On the standard 5.5J x 15 steel rims I can fit 195/65R15 or 185/65R15 tyres per the data plate on the drivers side door and the owners manual. The car came with 195/65R15 tyres. After 30,000 kms the tyres are OK but I will be looking for new ones in another couple of thousand km's. I wouldn't think that changing to 185/65R15 tyres would compromise on safety since they are a suggested option. But wondering if they would make any measurable difference to fuel economy.
Data plate attached for reference.

[2i30s]

with such a small drop in tyre size i think your only saving will be with the tyre purchase,not in fuel consumption.  my wife's cw runs the 195/65/15 and my hatch has 225/45/17 tyres. when we go on long drives our fuel consumption in each car seems to be very similar,but i/we only use our comparisons by using our fuel gauges because both our cars are base models. 

[Surferdude]

Don't think it'll make a noticeable difference in fuel economy Steve.

We've covered this slightly in the last day or so on here somewhere. The first thing to remember is that the 195 is doing 835rpm at 100kph. The 185 is doing 18rpm more at the same speed, so there'd be a little more fuel used.
There are other things to consider.

1/The smaller tyre will ride slightly more harshly as there is less sidewall to flex and absorb bumps, dips and other imperfections.

2/ You will have approximately 10mm less rubber with on the road - so slightly less grip. You need to remember it's not just the width but the fact that this 10mm carries right through the tyre's footprint from front to back.

3/ The slightly wider footprint helps keep the tread flatter on the road in cornering, thus reducing uneven shoulder wear.

They're the most obvious points in favour of staying with the 195/65R15.

There is one marginal plus to the narrower tyre. In the wet, slightly less tendency to aquaplane.

Thanks for the advice surferdude - makes sense to stick with the same.
What are your thoughts on the Hancook Optimax tyres that came with the car. Do they make a measurable difference to economy?

[Surferdude]

Thanks for the advice surferdude - makes sense to stick with the same.
What are your thoughts on the Hancook Optimax tyres that came with the car. Do they make a measurable difference to economy?

Steve, have a bit of a read through here as it documents my thoughts on the OE tyres. There are probably any number of options available.

Trev
https://www.i30ownersclub.com/forum/index.php?topic=9803.0

[Hati]

Surferdude, can you explain how the 10 mm narrower but same diameter tyre turns 18 rpm more? Unless I am missing something tyres of the same diameter will rotate the same amount at a given speed. The wider tyre, 195, will use slightly more fuel having a larger contact surface of course, but probably on the negligible side on a tank of fuel. The rest of your points however make a lot of sense

[accim]

This site/calculator is great for calculating the optional tires http://www.miata.net/garage/tirecalc.html

I've had winter tires 185/65/15 on my previous i30CW, also had summer tires 205/55/16 on the same car. The fuel consumption was about the same or maybe there was max 0,2 lit difference. Going 185->195 won't improve nor worsen the fuel consumption.. Maybe in theory, with the 185 the fuel consumption will be just a bit better (the 195 is also a bit heavier so it adds some little weight to "unsprung" mass), but in real life, there isn't any difference really.

On my new i30CW, the stock tires are 195/65/15 and I must say, there isn't that much difference between 185 and 195. If you ask me, I would go for either 195/65/15 or 205/55/16 on i30, especially CW. Feels right and looks okay. And in Slovenia, due to the fact the 195/65/15 dimensions is really widespread, they even cost less then 185 ones.

[Surferdude]

Surferdude, can you explain how the 10 mm narrower but same diameter tyre turns 18 rpm more? Unless I am missing something tyres of the same diameter will rotate the same amount at a given speed. The wider tyre, 195, will use slightly more fuel having a larger contact surface of course, but probably on the negligible side on a tank of fuel. The rest of your points however make a lot of sense

Hi Hati.

The simple answer is they aren't the same diameter.
The 185 is 621.5mm
The 195 is 634.5mm

As the 65 designates that the sidewall is 65% of the width of the tyre, this explains the difference in height.

[Surferdude]

And in Slovenia, due to the fact the 195/65/15 dimensions is really widespread, they even cost less then 185 ones.

Thanks for that accim.
You are quite right but I didn't mention it above because I wasn't sure if it was the same in other parts of the world as it is in Australia.

Two things happen. Beacuse there are apparently more cars using the 195 more are made which reduces the cost because of economies of scale.
Then, because more people are wanting to buy them the marketplace gets into more of a price battle, which also drags the price down.
It's also why 235/45R17s are considerably cheaper than 225s or 215s.

BTW. As you are probably aware, Sava is owned by Goodyear these days. And they make very good tyres both car and truck. We've had them in Australia for years and once they started getting into silica treads they were very good value.

[Hati]

The simple answer is they aren't the same diameter.
The 185 is 621.5mm
The 195 is 634.5mm

As the 65 designates that the sidewall is 65% of the width of the tyre, this explains the difference in height.

Always learn something new , thanks for that.

[rustynutz]

Trev has pretty well covered the downside of going to a 185, but just in case some haven't realised, the slightly smaller diameter will also alter your speedo and odometer a whisker....

[Surferdude]

Trev has pretty well covered the downside of going to a 185, but just in case some haven't realised, the slightly smaller diameter will also alter your speedo and odometer a whisker....

Yep.
On the 185's at 100kph, your speedo will be reading 102.1kph

Assuming it is accurate on the 195's

And to add, the speedo which already indicates higher than reality will read even higher.

That is to say, when doing 100 the wider tyres will indicate ~95 but the narrower (smaller diameter) tyres will indicate maybe, ~93 (this is a guess - CBF working it out!).

Edit: K, Surfer said it before me... same idea...

[Surferdude]

Hers's a pic

[Ace Demon]

2/ You will have approximately 10mm less rubber with on the road - so slightly less grip. You need to remember it's not just the width but the fact that this 10mm carries right through the tyre's footprint from front to back.

Which ignores that the chord will increase to compensate, unless the pressure is increased. Grip has only limited connection with width, given that width is only a reflection of the general increase in car weights. My 205s on a >1.3 tonne car are no "more" of a tyre than the 165s I used to have on <1 tonne cars. I am surprised that 185/70 is not listed as a better equivalent.

[Surferdude]

2/ You will have approximately 10mm less rubber with on the road - so slightly less grip. You need to remember it's not just the width but the fact that this 10mm carries right through the tyre's footprint from front to back.

Which ignores that the chord will increase to compensate, unless the pressure is increased. Grip has only limited connection with width, given that width is only a reflection of the general increase in car weights. My 205s on a >1.3 tonne car are no "more" of a tyre than the 165s I used to have on <1 tonne cars. I am surprised that 185/70 is not listed as a better equivalent.

I'm not sure what you are saying here.
Firstly, what is a "chord"?
And secondly, on the same size car and with the same size wheels, what I said isn't ignoring anything that I'm aware of.
Could you please explain further?

[Surferdude]

^^^^^
Plus, a 185/70 will not give as good steering response as the standard 65 series as there is more flex in the sidewall.

OK, I've waited long enough for you two... my understanding of the "chord" is the measurement from front to back of the footprint. What I think has been said is that all things being equal (pressure), the "area" of the footprint will be (near enough) equal for the two tyre sizes.

So apart from "flex" from the slightly taller sidewall one is the same as the other and in practical terms, equal.

I'll butt out now... I know nothing about tyres really...

[Surferdude]

Pip,
That's not true. I said above that the footprint difference would be significant - not massive, but significant.
I still don't know how a chord which is a geometric term, relates to the footprint of a tyre.
Tyres have "cords' which are the strength of the tyre, encased in rubber but they don't alter anything.

Many years ago as part of my training package I developed a series of tyre footprints by the simple expedient of painting a tyre tread with tyre black with the wheel jacked up, then let it down onto a sheet of A4 paper. I prepared about a half dozen including 13", 14" and 15" with some in sequential sizes. I then laminated them for use in training sessions.
The main purpose of this was to show just how small the footprint of a tyre is.
But it also demonstrated the difference in footprint sizes.
I doubt I have them any more but they also demonstrated the different shapes of footprints between different profile tyres.

Probably boring as hell to non-tyre people but it was an interesting exercise.

[Dazzler]

Probably boring as hell to non-tyre people but it was an interesting exercise.

I would find that interesting .. Pity if you still don't have them..

Although, probably a bit hard to illustrate on here...

[Surferdude]

According to the BND Tech Source Calculator, the following figures apply....

Contact Patch Width = 163mm (185) versus 169.6mm (195)

Contact patch Length = 146.1mm (185) versus 150.3mm (195)

Tyre Comfort rating = 7.6 (185) versus 9.1 (195).......... where 1 = bad and 10 = good.

Hope this helps

Summary.....195/65R15 has larger (both wider and longer) than the 185/65R15 and the 195 is noticeably more comfortable.

BTW. I've never seen this site before but it's pretty handy for more than just tyres. May be worth further investigation.

http://bndtechsource.ucoz.com/index/tire_data_calculator/0-20

[Ace Demon]

I'm not sure what you are saying here.
Firstly, what is a "chord"?

If you draw a straight line between two points on a circle, that is a "chord".

And secondly, on the same size car and with the same size wheels, what I said isn't ignoring anything that I'm aware of.
Could you please explain further?

Grip being a function of width is too simplistic. People talk about cars now having more grip than years ago because tyres are wider. There is no connection.

[Surferdude]

Ace, you're making statements which you aren't explaining in relation to the chord. You haven't explained how it will offset the change in width.

And in the case of grip being a function of width being too simplistic, you are drawing a long bow and, in fact IMO over complicating the issue. Again, please explain the statement.

But please remember, we are talking about the difference between and 185/65R15 and a 195/65R15 here. There is no suggestion of "years ago" being involved.

People talk about cars now having more grip than years ago because tyres are wider. There is no connection.

This statement is essentially correct but not in the context of this discsussion. It will create confusion for readers.
There are a whole range of reasons why tyres of today have more grip. But to suggest there is "no connection" to width is incorrect.

[Ace Demon]

According to the BND Tech Source Calculator, the following figures apply....
Contact Patch Width = 163mm (185) versus 169.6mm (195)
Contact patch Length = 146.1mm (185) versus 150.3mm (195)

I wouldn't accept this as gospel because it ignores weight and pressure. Just consider the difference between front & rear tyres.

[Ace Demon]

Ace, you're making statements which you aren't explaining in relation to the chord. You haven't explained how it will offset the change in width.

A tyre patch has length and width, not just width. The area will not fall by loss of width.

People talk about cars now having more grip than years ago because tyres are wider. There is no connection.

This statement is essentially correct but not in the context of this discsussion. It will create confusion for readers.
There are a whole range of reasons why tyres of today have more grip. But to suggest there is "no connection" to width is incorrect.

I am trying to suggest that notions of width (on its own) and grip should be erased. The old/new simply shows that width has increased for completely unrelated reasons. For purposes of discussion, all else being equal, I think it is very unlikely any grip difference between small size changes would be found.

[Surferdude]

According to the BND Tech Source Calculator, the following figures apply....
Contact Patch Width = 163mm (185) versus 169.6mm (195)
Contact patch Length = 146.1mm (185) versus 150.3mm (195)

I wouldn't accept this as gospel because it ignores weight and pressure. Just consider the difference between front & rear tyres.

???

I don't understand why you keep needing to complicate the issue.
It's simple.
The chart is accurate given everything else is equal. It's a normal way of comparing things.
Apples with apples and so on.........

Ace, I accept you have a pretty good understanding of the principles involved here butplease try to keep this discussion on topic.

If you want to discuss the variables involved in all this, could I ask you to start a separate topic, because I do find your contributions interesting and worth pursuing.

Trev

[Ace Demon]

I don't understand why you keep needing to complicate the issue.

Because it is complicated.

[Surferdude]

I don't understand why you keep needing to complicate the issue.

Because it is complicated.

No. It's not. And you continue to make statements without any evidence.
Which isn't fair to other readers.

[Shambles]

It's losing me, that's for sure 

[Dazzler]

Ace,

Trev has about 40 years in the tyre industry many as a Manager.. so unless you have more than that I wouldn't be contradicting him if i was you.

The way I have read Trev's comments on here he has been talking contact area (with the road) not JUST width...