Speed calculation

How long is the split? Laps are usually 5 km, but splits can be any distance. Could you share the full activity? 

It says "Km 86-90". Shouldn't it say 85-90 if it was a 5 km split? If I do the math backwards from 28,88 km/h and 10 minutes I get 3 miles.

I catches the screenshot from the real-time activity I was monitoring. In real time, garmin shows splits from 5*N to 5*(N+1) km, as you can see from the header of data I shared. Time, distance and average speed are not congruent. 

9078975 said:

garmin shows splits from 5*N to 5*(N+1) km

We still do not know how long the split was. It the split was ended at the distance of 4.81 km, it would still be km 86 - 90, but the speed would be indeed 28.8 km/h. Please share a link to the activity, only then we can try to guess where the value comes from.

9078975 said:

garmin shows splits from 5*N to 5*(N+1) km

Then why isn't it from 85 to 90?

With N=17 => 5*17=85 and 5*(17+1)=90

Your answer suggested to me what the calculation problem in LiveTrack might be: an unmanaged conversion mile/kilometer. To recap: LiveTrack reports the runtime information of a ride, partitioning the path, by default, in "5 kilometer" segments (in countries using metric system). I suppose the partition segment length in countries using mile system is "3 miles", isn't it? 3 miles is approximately 5 km (4,82803). If the average speed  is calculated, by approximation, on a 3 miles segment, the 28.88 km/h is explained, but it is not correct. As far as I know, LiveTrack information are available only during the run. I will check again the speed calculation next time I'll have the chance to do it. 

Units conversion isn't always working good. Which units are used by the athlete that logged the activity? If miles is being used, all splits will be in miles. You are using metric, so the values will be converted and rounded or truncated to integers.

Next time try switching between metric and statute in upper right corner of the LiveTrack screen and see what values you get.