I just picked up some ridiculously cheap 16 inch alloys and need some tyres. Im currently running 195/50/15's now im stepping up to 16's do i go for do i go for 205/50/16, 205/45/16 or 205/40/16's.

50's or 225 45's

205/50 or 225/45

His coupe is a 20V so fitting 20VT sizes may not be the best idea for his car as both will increase the gearing by around 6%.

205/45/16 will be closer to the original diameter.

Here's a tool to calculate the effects of changes.

Petep, it doesn't always work like that. It's the effective rolling radius you need to consider; i.e. when the tyres loaded.

Same weight, same deflection. Always assuming that all tyres of the same dimensions have similar sidewall stiffness.

Which is why the same pressure, too...

I'm not sure I follow your assumptions, the tyres are different centre diameters and widths. Your calculation was based on an unloaded tyre. As I've stated before you need to consider the tyre loaded. As you've calculated an unloaded difference of 6% I'd be inclined to think Fiat would've kept the effective rolling radius the same between na and turbo.

Surely 195/50 15s are incorrect for the 20vna - that equates to 576mm diameter which is way off the 20vt diameter of around 610. I think the original 20vna tyre size would have been 195/55 15

205/50 or 225/45 as stated here already are the correct sizes for 16" wheels on the coupe.

20v NA had 205/50 R15 as standard, 20vt had 205/50 R16 or 225/45 R16.

But Fiat don't, it is close, but not identical.

Looking into it further it appears that Sheep's car has the wrong size tyres fitted on his present 15" wheels.
The tyres Sheep has on his car are 195/50/15.

From the handbook figures the 16v runs 195/55/15 which gives a diameter 596mm, the 16vt is 205/50/16 = 611mm a difference of 2%

If, as I believe is the case, the correct 20v tyre size is the same as that of the 16v, then I would be fitting 205/50/16 on the new wheels..

Wouldn't 45's be closer to the original size?

205/45 R16 would be closer and 225/40 R16 would be identical to the original.

But back to my effective rolling radius argument; with the car sat on its tyres I suspect the effective rolling radius to be near identical. As I've said before you can't establish any kind of suitability from working with an unloaded tyre.

I don't disagree with your conclusion, I'd use the stock size also. But your method used to determine circumference is flawed.

If you want a couple of very good 205/45-16 Yokohama AD08Rs I have a pair for sale

I'll leave it to others to decide if the size is right.

Sorry sheep didn't see it was an na, drop 5 off the side wall to get as close as possible to the na 15' size.

So looks like 205.45 or 215.45.16's. Andy could you pm me a price for your tyres delivered.

It's not flawed, it is purely geometry on an unloaded tyre which is the only possible constant which can be used.

As soon as you fit it to a car, then variables such as weight, sidewall stiffness and tyre pressure come into play, so then the actual rolling circumference will be affected. But at different rates.

It is because you only considered the unloaded circumference when calculating. Which, as I've said previously, has little bearing on how the effective circumference will be.

What use is knowing the unloaded circumference when considering gearing?

European law... outside circumference should be within -2% and +1,5% of the original circumference of one of the tyre sizes the car was homologated with.

This is checked at MOT (not in the uk I'd take a guess).

Track width when fitting different size tyres or rims should fall within a tolerance of 2% for cars and 4% for all terrain vehicles.

Loaded or unloaded is not taken into account, and would vary more due to tyre pressure and sometimes even wear.

Fundamental, because it is the basis from which you can work out loaded circumference.

Everything in physics and engineering has to start from a known position, you then bring in additional data when making calculations for varying conditions.

So i have the wrong size tyres on my 15's at present they should be 195/55, so i should go for 205,50,16 then.

Sheep, pop onto kwikfit site and it will tell you the standard size, then knock5 off for the inch increase.

Petep, your first post made reference to an unloaded tyre and its effect on gearing. As I stated previously the effective radius is what we need as the tyres effective radius varies with the dynamics of driving, and thus its effects on gearing change. Which we both agree on?

As far as I'm aware there isn't a way to calculate the effective rolling radius without having access to the tyre's data. We just don't know how the tyres will behave at different speeds/temperature/load.

I'd therefore assert that using the unloaded tyre circumference is, in the context of this thread, an unnecessary datum.

So let me get this right: we can't calculate the effective rolling radius, and the unloaded tyre circumference is "an unnecessary datum"...what size do you suggest the OP uses? And what do other people use when wanting to change sizes?

The workshop manual states 205/50-15 for the 15" n/a wheels and 205/50-16 for the 16" 20vt wheels, so I think your statement isn't right.

And whilst (when changing sizes) we'd all like to keep the "radius" the same* there isn't an infinite choice of sizes, and with some more common sizes being significantly cheaper, this will often sway the buyer's decision to accept a few % difference either way.

* Me? IIRC I've got 225/50x17 which eliminates completely the speedo error and fills the arches nicely (but does rub a bit on the wheel arch liners )

DaveG, do you understand what I mean by effective rolling radius? As I think you've misunderstood my point. I think the OP should go with a 205/50/16 as per the turbo, and one of my previous posts. But not based on a calculation that doesn't consider the tyres circumference in use. It's different to the unloaded circumference so we can't say with any certainty how the new tyre size will affect the gearing. I base my opinion on the fact we have a size that is known to work on the Coupé and it's speedometer.

I am saying "we" can't calculate effective rolling radius. We don't have the data. A large tyre manufacturer, they can, using historic data and computer modelling. We don't have that.

My point is that quoting an unloaded circumference is misleading. What I suggest is that people make an educated guess, much as I did with my 215/35/18, and presumably you have with yours.

I'd be interested to hear why you don't agree that the effective radius between the turbo and na tyres can't be the same. Petep worked out the unloaded circumference difference between the two above and got 2% difference.

I thought 225/40/18 was stretching it a bit!
Mine will be an inch bigger than the standard size but yours will be nearly 2 inches.
Does that not compromise the handling?

I don't follow? You've got bigger tyres than me.

I meant the sidewall will be bigger allowing more movement when cornering.

I still don't follow, your sidewalls are bigger.