Re: wheels/tires for Coupe GT

[paul.heneghan@bbc.co.uk (Paul Heneghan)]

From: Allan Jones <ampj@tiac.net>


>    It may, but what will my speedo read. That is what I am after....a plus
>or minus % so that I can quite quickly calculate my true speed when
>riding on 195/55 tires. It equals a negative 1.3 % change.

According to my calculations, the diameter of 14/195/60 is approx. 589.6mm. 
 The diameter of 14/195/55 is approx. 570.1mm.  This is a -3.3% change.  I 
am using the formula D = WD + (2 x TW x AR).  WD is Wheel Diameter, TW is 
Tyre Width, AR is Aspect Ratio.

> . . . my top speed may actually increase due to a smaller tire, but I
>believe that to be false(I_could_be_wrong).

If your car has a very tall top gear, then you might find that you are down 
the left hand side of the power/torque curve.  As an example, lets say you 
do 5800rpm, your gearing is 0.314581 - this will give a (top) speed of 
126mph if using 14/195/60.  If however, you change your tyres to 14/195/55, 
then you will be doing 6000 rpm for the same speed of 126mph.  If your 
engine is a multivalve type or race tuned, then it will probably be 
developing a lot of power at high revs.  If it can produce 5% more power at 
6000rpm compared with 5800, then you will be using a more powerful power 
band of the engine and you could probably get another 2% increase in (top) 
speed (129mph).

This effect is seen in many cars with a 4+E gearbox.  Top speed is usually 
reached in 4th gear.  The 5th (or E gear) is a cruising economy gear.

Paul
paul.heneghan@bbc.co.uk
1984 Audi 80 quattro
1989 Audi 100 Avant