Spider Speedo

[The Count]

Have noticed a quite significant difference between the speed indicated on the speedo on our 1999 Spider T-Spark 2.0 compared with the GPS speed readout.

Around 80 to 100 kph there is a variance of around 18kph between the 2. With the speedo showing 100 kph, the GPS indicates 118 kph!

This could mean a speeding ticket if it's accurate so, how can I check which one is right?

Any ideas welcome

[Cool Jesus]

I can't imagine tyre size to equate to such a variance. Could be a number of issues from the sensor on the gearbox, the speedo gauge? I also thought of an engine having been swapped out from CF1 to CF2 but I believe by late 98 they all came with the plastic rocker covered twin spark CF2 engine, so there should be any disparity between speed sensor and the gauge. Just another thought, the ABS may also play a part in speed readings. I'd have to have a closer look at the manual, but someone else may add to this.
Best bet to start of with is to hook up a diagnostic to check for errors.

[Norseman50]

There is a good chance that the speedo has a very small potentiometer that is internally mounted and used to adjust the actual readout.  If you decide to remove the unit yourself and manage to locate that small pot, be aware that it doesn't take much turning (less than 45 degrees) to change the displayed speed noticeably.

If you remove the unit but don't feel comfortable opening it up and messing about with the internals, just take it to any speedometer repair shop in your area and tell them the results you have already achieved (your speedo reads 100 when your GPS reads 118), and they will be able to correct your speedometer on their bench so that when re-installed in your car it will be accurate to within 1 kph of the GPS or actual.