Bonefishblues wrote:Vorpal wrote:pwa wrote:I'd just like a speedo that tells me I'm doing 25mph when I am actually doing 25mph. A speedo that gives me a figure that is not correct is wrong, and I would have expected something better by now. As it is, we all go round knowing our speedos are overstating our true speed and doing mental arithmetic to get a more accurate figure. I suppose that might be good mental exercise though.
Well, except that the same 25 mph on a nice summer day might read as 27 mph on a cold, frosty day. Which one do you want to be right?
edit: yes, that's a slight exaggeration, but variations of more than 2 mph can be caused by varying conditions
How would that be?
The contraction of tyres and air pressure on a cold day can change the pressure as much 10% versus a summer day (the rule of thumb is 2% per 5 degrees C; that's not perfect, but it works pretty well). The cold will reduce tyre flexibility. The cold, frosty road surface will have reduced friction. The combination will result in a changed speedometer reading. 2 mph is my estimate, probably conservative, based upon experience 30 years or so ago.
I will see if I can find documented evidence when I get a chance.