Useful thread thanks.
This is my problem:
My sat nav (Garmin) indicated speed is with in a knats of my Mondeos (terrible confession) speedo at all speeds.
My sat nav is almost as close on my wifes (2000 W) 156 Twinnie speedo.
= I suppose sat nav is accurate.
Originally bought from Lythgoe motors in 2001, an Alfa dealer in Rochdale, it had (past tense) the Retainacar stickers and the Alfacare documents and was registered as new on the V5 - so assume a UK car. Therefore speedo should be localised by Alfa.
So assume speedo is correct.
It had only done 24K miles when I bought it, so I paid £180 to verify this and had the TWO computers read the same. The main one is in K's and matches after calculations the odometer in miles. So all there seems to be good.
When I bought this car it was shod with 235 40 18's on an 18" rim (obviously) and that was why I assumed the speedo was inaccurate, but when I fitted 17" Teles I expected the speedo to read (or nearly) correctly, but it does not. It still says 120 at the sat navs 110 and 66/70 30/34 with speedo always showing the HIGHER reading. So it says I'm going faster than I am. About 10%. Now the Teles have 215 45 17's on them instead of the recommended by the Alfa handbook 225 45 17's. Why the width should make a difference is beyond me, but according to the above linked calculator it does.
Using the calculator I have tried all the combinations of 16, 17 and 18" and my tyre sizes and I cannot get near the answer. Even if I use the Twinnies 15" steel wheel numbers, I can't get this level of inaccuracy.
So the question is this:
How do I get the speedo more accurate than + 10%? Or am I being anal? Are 916 speedos designed to massage our ego's as Yamahas R6 rev counter was?