Questions about tire stuff....

[Ramiro Reinoso <ramiro@ctt.bellcore.com>]

>
>   Date: Tue, 10 Aug 93 08:07:08 EDT
>   From: mlr@critter.cv.com (Mike LaRosa )
>
>   HI everybody.....
>
>   Got a question regarding tire sizes,  I have an 89 100, with 205/60/15
>   Michelin mxv2 tires that are quickly approaching the surface texture of
>   a boloney skin.....:-)  Some of you may remember me asking questions
>   last year about which tire to replace these with, well the time has
>   come.  I've decided to go with either P500 or COMP T/A HR4's but my
>   one nagging question is I'd like to go a little wider, to maybe
>   225.... But what's this gonna do to my speedo ?  How much faster do
>   you think I'll really be going when the speedo say's 60 mph ?
>
>   Mike L.
>

The question is "How much slower do you think you'll really be going
when the speedo says 60 mph ?"

The answer is 57.9 mph.  The 225 wheels are going to be 3.5% smaller
than the 205 wheels that you currently have.  Therefore, the
speedometer will show higher speeds than the actual speed that you're
traveling.  Your speedometer is calibrated to show 60 mph at 14.20
rev/sec.  But with the smaller wheels, at 60 mph (actual speed) your
wheels would be rotating at 14.75 rev/sec.

So when the smaller wheels are rotating at 14.20 rev/sec, the
speedometer will read 60 mph (it's been calibrated for 14.20 rev/sec =
60 mph), but the actual speed is only 57.9 mph.  When your actual
speed is 60 mph, the wheels will be rotating at 14.70 rev/sec, and the
speedometer will show 62.1 mph.

These are the basis for my calculations; please correct them if
I'm wrong.  Diameter of wheels:

205/60-15 = 621 mm (this is what you have now)
225/50-15 = 600 mm (is this what you want ?)

225/55-15 = 622.5 mm (bingo, this is what you need)
225/50-16 = 625 mm (you can use this too; need new rims though)