[urq] Speedo Accuracy

[Ed Kellock]

Interesting.  If we knew the baseline speedo gearing was correct, a
larger tire than stock should yield a higher speed than indicated on
the speedo.  I wonder if they didn't update the speedo gearing with
the change in stock tire size, or if they were slow to do so.  Your's
is an ''84 right?  What is the stock size indicated on your door jamb
(fuel filler lid?) plate?  In the owner's manual?

Between the 205/60 and the 225/50 there is a 3% speedo error, with the
225 being the smaller diameter.  This would yield a speedo error in
the right direction, but only .9 mph so the radar sign would seem to
be reading low.  However it has been my experience that SP8000's tend
to run large in relation to their nominal size, which would close the
gap even more.

Between the 215/50 and the 225/50, the latter is 2% larger so and
indicated 30 would actually be 30.6 mph.

My dad (a retired AF test pilot) always used to calibrate the speedo
on the road with mile marker signs and his watch.  I haven't seen any
in a while, but they used to have series of 3 to 5 signs that were
placed just for that purpose I believe, at least out in CA in the
70's.

I've seen some discussion of speedo gearing in relation to the change
in tire size specification, but I don't recall a final answer.

Ed


On 6/16/05, Brandon Rogers <brogers at terrix.com> wrote:
> Guys-
> Data point here - I had the chance recently to drive by ( a couple times) one of those neighborhood radar signs - you know - that tell you how fast you are going - an appeal to your conscience..)  anyway I kept the speedo right at 30 and got a readout of 27 on the radar sign - a couple times.  This is with new (3000 miles?) Dunlop SP 8000 225/50 15 on 8 x 15's.  Now if I can just remember to keep the speedo at about 82 when I want to go 75.  I'm assuming it won't always be 3 off - but it will be about 6 off at 60 and so on...anyone know different?
> 
> FWIW my A4 was right on.
> 
> Brandon