profile calculation

[.]

I am getting a new set of tyres and I want to know how to calculate the difference they will make to my speedo.

 

The standard tyres are 65 profile on 14 inch rims.

I will be getting 45 profile on 14 inch rims.

 

 

so the Radius will go from 177.8mm + 65mm = 242.8mm

to 177.8mm+45mm = 222.8mm

 

if my calculation is correct this means when the speedo reads 100KMPH I will actually be doing 91.8 KMPH

 

Does that sound correct?

 

[Sirpent]

I am getting a new set of tyres and I want to know how to calculate the difference they will make to my speedo.

 

The standard tyres are 65 profile on 14 inch rims.

I will be getting 45 profile on 14 inch rims.

 

 

so the Radius will go from 177.8mm + 65mm = 242.8mm

to 177.8mm+45mm = 222.8mm

 

if my calculation is correct this means when the speedo reads 100KMPH I will actually be doing 91.8 KMPH

 

Does that sound correct?

 

Think this may help

 

Cheers

 

John

 

http://74.125.153.132/search?q=cache:9aNiI86LbSAJ:www.alloywheels.com/tyrecalc.asp+calculating+tyre+profile+speed+change&cd=3&hl=en&ct=clnk&gl=au

[Zeddophile]

Another calculator with metric speed option....  Needs Java to work though.

 

http://www.miata.net/garage/tirecalc.html

 

 

And for what its worth, your calculations are a long way out, unless you have 100mm wide tyres?

 

I'm not sure what is marked on your tyre currently, I'll guess it is something like this:

 

195/65 R14

 

In this case, the first number (195) is the tyres' width across the tread face in millimetres.

The second number (65) is the height of the tyres' sidewall, BUT it is expressed as a percentage of the tread width.

The R means a radial construction tyre, and the 14 is the diameter of the wheel in inches.

 

SO. You have already worked out the radius of the wheel, that being 177.8mm.  Now to get your sidewall height to add to that, you need to take 65% of the 195mm width.  195 x 0.65 = 126.75. 

So the actual radius of the original wheel and tyre assembly is 304.55mm.

 

From there obviously you need to work out the circumference of the tyre, using that radius you've worked out, and then the circumference of the new tyre (working out a new radius for it!), and then compare the two and find out the percentage difference. 

 

Or you could just jump into an online calculator, and play around till you find the tyre size you want.  I use the one I posted when I want to know what size tyres I could use, stick the original size in on one side, then try different ones on the other until I find the closest one.

 

Incidentally, assuming your current tyre is 195/65 R14, to keep the same overall diameter at its closest with a 45 aspect ratio on a 14 inch rim, you'd need a 275/45/14    Time for some flares?

[Zeddophile]

thats hilarious, I am sure it is pure chance but using your calculation I still come up with 91.32 (I was only 0.5 KMPH out)

 

I am going from 215/65/14  to 215/45/14

which means the radius goes from 247.675 to 226.175 which is 91.32%

 

Thanks for your help

 

I get a radius of 317.55mm and 274.55, not sure what you've done there....

 

Banging those tyre sizes into the calculator on that site I posted says that when the speedo shows 100km/h, you're actually doing 86.5km/h.  If you use that, it also gives you the radius/circumference and a bunch of other numbers, so you can check to see if you're on the right track....

 

Incidentally, how low is the lowest point underneath your car?  The new tyres are 43mm smaller in radius, and that is going to translate directly into ride height - in other words, you will have 43mm less ground clearance.  Not to mention they will be 86mm smaller in diameter, and will look really tiny in the wheelarches....

 

I presume the idea is to gain better acceleration on the track, which you definitely will, but I wonder whether you might be going a bit too far with the downsizing.  Only you know how much clearance you actually have under the car, and whether its going to be an issue or not....

 

*edit*Also - I may or may not have taught you something today, but you just taught me something too - I didn't realise you could work the speedo error out just from the radius of the wheel + tyre assembly, I assumed you had to work out circumference first.  Seems not!  Thanks, that's a piece of knowledge that's getting filed away in my head for later!

[Zedman240®]

Best site I have found for working out things like this:

 

http://www.carbibles.com/tyre_bible_pg2.html