Tyres: Alternatives to KU31s

[mazdarati]

I think you mean Kumho Ecsta LE Sport KU39. I have them on my 135i and agree they're a good tyre in terms of grip and wear rate - but a bit noisy. However they're only available in 17"-19" according to the Kumho website. They're also a bit more expensive than the KU31s.

No No,
http://www.tirerack.com/tires/tires.jsp?tireMake=Kumho&tireModel=Ecsta+SPT
Just the Sport (SPT) model.

I've got them in a 195/50/R15

KU31s, they can be had for $80ea on ebay in that size.

The KU31s is what this whole thread is about, or at least replacing them with some other manufacturer/model. Which seems to be a difficult task as the conversation seems to keep returning back to KU31s and there awesome price and handling.

My mistake, I was unaware that KU31 was code for Kumho Ecsta Sports...feel like im pretty cool now, knowing the lingo

[project.r.racing]

I'm onto my third set of KU31s. Very happy with them. Normally get 40-45,000km out of a 215/35R18 tyre.

[JBT]

Tyres do not effect the the gearbox rpms in relation to the engine rpms.

Correct. However, one assumes that the car will be dríven on a road and not just sit on blocks, stands or a hoist with the bowels being revved out of it. So, considering engine RPM in isolation from actual road speed or indicated speed is irrelevant.

I personally use engine rpms as my shift points and not road speed/real speed to know when to change gears in motorsports I do. So it is relevant to me. Maybe you should try it sometime? Might keep you car in a more powerful rpm range and get you faster time...

Now, now...play nice. After 45 years of driving many and varied vehicles in normal and competitive conditions, I think I have a fairly good idea of how to get the best out of my car(s) using a tacho and knowing what revs and gears to use and when. But we digress...we were discussing the effects of tyre size

But tyres do effect the drive wheel speeds in relation to the engine rpms.

Incorrect. The wheels will spin at precisely the same RPM for a given engine RPM and the speedometer (if the speedometer output gear in the gearbox has not been changed) will indicate precisely the same speed. However, the actual (true, real) speed of the vehicle will differ for a given RPM if the wheel/tyre rolling diameter is changed.

Happy? It was a typo.

This has been fun but, no, not happy. Tyres do not affect drive wheel speeds in any way shape or form with respect to engine RPM. However, they will affect vehicle actual (not indicated) speed.

[Silvia]

Oh hi..for some reason was not notified of the extra contributions coming in on this subject..thanks guys.
So , Silvia which is the car, incidentally, needs new tyres. wheels are 16" stock and nice looking I think. Stock size 205/45 16 doesn't have too many choices but KU31 is one and sounds good except $170 each fitted is the best I've found (leaving out Tempe tyres - too many warnings)
Now, when I put Potenza RE001's on my turbo Forester they didn't make stock size so were a little smaller and when speedo reads 103kph car is actually doing 100kph.Also revs more in 1st..faster but noisier.
I want the opposite effect with the MX5
I find my NB revs too high too soon in first gear (I tend to drop early into 2nd which feels torquier and longer running(i.e. more comfortable) and could cruise better at 110kph in slightly lower revs too.
So, a slightly bigger circumference tyre (say 1900mm instead of the stock 1856mm) will have the car running longer in 1st gear before reaching say, 5000rpm, and at 110kph it will be revving a bit lower than now too ....
Now, if a tyre size conforming to these requirements is also an easily found size instead of a special order (more expensive) Well, how good would that be?

[bigdog]

I run 205/50x16 Michelin PP2s on my NB8B - according to GPS the indicated speeds are almost spot on, about an 8% change over 45 profile. The extra sidewall height is more comfortable on the road too. Check the dimension specs on the tyre brand website for the tyres you choose as each brand is slightly different. My tyres will just touch the shock tower and the inner plastic guard lip on full compression, on stock suspension and wheels you should be fine.

[project.r.racing]

Bigger tyres wont effect any change in how you reach 5000rpm. It will only change the vehicle speed at 5000rpm.

Two optiosn for bigger tyres are 215/45 or 205/50. The 215/50 is 9mm(1.5%) bigger and the car will rev 1.5% less at any given speed. The 205/50 option is 21mm bigger and will rev 3.5% less at any given speed.

So example, if you were doing 2500rpm @ 110kph. then on the new tyres, the engine will rev at 2462/2412rpm (1.5/3.5%).

Two notes I'd add. The 215/45 option would only suit a 7 inch or wider rim. The 205/50 option is 21mm bigger rolling diameter, DOT only allows a rolling diamter increase of 15mm, so you'd have to check the legallaties of that size against what other oem options were availble for other NB models.

With the less revving, then speedo will also be showing slower the same percentage.

[Silvia]

Thanks guys. the 215/45 would seem to be too little change to be worthwhile. But the 205/50 sounds good ,specially if they ride a bit softer, as the 45 series on mine does bang and bounce a bit on our terrible local roads .
..now to check the brand and price options for 205/50.
Is the Michelin PP2 good and well-priced, big dog?

[JBT]

As an example of tyre size vs speed, I have 215/45 x 17 on the NC instead of the original 205/45 x 17. This makes my actual speed at an indicated 100 km/hr almost exactly 100 km/hr instead of an actual 95 km/hr at an indicated 100 km/hr. Of course, the indicated revs at an indicated 100 km/hr have not changed. I use the same revs to change gears as before, but the car is moving at an imperceptibly faster actual speed at each change point.

Silvia, your indicated low speed for high revs situation will not change with a change in wheel/tyre rolling diameter. You will also not notice any actual/physical speed change unless you go to some ridiculously huge wheel/tyre combo or change the diff ratio. However, you may notice slightly tardy acceleration as the rolling diameter increases.

[Silvia]

I thought "actual/physical speed change" was the same as "slightly tardy acceleration"
....both seem to mean speed change yet you are saying they are not related?
My experience with the XT Forester when slightly reducing tyre circumference was a noticeable change in performance - a bit faster to accelerate but running out of useful torque ("puff") earlier. Likewise , the motor revved higher at cruising speed 110kph - more fuel used too.
I would expect the opposite is true when increasing tyre circumference a bit.
As a bonus, changing to 50 series should make my ride a bit gentler too...no bad thing as I'm finding the 45 series unsettles the car on bad surfaces.
I see KU 31's in 205/50 16 are $104 on Ebay (+$70 freight,$60 fitting) he said , getting back a bit to topic

[Guran]

I thought "actual/physical speed change" was the same as "slightly tardy acceleration"
....both seem to mean speed change yet you are saying they are not related?
My experience with the XT Forester when slightly reducing tyre circumference was a noticeable change in performance - a bit faster to accelerate but running out of useful torque ("puff") earlier. Likewise , the motor revved higher at cruising speed 110kph - more fuel used too.
I would expect the opposite is true when increasing tyre circumference a bit.
As a bonus, changing to 50 series should make my ride a bit gentler too...no bad thing as I'm finding the 45 series unsettles the car on bad surfaces.
I see KU 31's in 205/50 16 are $104 on Ebay (+$70 freight,$60 fitting) he said , getting back a bit to topic

True but you should also consider that they may not be street legal for your car.

[JBT]

I thought "actual/physical speed change" was the same as "slightly tardy acceleration"
....both seem to mean speed change yet you are saying they are not related?
My experience with the XT Forester when slightly reducing tyre circumference was a noticeable change in performance - a bit faster to accelerate but running out of useful torque ("puff") earlier. Likewise , the motor revved higher at cruising speed 110kph - more fuel used too.
I would expect the opposite is true when increasing tyre circumference a bit.
As a bonus, changing to 50 series should make my ride a bit gentler too...no bad thing as I'm finding the 45 series unsettles the car on bad surfaces.
I see KU 31's in 205/50 16 are $104 on Ebay (+$70 freight,$60 fitting) he said , getting back a bit to topic

No, I have never said speed change and acceleration were unrelated. What I have been saying is that with a change of tyre profile and/or wheel diameter you would need a huge change to actually be able to detect the change in actual speed - i.e. I'm stuffed if I can pick the difference between 100 and 105 km/hr if the speedo is reading 100 km/hr and the tacho is indicating 2700 RPM in each case. Similarly, if the actual vs indicated speed difference is linear (5%), it would be very difficult to notice the actual speed difference between changing into second gear at say 40 km/hr vs 42 km/hr at an indicated 40 km/hr at the same RPM in both cases. However, if you have a very finely tuned sense of acceleration, you may, repeat may detect slightly slower acceleration. With the NC, I have noticed no change in economy with the larger rolling diameter. nor have I noticed a difference in acceleration. It would have to be done against a stopwatch.

It would also be a personal thing as to whether the ride will feel much different with a higher profile tyre. I sincerely doubt you will pick any changes for the better regarding the car feeling unsettled on bad surfaces. You might also find a that the handling of the car deteriorates slightly with a higher profile tyre. I've changed tyre profile on various cars and noticed no real difference in ride quality. Tyre brand made a difference though.

[Silvia]

thanks Guran, the difference in circumference between 205/45 and 205/50 in 16" is about 60mm -not enough to worry about legally considering there might be 50mm difference between a new and used up tyre in one size.

JBT your "rolling diameter" is confusing as I would think the diameter is always similar, rolling or not. are you meaning circumference, a measurement I find that cuts through a lot of bull.... ?
I'm thinking if I get new tyres then it would be good to try to address my concerns at the same time - (high revving in 1st and 110kph) and rough ride over corrugations.
Going to 50 series may address these issues, seems cheaper ($104 for KU 31), and could even wear longer.
There doesn't seem to be a downside.....

[manga_blue]

I haven't looked back through all the posts on this topic, but has anyone thrown a link to a wheel and tyre size calculator in here yet?
http://www.rimsntires.com/specs.jsp

[Silvia]

Yeah pretty good and comprehensive. Needs updating re. tyre brands and sizes though.
Miata seems to have a popular one....just tyres/tires though.

[Guran]

thanks Guran, the difference in circumference between 205/45 and 205/50 in 16" is about 60mm -not enough to worry about legally considering there might be 50mm difference between a new and used up tyre in one size

I realise you're in Queensland but can't find the quote for TMR. RTA's Vehicle Standards Information No.9 states ...
"The outside diameter of the wheel and tyre combination must be no more than 15mm over the largest diameter wheel and tyre combination specified for the vehicle and not more than 15mm below the smallest diameter wheel and tyre combination specified for the vehicle."
I understand this is now a National requirement, so TMR requirements should be the same.

You claimed that your stock size is 205/45R16. If that is the case, 205/50R16 is 22mm larger in diameter and therefore not legal on your car. A stock 205/45R16 tyre that is worn to the minimum legal tread depth (1.5mm) will have lost about 13mm in diameter (ie starts with 8mm of tread depth), which is much less than the diameter change that occurs if you swap to 205/50R16.

Rather than changing tyre sizes, if you want to change gearing, you might be better to look at an alternate drive ratio for your differential.