Tyre maths & On-line Calculators

weaponofchoice

Forum Member
Joined
Nov 12, 2013
Messages
11
Location
Beaudesert
Car Year
2007
Car Model
07XT Foz
Transmission
5MT
Hi all,

I'm looking at changing my SG 07 FXT tyres to 16 inch rims.

I have 225 55 17s at present so up slightly from stock. I wish to go to 215 65 16.

My question is, on tyre size calculator my current tyres have sidewall of 124mm and the 16s I want are 140mm.

Given that 16 inch is 1 inch smaller rim - say 25mm. 140mm - 25mm is 125mm making it equivalent of 1mm larger sidewall than the 17inch rim.

So basically the tyre will be 1mm taller than my 17 rim??

Can't quite get my head around it but it seems going to a 16 rim of this tyre size will have no impact on lifting the car any higher, only more ballooning of tyre when let down?
 
You forgot there are two sidewalls per wheel diameter.

Tyre Diameter = (rim diameter) + width * profile * 2

so:
225/55R17 = 17*25.4 + 225*0.55*2 mm = 679.3 mm
215/65R16 = 16*25.4 + 215*0.65*2 mm = 685.9 mm

ie: about 17mm larger. (8ish mm larger radius)
 
Hmm yes those are the correct diameter in mm.

The 16 inch size is 8mm larger overall diameter as you said 686 and 679.

Is there a way to work out how much "higher" the 16in rims will make the car if any higher at all?

If the diameter is measured from outer tread to opposite outer tread, would 8mm more mean car will be 8mm higher?

Cheers
 
Cool thanks ratbag!

Kind of makes sense now ��
 
weaponofchoice said:
Hmm yes those are the correct diameter in mm.

The 16 inch size is 8mm larger overall diameter as you said 686 and 679.

Is there a way to work out how much "higher" the 16in rims will make the car if any higher at all?

If the diameter is measured from outer tread to opposite outer tread, would 8mm more mean car will be 8mm higher?

Cheers
Click to expand...

The car 'sits' on the axle.. so will be the difference in radius (4mm) higher.

I can't subtract - as you say, 686-679=7, not 17 !
 
Thanks duncanm yes of course... The radius method makes sense for a quick idea of height compated to other tyres. Hmm 4 mm is not much :(
 
Calculators are useful, but they are not entirely accurate.

Specific tires can vary significantly from what the calculator predicts. Then, installed, they will be shorter. A few months later they will be even shorter, when they attain their longer-term shape. This, at least, is what I have observed on my last two sets of Geolander AT-S.

Another way to look at this may be the revolutions per mile. Tires that have the same nominal diameter can differ in a relatively significant way in terms of revolutions.

Example: my stock size is shown as 27.6" my current as 28.5. The tires I will probably get next are 28.1". However, in real life, the AT-S I have had ran smaller than either 27.6 or 28.5 while the Cooper AT3 I want next, nominally 28.1" is actually 28.3". Now, if this were the other way round, the Cooper would have been 27.9" which would have been too small for me to care. But because it is 28.3 vs 28.5 for my current AT-S, which also happens to run smaller than advertized, the difference is negligible. Ditto for sidewall.

Therefore, if you are trying to understand exactly how specific tires compare, you need their precise measurements and not the generalized one from a calculator.

This would matter especially if you are trying to fit the largest possible tire, which I think is a mistake anyway. In my case a stock car can fit a maximum of 29". However, our Tribeca, which has 29" as OEM size, is running on 29" performance tires that are 28.25" ! On the other hand, some 29" AT tires are 29.2-29.3.

As for going down, if I could fit 16s over my 12.4 brakes and big calipers, I would be facing:

225-65-17 to 225-70-16 for virtually identical diameter (give or take 0.1") but A LOT more sidewall, hence the advantage of the 16".
 
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