As a slight digression from the "How fast have you gone ?" thread ... this weekend I tried to calibrate the speedo (I'm on 19" wheels with Pirelli Zeros) using my ancient and rather crappy SatNav (a Medion in a modified Pocket PC; about eight years old). Speedos always over-read (keeps you within the law) but it's nice to know by how much.

This is one advantage of a glass sunroof; even closed it's a hole for the GPS waves to get in. (And I don't think the Jag SatNav shows speed ... or does it ? If it does please tell me - I'm still trying to work out how to work it).

Anyhow my Pocket PC SatNav seemed to indicate that my speedometer over-reads by about 4 at 21 mph (that's actually the cruise Set-speed which agrees with the speedo reading) , rising to an error of about +6 mph at 70 mph.

I then tried timing the car over 1.6 km using the 100m posts on the motorway (the old Severn Bridge was closed this weekend, so the couple of miles approach was VERY quiet and I could safely drop down to 30mph without causing anyone any hassle (I still kept a weather eye on the mirror !)

Interestingly these figures seemed to indicate an error of only about +2mph at 30mph, rising to +3½ to 4mph at 70. (and yes I did allow for 1.6 km not being exactly 1 mile; it's 1,749 yards 2-and-a-bit feet to be precise-ish)

Dave in Gloucestershire.

 

May be the easiest way is to follow a truck as they are governed pretty accurately to 56mph, and on the motorway most will be flat out. (that is if you really need to know)?

 

Do you have a smartphone? You can download a number of apps that will give you readouts from the onboard GPS, including speed.

 

Hidden menuu in the nav system. Press and hold the 'Menu' and 'Telephone' keys simultaneously, when the keypad screen appears release the keys, enter the code 917 and press the 'Enter' key on the screen. Will show you gps speed in kph and mph. My XJR is out 9 kph at 100kph. There was a thread not too long ago that you can search for, something like 'how to trick your car' that shows this.

Stu

 

Cheers Stu - I'll give that a try tomorrow - Dave