal rim width in inches W, the suggested tyre width (in mm) would be given by 27.86 * W + 22.8

A suggested lower limit would be 27.94 * W A suggested upper limit would be 33.87 * W

For a 7" wide wheel, this would give a suggested tyre width of 215. The lower limit would be 195 and the upper limit would be 235. This fits pretty closely with the sizes that are offered for wheel and tyre packages.

Anyone any further thoughts/comments?

Al

Reply to
Al Reynolds

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