Join Date: Jul 2005
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Short of having the speedo and cruise calibrated (at great expense I'm sure, and I wouldn't even know where to go to get it done) your best bet is to compare it to GPS on a straight, flat road. GPS should be accurate to within less than 1mph. Effectively, it can be taken as gospel.
If you don't have access to a GPS unit that shows speed, then at a pinch you can use the sat-nav and a stopwatch. Start the clock when it shows a specific distance from your destination, and stop it as soon as that has dropped by (say) 5 miles. Obviously this only works on a clear-ish road where you can be sure of maintaining the exact speed on cruise over a given distance. It's also safer to get a passenger to watch the numbers count down and operate the stopwatch.
I compared mine with my TomTom 700 and I discovered that 70mph GPS on my car is 73 on the cruise and about 75 on the dial. That's on 16" wheels though. If you have 17"ers then the overall rolling radius (even with the lower profile tyre) is slightly greater, and so your speedo should read lower for any given speed.
In my experience, SPECS won't 'do' you even if you average an indicated 10mph over the limit. I regularly used to average an indicated 50 through the 40mph Thelwall M6 works when traffic permitted, and never once got a ticket.
Currently in Horatio's garage:
2010 Laguna III Initiale.