Do Lower Profile Tires affect Speedometer Reading

[Franco Barber]

Charles,

>From the tire width/ratio/rim size you can calculate the tire
circumference, and thus the effect on your speedometer.
The formula is simple math, and there are various tire calculators
on the web.

For the four sizes you mention:

Tire-spec  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
205/60-15     123mm   314mm     627mm         1970mm        817        0.0%
195/65-15     127mm   317mm     634mm         1993mm        807        1.2%
185/70-14     130mm   307mm     615mm         1931mm        834       -2.0%
195/55-15     107mm   298mm     596mm         1871mm        860       -5.0%

So, if 205/60-15 is your OEM size, than a 196/65-15 tire has a
circumference only 1.2% larger.  Your speedometer reading will
be lower by 1.2%, which you will probably never notice because
the error in your speedometer under normal conditions could be
as much as 5%.

The 195/55-15 size you mentioned early on is a whopping 5% smaller,
and this might start to actually affect your speedometer readings
significantly.

Last comment, as said by others, for snow tires, a narrower tire is
better than a wide one so that the tire can sink into the snow a
little deeper and find some grip, so between the 205/60-15 and 195/65-15,
the 195/65-15 would be the better choice for a snow tire because it's
just that much narrower.

The 185/70-14 is narrower still and only 2% off in size, so it might
be an OK choice, but now you're down to a 14" rim and you've got to
figure out if a 14" rim will fit over your brake calipers,
and as you said you also have to check that the load rating on the 14"
tire is high enough for the weight of your car + your passengers.

Franco


On Wed, Nov 09, 2005 at 08:03:48AM -0500, mboucher70 at hotmail.com wrote:
> I'm looking at a pair of 'performance winter' tires.  The size is 195 55 R15.
> 
> My car's original equipment is 205 60 R15.
> 
> I've read that for winter you should change the sizing by one each - that is either
> 
> 205 60 R15 or
> 195 65 R15 or
> 185 70 R14 if load capacity allows.
> 
> I'm a bit confused by this rule.  I understand that the tire profile change is ofset by a rim size change.  So then, wouldn't 205 60 R15 and 195 65 R15 give different speedometer readings, since the rim size is the same but the profile is different?
> 
> Bottom line question is will moving from 205 60 R15 TO 195 55 R15 work on my car (1990Audi100) or will it change my speedometer readings?
> 
> Other question, where is a good place to get inexpensive winter rims (Wal-Mart, etc)?  It uses 6Jx15 rims.  Are these standard rims, or specialized Audi rims?  I've noticed that most Audis use the 5-bolt tires, but mine uses a 4-bolt.
> 
> Thanks
> 
> Charles
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> 

-- 
Franco Barber                   feb at febsun.cmhnet.org
95 A6Q w/Gamma CD mit RDS!      Columbus, Ohio  USA
95 A6Q 5spd w/HCB-30 bluetooth car kit!