tyre sizes

Hello,

The Astra handbook lists seven different tyre sizes that can be fitted to the car. They are:

185/65r15 195/60r15 215/45r17 225/40r18 195/65r15 205/55r16, which is what is on at the moment, I think 225/45r17

I haven't worked out what circumferences all these would have. Are these sizes all "plug and play", i.e. can you swap them without any adjustment, or do they have different circumferences and would I need to get the car's computer reprogrammed to tell it what size rim was on each time I swapped wheel sizes?

I ask because I'm thinking about getting some winter tyres in the summer, before the prices go up ;)

The manual says not to use 17 inch rims or above with winter tyres but it does not recommend any one size. I'm wondering whether to get winter tyres that are the same size as the ones that are on now, which would be simplest but the 15" rim size are slightly cheaper, so I am wondering if I bought 15" rims and went with 195/65r15 rather than

205/55r16 whether I would have to do anything to recalibrate the speedo for example.

Thanks, Stephen.

Reply to
Stephen
Loading thread data ...

the two you mention at the end would be very similar in rolling radius. I doubt you could tell any difference, but you could always check with a GPS

Reply to
Mrcheerful

The scribbles on the back of a handy envelope say they're all within 3% of the same rolling radius. Legally, your speedo only has to read within

10% at 30mph, and not underread at any speed.

Try this:-

formatting link
To save wear and tear on your fingers.

Reply to
John Williamson

formatting link

So realistically it could put you over or under the legal requirements

Reply to
steve robinson

Most likely just upsets the stability of the car if you want to use a bigger balloon tyre. ie using a 65% profile vs a 55% profile. You have to use the correct width rims to suit the width of the tyre.

Most seem to fall within the same radius size except that 195/65-15

Most speedos are out of wack nowdays none of mine read accurately 4kph slow one checked with the GPS And its a bugger calculating mph to kph,

Reply to
Rob

formatting link

Reply to
Rob

formatting link
> To save wear and tear on your fingers.>

To be *totally* certain, load the vehicle to its normal weight, with the tyres you will be using on it, and take it to your local tachograph centre, where they can check your speedo to within 0.5% on their rolling road, then go to a main dealer who can apply the appropriate correction to the car's computer. Or put your new tyres on, go to a measured mile, (Handily located on a number of major roads near you, and used by the police to calibrate their VASCAR type equipment), travel along it at an indicated 60mph, time it, and divide 3600 by the number of seconds it take to travel the mile to get your actual speed in mph. Have you checked your particular case using the calculator I gave a link to?

I plugged your two desired tyre sizes into the calculator that I linked to, and they say that the error from one tyre to the other is less than

2%, (+1.4/ -0.7kph respectively at 100kph)so does *that* answer your question? Tyre rolling radius will vary by more than that in day to day use with wear, varying inflation pressure and varying loading, too, so you may want to take that into account.

Unless you are a driver who routinely drives at the speed limit plus

10%, plus 1 mph to stay within the ACPO guidelines, you'll be legal and safe from prosecution using *any* of the listed tyre sizes as long as you drive with your speedo at the posted limit. Your ultimate defence in this case is the car maker's recommendation. As long as you have followed that, then you have done all that will be expected of you. If *they* considered that the computer needed recalibrating when you changed tyres within the list, it wuld say so in the manual
Reply to
John Williamson

In message , John Williamson writes

LOL! Or use a GPS. Preferably held by someone other than the driver, of course. They are much more accurate than a vehicle speedometer and will give a direct comparison.

Reply to
Gordon H

In message , Rob writes

My GPS(s) can all be set in either Imperial or metric units.

Reply to
Gordon H

formatting link
>> To save wear and tear on your fingers.>>

Have you ever had sex?

with a woman i mean.

Reply to
Nige

formatting link
>>>

Yes, thanks. Not only that, I didn't pay. Have you?

Reply to
John Williamson
[SNIP]

More to the point, have either of you ever snipped?

Chris

Reply to
Chris Whelan

Thanks for your replies. I found them very useful. There were two Steve/Stephens, I think you got us mixed-up.

Reply to
Stephen

I never knew that. So are cars always more stable with lower profile tyres?

Reply to
Stephen

it would make little difference in road use, esp. in bad conditions.

Reply to
Mrcheerful

Side walls flex more.

Reply to
Rob

Does the cost of the 15" rims offset the difference in tyre price?

Do you have a model of car that has bigger discs and calipers (ie a faster model) than those with 15" wheels - will they physically fit on the car?

IMO you should just use the tyres and sizes that were specced for my model of car when it was designed..

Reply to
Mike P

Yes lower the profiles - higher the price.

Have a look at the difference of the tyres mentioned as to the specs and compare the prices (of the same manufacturer of course)

formatting link
Interesting exercise.

Reply to
Rob

I am planning on buying new rims anyway, so that I can have a summer set of tyres and a winter set on their own rims. I am only going to get steel rims, possibly second hand ones, so I hope the cost of either 15 or 16" will not be too excessive.

It's just a standard 1.4 Astra as far as I know. Nothing sporty.

But all of those tyre sizes were spec'd by the manufacturer and all are listed in the handbook!

Thanks, Stephen.

Reply to
Stephen

So pick the cheapest combination of M&S tyre tread and rim you can find on that list for the cold and wet season, and the highest grip/ acceptable wear combination for your current rims for Summer fun. Low profile tyres grip better, but are less comfortable. As to grip, with a decent make, on a 1.4 Astra, you'll probably only really notice the difference if you're going stupidly fast.

Having said all that, if you live in most of the UK, there's no real reason (IMHO, etc...) to have Winter and Summer tyres. If you live where they do make a difference, you should possibly be considering something with four wheel drive.

Enjoy!

Reply to
John Williamson

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.