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.

Sorry Szkom. It was in response to Dave G.

According to kwikfit i should be running 205/50/15's as standard.

I am not sure I would ever trust KwikFit to be correct.

No, I think I understand you perfectly.

AFAIK, the 20v na and 20vt have speedo's that are "calibrated" differently.

Yes, but both ours and your "educated guess" is based on the "misleading" unloaded circumference/radius. My point is that it's not exactly correct, but it's certainly not misleading.


petep was referring to the 16v na (195/55-15) and 16vt (205/50-16) where the larger tyre is larger by closer to 3% (2.7%). The OP has a 20v na and is presumably fitting 20vt wheels, where the difference in unloaded circumference is 4%.

On the 20v na/ 20vt, that 4% difference translates to a difference in OD (or “height” if you will) of 611 – 586 = 25mm (1”), so that the same car would sit 12.5mm (½”) higher on the 20vt wheels/tyres. Now I concede that the 20vt engine is heavier than the 20v na, if only because of the turbo and extra radiator fan, but at the same time the recommended tyre pressure increases from 2.5 to 2.7 bar. So I can’t really see how the effective rolling radius on the same 50 profile tyres could be the same. Similar maybe, in the same way that unloaded circumferences are similar, but not the same. But without all this special tyre data we’ll never know, so we’ll just have to agree to disagree.

Just as a footnote, according to the workshop manual, the 16v na could come with 205/55-15 as an option which is larger than the 195/55-15 by 1.8%. Likewise, the 16vt could come with 205/50-15 instead, which is 4% smaller different. And maybe these will all have the same effective rolling radius, maybe not.

At the end of the day, we can’t care too much about something we can’t estimate, but we can care about something that we can, however misleading you think it is.

Oh and finally

Compared to 205/50-16, your 225/40-18 is around an inch “bigger” (OD) or ½” bigger (radius) and my 225/50-17 are indeed almost an inch bigger in radius.

I chose that size deliberately to reduce the speedo error on my car with 205/50-16 (an indicated 74.9mph at a true 70.0mph from GPS, YMMV) to precisely zero, whilst at the same time filling out the arches as I mentioned, perhaps raising the car a little to overcome the “a bit too low?” drop on Eibachs, and also introducing a degree of “comfort” by maintaining the 50 profile instead of going to 45 or even 40. Similar reasons that other people have used in going to 205/55-16, which can be significantly cheaper than 205/50-16.

And with a width of 225, the same as I have used in the past (on 16” wheels) I thought I would be OK on clearance, and in general I am, but there is some rubbing on the front “air grid” part, and on the small cover over the aux belt tensioner or whatever it is. I’m no handling expert (nor, clearly, an expert in effective rolling radius) but it seems to handle pretty good to me.

Dave, hopefully no offence caused. Likewise for Petep.

I think there's a lot we will have to agree to disagree on. But equally I think we're mostly on the same page.

I didn't calculate the circumference/radius/diameter of my tyres. I simply deduced that I'd likely need a smaller tyre profile with 2 more inches of diameter on the rim. I then went for the one that would likely fit (eye-balled), but not behave like it would be painted on. As it turns out a reasonable guess as my speedo tends to read just under.

Where I disagree is that it's not the same 50 profile between the two tyres except in a perfect cross section. ones a 16 inch, the other 15. There's simply more rubber per radian on the 16 inch. Therefore able to deform more under load.

I'm not suggesting we need blinding accuracy; why when a stock car under reads by 7% (using your figures)? But this thread has lots of suggestions of alternative sizes based on an unloaded, static dimension that are comparable to the OP's unloaded, static tyre. I'd like to think you'd agree that as the effective radius varies when in use it's difficult to state that tyre A will be comparable to tyre B without using the "special" tyre data.

Hopefully you can see why I consider the unloaded static measurement of a tyre misleading in context of the OP's question.

I hadn't considered the speedo error!
Like you I wanted to fill the arches a little more than they are at present and give a little more clearance over the many speed humps around here.
That's why I opted for the 40 profile instead of the 35.

Hmm seems to be 2 topics running here....

The kwikfit site is accurate for std fit wheels, as the coupe didn't have a million choices of wheel then it's right.

In simple terms for every one inch increase in rim drop 5 on the tyre wall, same for every 20 in width.

Sheep 45 16 on a 205 are as close as you'll get, 195 will fit on the vt rim also and be closer. Check your speedo to sat navigation on the flat if being a bit out worries you, you can always compensate once you know.

Thanks for the replys guys, im going for some 205/45/16"s. Everyone has assumed i picked up some cheap 20vt rims, infact i picked up some TSW evo 6 spoke alloys for a bargin price of £26 on ebay. These are a temp replacement as i picked them up so cheap. Thinking of getting some spacers for them do i need hubcentric? The et on these wheels are 35 so only looking at 5mm or maybe 10mm at the very most.

Actually I have 225/45-17 fitted and not 50 profile, so not quite as big a deliberate oversize

Decided not to go for the 16's as they still look to small, so now on the look out for 17's and 18's qustion is what size tyre on the 17's. 18's i will run 225/40/18.

I'd go for 215 40 17 or 215 45 17 as they are easy to find sizes.
Mine have 205 40 17 on at the mo and they look very small in the arches.
It also depends on whether you want comfort or sharp handling.

Fancy a set of dark grey Rota Circuit 10's?

Can you post up a picture of the wheels, discriptipn of tyre and wheel condition and then pm me a price.
Cheers

Tyres are all budget and 3 are at least half worn with the 1 other being quite new (6mm left)

Wheels are coming off this weekend and I will be refurbing the lips as they have some minor kerbing.
The rest of the wheels are virtually unmarked.

Not sure on the width and ET until I get them off the car but I am running spacers with them.

Here's a pic of them on the car:

How much for the wheels.