Average Speed Reading

[jhconnolly]

When I cruise at 80mph in cruise control for an extended period the trip computer reads 75mph, and yes I did reset the average speed indicator.

Do the speed readings on the dash and trip computer have different sources or something ? Thanks, I drive a 220D.

[K-Fal]

Your clock is always out by about 10%. this is standard practice by all manufacturers. Your trip computer and average speed calculation is accurate.

[fonserg]

for my is220d 7% when speed 100 km\h gps and avg speed show 93 km\h, honda accord 5%, wolksvagen crafter 3%

[Sleepypete (FB)]

for my is220d 7% when speed 100 km\h gps and avg speed show 93 km\h, honda accord 5%, wolksvagen crafter 3%

I still need to gps-calibrate my CT's speedo ...

It's the different ways of measuring/calculating the speed :

Speedos tend to do it by having a gear driven from the final drive. I.e. after the gearbox. Inaccuracies come in due to things like tyre diameter and the way the speedo-drive is turned into an indication. Older speedos used to work against a spring which would resist differently at varying speeds. Newer ones will work it out with optical decoders or similar.

Satnav/GPS speed - GPS works by giving precise time and position. If you get enough position samples, find the distance between them and divide by the time difference. That gives a time averaged velocity. Note - that's not identical to instantaneous speed because you may have gone through a slight arc and it's average speed over something like 10 seconds ... There may also be some correction going on through :

GPS reports a position that isn't on the road

Satnav say : "I don't believe that, I ain't no 4x4"

And satnav corrects the position to be on the road

What I need to do sometime is set up my old satnav in my CT when I'm likely to do a steady run at 70(ish). I.e. aim for 70(ish) on the motorway, let the speeds settle and check what the GPS is claiming my speed is.

Puma - 75 on speedo = 68mph on gps

Focus - 75 = 72

Dad's Volvo - 75 = 75 (watching for speed cameras ...)