My focus tdci has 195 60 r15 tyres and I'm thinking of going one size up to 205. (Don't want low profile, just more width).

Is it possible to fit wider tyres, and what would be the effects on fuel economy, acceleation/top speed, road noise and handling?

OK, so I wrote up a massive reply, and then it disappeared into oblivion. So here is the short of it...

Going to:
205/60/15 - It will gear the car UP 2%, so your speedo will say your doing 60mph, but you'll actually be doing 61.2mph (If its correct to begin with). Fuel economy and top speed are likely to be better by 2% and acceleration will be about 2% worse.

205/55/15 - It will gear the car DOWN 1.4%, so your speedo will say your doing 60mph, but you'll actually be doing 59.2mph. Fuel economy and top speed are likely to be worse by 1.4% and acceleration will be a bit better.

The latter will provide a tad better handling than the first. Hope this all helps (and I hope the forum doesnt delete THIS post.... grrrr...

Thanks Si_NZ. Its happened to me before - what I do now is press ctrl and c before I press the post button if I've written a lot.

I would have thought the first option, with the same diameter tyre as my 195 would have no effect on top speed and speedo reading. So are both options illegal if it gives out an inaccurate speedo reading? So you would need to change the wheels to offset the different tyre size?

In light of this, I don't think its worth it. I just liked the idea of giving the car more presence with the tyres. I've seen corsas with astronomical tyres but they just look daft.

u can also use right click and back

No problem, and yeah, I did press ctrl and c, the second time... haha.

The first option would be the way I would go too. Better economy is always good.

It does have an effect on top speed, and speedo reading, as the height of the sidewall of the tyre is the "60" or "55"; a percentage of the width (eg 205mm) of the tyre (sorry I dont know how to word it better than that).

On a 205/60 tyre, the sidewall will be about 123mm.
On a 205/55 tyre, the sidewall will be about 112.75mm.

Not a big difference, but from dropping one profile, you've decrease the total size of your tyre by nearly an inch!

You wont need to change your wheels to go up a size in tyre, anymore than one size, you might.

not sure if i agree si.
an extra bit of 5% extra width will add more drag, but this is also related to compound tack.
otherwise r15/60 will be the same diameter and gearing etc.

Ok, fair call, the 5% extra will add a small amount more drag...

Are you saying that 195/60/15 and 205/60/15 are the same diameter and gearing?

quote:

Originally posted by Si_NZ:
Ok, fair call, the 5% extra will add a small amount more drag...

Are you saying that 195/60/15 and 205/60/15 are the same diameter and gearing?

erm this is not my area of expertise so im getting a bit nervous, so put me right if you like. I thought 195 and 205 refer to width, and the 60 the diameter (which is staying the same).

I always used to think that was the case, but the 60 for example is the percentage of the width (205mm), which determines the sidewall height.

Maybe a play around with this tyre calculator will help clarify what I dont know how to put into words...

i wouldn't pay any attention to those % differences in economy and acceleration. there is no science to back those up.
but smaller diameter will shorten the gearing and bigger will lengthen it. shorter gearing can improve acceleration generally. longer is better for economy once once at top or cruising speed.
the wider tyres will have more friction on the road and more air resistance to push against.
same applies for larger diameter but in a much smaller scale. wider tyres are heavier.
if you are having traction issues then go for it, but first make sure you are already using the best quality sports tyre your money can buy, this will give you the best performance increase rather than going for different sizes.

quote:

Originally posted by beach:
i wouldn't pay any attention to those % differences in economy and acceleration. there is no science to back those up.

Ok, there may not be science, but I have had experience with changing tyre sizes, up to you if you choose to pay any attention to my findings.

I went from 205/65/15's to 195/55/15's on the front of my FWD car (they were free, and very nice Pirelli's) and this put my speedo out by about 6mph at 60mph (showed 110kph on the speedo, but I was actually doing just under 100). And it used about 10% more fuel, driving over the same route several times, with cruise control engaged.

Acceleration was a lot better, but it made the car rev quite a lot higher at the same speed. It did however make it handle very nicely.

quote:

Acceleration was a lot better, but it made the car rev quite a lot higher at the same speed. It did however make it handle very nicely.

Correction...

The 195/55's made the car rev quite a lot higher than the 205/65's did at the same speed.

205 60 r15

I always thought

A: 205 = width
B: 60 = sidewall height
C: r15 = inch diameter of wheel

So the value of B will vary and depends on A. ie. 60 is not a fixed value but a % variable.

the 10% is a coincidence for both readings.
it doesnt reflect on the formula of 10% difference is speed = 10% difference in fuel economy, which is not true over all variables.

quote:

Originally posted by HP75:
205 60 r15

I always thought

A: 205 = width
B: 60 = sidewall height
C: r15 = inch diameter of wheel

So the value of B will vary and depends on A. ie. 60 is not a fixed value but a % variable.

thats a very common mistake that people make.
not realsing its a ratio value and not a measurement.

every day is a school day, cheers si & beach

For such a small change, i wouldn't bother, tbh.