National Tyres

No I won't. you bugger.

Arthur i ...in high dudgeon.

That's one reason. Another, quite vital, point, is that they'll be too small to fit onto your wheels. This is not a good thing.

Right, you can work it out. The number in the middle (55 or 65) is the profile. That number indicates the height of the sidewall as a percentage of the width of the tyre. So the overall rolling radius is half the diameter of the wheel, plus the sidewall height.

Let's take the 185/55 R15. Rolling radius is 7.5 inches, sidewall height is

Doing things the same way, the rolling radius of the other tyre would be

That's really quite an insignificant difference, so I really wouldn't worry that much. The RR of the 15" tyre is 0.24% greater than the 14" tyre, so if the speedo was correct in saying 70mph for the 14" tyres, you'll actually be doing 70.168mph with the 15" tyres. And let's say you were daredevil enough to test out the top speed of your car, which is probably around 120mph indicated - you'd be doing a devastating 120.288mph.

Let's take it down to more urban speeds - 30 on the speedo will equal

The ones you've got. The tyre man is obviously s**te at maths, or just doesn't know his arse from his elbow, or both.

Though 185/60/ R15 is a much more common size, but it'll be a bit more out - around 3.4%.

So let's look at what that means (first figure is the speedo speed, second is the real speed, assuming the speedo was right to start with):

So you'll be fine with them, but don't allow too much margin for error if you're above the speed limit, as your speedo will already definitely be under-reading a reasonable amount.

But in short, it looks like whoever put those tyres on your car knew their stuff, as they're pretty much a bang-on match as far as RR and speedo accuracy goes.

At a true 70 mph the Speedo will read 70.17mph , you would struggle to get closer than that.

Thanks again, Steve. I just didn't search the etyres pages as well as you did.

As you may have noticed I'm a trying to buy cheap but on the other hand I occasionally have to tote my 3 year old about. So my self respect tells me to steer clear of the mega cheapos. Are the Firestone F700's a decent choice?

Arthur

Should be OK - never had Firestones myself - but I'd pay the extra 7 quid / tyre for Dunlop SP200s. I have them on the front of my Alfa 75, which gets major twisty road stick when I use it, and I can't fault them.

the 185/55/15 are the factory option with alloys

IIRC, that's the size my old 306 XSI had, and also the size fitted to the front of the series l Lotus Elise.

I notice they do Toyo in my size, are they any good?

I refuse to buy anything French.....so no Mitchelin

Which could be made almost anywhere in the world - including South Africa.

Chris