Choice of Tyre Section

Generally, yes (the latter).

Not always, as they go out of fashion they don't make as many so the price goes up. I paid more for some Morris Minor tyres for a trailer than I'd pay for use on our main car. ;-)

The goal is to try to keep the same rolling radius so that you don't affect the gearing / instruments / speedo and so if you play with the profile (percentage of the nominal tread width (in mm)), that will in turn set the rim diameter (in inches).

There are quite a few online calculators that may help.

And remember if you do change the wheels you may need to notify your insurance Co.

Oh, if you go for a higher profile tyre and a smaller diameter rim, make sure it will still fit over the gubbins (like brake calipers etc).

Cheers, T i m

If you change the profile of a tyre to greater on the same size wheel, you'll also change the rolling radius, making the speedo read low.

Quite a few cars give you a choice of wheels sizes and tyre profiles all of which have the same rolling radius.

If you simply bit high profile tyres to large wheels (if you can even get them) you may end up with them rubbing on the wheel arch, etc.

Generally, most go for the largest wheel/low profile tyre since that says top of the range. So a smaller wheel for the same model can often be bought cheaply.

My last BMW had several wheel diameters over the life of the model. Mine being an early one had the smallest. Bought a set of wheels and excellent top brand tyres in my size off Ebay for not a lot - the person selling had 'upgraded' to larger wheels.

You need to change both wheels and tyres, such that you end up with the same overall diameter.

Thanks for that. No, I'm keeping the existing wheels.

Yes, as Dave and Tim have made perfectly clear. Looks like I'm stuck with what's on there, then!

(As mentioned), many vehicles can / do come with a range of wheels with the bigger / low profile jobbies often appearing on the more expensive / sport models.

In most owners handbooks it will show a range of wheel, tyre sizes and pressures, often depending on the engine size and trim level (or load capacity if a van etc).

So, it's quite possible (also as mentioned) you could probably pick up a set of smaller diameter wheels pretty cheap from someone upgrading to some bigger ones (with lower profile tyres etc).

You may be able to sell yours, get replacements and earn some money. ;-)

The only thing is you would need to make sure the overall rim / tread width was roughly similar (and the centre spacing / offset etc) and the tyre load / speed rating up to it.

I was talking to a mate who was out with his son the other day in his fancy BM with it's low profile tyres. They got a puncture, no spare and ended up sitting in the car on the had shoulder till the emergency services turned up (because it was so cold).

Apparently he has suffered all sort of wheel damage on potholes etc. ;-(

Doesn't sound like a 'good idea' to me, ;-(

When we built the kitcar we went from fairly small diameter wheels and tyres (12 - 13"?[1]) to 14" with nearly full profile (80) and wider M+S tyres. This increased the circumference by about 1/3rd so we had to fit a higher ratio diff to bring the rev / mph roughly back where it should be.

Cheers, T i m

[1] 78 1300 MkII Escort Saloon?

Not necessarily. Use the tyre calculater to change wheels.

Harsh suspension could also be due to faulty brackets under the car.

I'm not going to spend much on it anyway. It's not worth it when you change your underpants more frequently than your car - as I invariably do. It's no exaggeration to say I've owned hundreds of cars since I began driving back in the good ol' 1970s. A mate of mine is an MG nut. He bought his first one at the age of 15 60 years ago and still has it to this day (and loves it to bits). I wish I could be like that. :(

Take a look at the handbook. You may find that a narrower tyre, with a deeper sidewall on the same wheels was an option on some models. Many years ago, when I had a Sierra, I found that 165, 185 and, I think, 195 tyres were fitted to the same wheels, variations in aspect ratio giving the same rolling radius.

SteveW

I have a 1995 Saab 9000 2.3 200bhp full turbo auto leather. Love it. Amazingly smooth engine, Saab made a special advancement on balancer shafts and won an award for this engine. Plus solid build quality.

I think you only get that option when you aren't already stuck with a very low profile tyre?

If you currently run say a 175/70/15 you could probably also get away with a 185/65/15 or, if the rim would take it, a 205/60/15, or for a bit more error, a 195/60/15 or a 195/65/15.

When you are already down to the 35 profile tyres, you don't have many directions to go that won't end up with the tyre being that much bigger and the profile any better (inside a 3% error anyway).

Cheers, T i m

It must be something very extreme for "little more than 30mm of rubber between an alloy wheel rim and a curb" In fact I really think you won't find any road tyre with a section height even approaching that as the lower you go in profile the wider the section width.

For example

(you can ignore the rim sizes but they are there because these are plucked from real world tyres available to buy today)

165 x 45 x 15's have a section height of 75mm 165 x 80 x 15's have a section height of 132mm 155 x 80 x 13's have a section height of 124mm 185 x 55 x 15's have a section height of 102mm 195 x 55 x 16's have a section height of 107mm 205 x 35 x 18's have a section height of 75mm 215 x 30 x 20's have a section height of 64mm 205 x 45 x 16's have a section height of 92mm (that was the lowest on any production car in 1994)

If you can find a tyre with a lower section height (in mm) than 64mm, either fitted as standard or available in the aftermarket from a reputable manufacturer with type approval I'd like to know!

If you can find one with "little more than 30mm" I'd eat my hat and grate my front brake pads on my muesli.

No reason why you can't change from for example a 185 x 55 x 15 tyre to a 195 50

15 on the same rim (that example being for Lotus Elise S1 fronts)

The distance per wheel rev is 1836mm for the former and 1809mm for the latter, the diameter within 9mm and it keeps the speedo 'legal'

Both tyres were type approved for fitment by the manufacturer

A few years ago a mate of mine who has amongst other activities a business refurbing alloys bought another new 911 'for his wife' and laid on extra staff on the basis of ongoing repairs to Audi Q7's BMW X5's and the like with their ridiculous 20 & 21 inch rims and tyres resembling rubber bands.

That they often bent out of shape when you ran over a single piece of pea gravel seems to have been overlooked by the 'designers' sat at their desks in the factory.

I hope your mate goes on to make a ton of cash from this business; it's the only way the dumb-arse car buyers will learn.

AFAIK, the only advantage of skinny tyres is better roadholding around corners on nice, smooth racetracks. Maybe you could get away with such tyres on the lovely public roads we had back in the 1970s, but today's

3rd world pot-hole riddled assault courses? Forget it!

I saw a Range Rover the other day, driven by a middle-aged gent. It was nice and clean, but had special large alloy wheels and ridiculous tyres, just like the afore-mentioned rubber bands. It's a Range Rover, for Christ's sake!

it is a 'life style'

Yes, it's a Range Rover being misused as a status symbol. The chap probably never ever takes it off-road. Hence the nickname 'Chelsea Tractor' for such vehicles. :)