Will a 6% increase in rolling circumference make a huge diff

[Adam_NAclubman]

erence?



I think I'll go and get a pair of new 195/60/15's for two of my Buddyclub P1's and instead of shelling out for a second pair (when I dont even know if they'll fit without rolling the crap out of my guards) I'd like to use a pair of 195/60/15's that I've got sitting around for the back. They should be about 6% bigger in terms of rolling diameter than the 185/60/14's that are on the car atm and they will be replaced with 195/50's when funds permit, but is it a bad idea to use the 60 profiles?

I know that I'll have to stick to say 55 on the speedo in a 60 zone, but apart from speedo error, it should be fine shouldnt it?

[Benny]

Apart from the difference in speedo readings, you will also notice a drop in acceleration.
Even with my SP, when using the higher profile R spec rubber, I can really notice the difference in acceleration, and this difference would be amplified if you have a NA motor.

[spitty]

Dont want to hijack Adams post?? I have 195/60/14 on my NA8 . They were on when i bought it but i think the sidewall \"moves \" too much. Would195/50/14 give me a sharper response? Sorry Adam

[StanTheMan]

I would have tought that would make a huge difference.
the hight is .....like..... an inch taller than stock.


I wouln't touch them....with someone elses....

[adamjp]

If you email me with your email address I will send you a really cool MS Excel spreadsheet I started building in 1987 (Lotus 123 on MSDOS 2.1 back then). It has blossomed a bit since then, but will work all of these things out for you without any problems at all.

Roderic has a copy and sounded impressed.

And yes - I have been an engineering geek for a really long time.

I should also add that my NA6 speedo is so irrelevant to the actual speed the car is doing (with stock rims and tyres). I use a GPS for long distances - at 110 actual, indicated is more like 120.

[spitty]

Thanks Stan!! Thats probably why my Camry{oops } handles as well as my MX New tyres i guess.. Mrs Spitty wont like that

[Bevan]

If you email me with your email address I will send you a really cool MS Excel spreadsheet I started building in 1987 (Lotus 123 on MSDOS 2.1 back then). It has blossomed a bit since then, but will work all of these things out for you without any problems at all.

Roderic has a copy and sounded impressed.

And yes - I have been an engineering geek for a really long time.

Is that similar to the calculator on this site?

I should also add that my NA6 speedo is so irrelevant to the actual speed the car is doing (with stock rims and tyres). I use a GPS for long distances - at 110 actual, indicated is more like 120.

I find the same thing in my NB8B. GPS says 110 and speedo says about 117km/h

[StanTheMan]

Spitty....I was actually referring to adams tyre sizes.

If you want sharper response.....start with an allinment. I personally have 0 toe on my front......but next time i will change that so some reasonably amout of toe out.
If you want to go 50 profile.....go to a 15\" rim. you can get reasonable response from most tyres....even with the A539's with the right allingment.
If you go 195/50x15 this will get you within 0.3% of the original tyre size.

(0.3% correct me if I'm wrong)

[irwin83r]

everything you need to know about wheels on this site.

has all the calculators your after and will even do a search for available tyres in a size

its american based though so some things dont quiet ad up.

http://www.1010tires.com/tech.asp

[adamjp]

Bevan,

No it is not similar to the one on that site.

It is not exactly hard math, just a bit complex. I originally built mine to prove to friends with LJ Toranas that they were not really doing 200km/h as the speedo said due to the low profile tyres they had fitted.

Over the years it has grown to include many useful calculations relating to tyre sizes, particularly useful for what-if scenarios. It is not precisely accurate as it uses a calculated tyre size, which is NOT really accurate, but better than a guess.

[Adam_NAclubman]

I was mainly interested in whether the diameter of the tyre will cause any clearance issues, changes to indicated speeds and an effect on acceleration are pretty obvious.


Stan, it won't add an inch to diameter, thinking about it it should only add about 6mm to each 'end' of the sidewall (ie top and bottom of the wheel) so it'll only add 1.2cm to the diameter in total which is half an inch or thereabouts?

I just want to get the wheels on the car, though I guess the fact that they'll be on 7 inch rims might reduce the rlling circumfrence a bit as they'll be stretched a tad

[StanTheMan]

Ok. you are thinking of using 195/60 series on a 15 inch rather than a 14 inch.
to me that sounds like you are using the same measurements except you are changing the rim from 14inch to 15 inch.....which adds an inch.
Standard height of the 195/60x14 is 577 mm
height( or Dia) of your spare set of tyres are is 615mm 1 inch = 24.5mm...right?

if you still don't believe me.....put them next to each other.
you have then sitting in the back yard.

sorry i don't know if they fit. I just would never have considere that size of tyre for an MX5. I thought it would have been far too big.

[adamjp]

According to my theoretical model, you will add 37mm in diameter by going from 185/60-14 to 195/60-15. That is 3/4\" in radius which you need to consider on your guards. Overall it is only a 6% increase in Circumference, and therefore speed at any given RPM.

Your results may vary somewhat due to tyre construction (15%) but the numbers are still between 1\" and 1.5\" in dia. Stan's super simple (but cool) maths gets us into the ballpark quickly, then you add the growth in sidewall (10% of 195mm).

Of course it will be obvious when you put the two tyres beside each other. What you do to your car is your business, not mine (or ours), we can provide opinions when asked, but the decision to make a change is yours alone.

[Techno]

erence?



I think I'll go and get a pair of new 195/60/15's for two of my Buddyclub P1's and instead of shelling out for a second pair (when I dont even know if they'll fit without rolling the crap out of my guards) I'd like to use a pair of 195/60/15's that I've got sitting around for the back. They should be about 6% bigger in terms of rolling diameter than the 185/60/14's that are on the car atm and they will be replaced with 195/50's when funds permit, but is it a bad idea to use the 60 profiles?

I know that I'll have to stick to say 55 on the speedo in a 60 zone, but apart from speedo error, it should be fine shouldnt it?

I think that you might like to check the calculations. I make the difference to be 2.1% in circumference and an increase in tread width of 3.5%.

http://www.tirerack.com/tires/Spec.jsp?make=Bridgestone&model=Potenza+RE92&tirePageLocQty= shows an example for the Potenza RE92's.

The NA's were engineered for a high profile tyre ... added safety by warning the driver of an impending slip. Those with low profile tyres can tell you that there isn't the same warning signs.

[Garry]

The NA's were engineered for a high profile tyre ... added safety by warning the driver of an impending slip

Coupled with recomended tyre pressures of 26PSI it does wonders for a good ride and masking scuttle shake as well.