This is an interesting problem and I think I got a solution for you.

Speed is the difference in position per time interval. This means that if you can measure travel distance and time, you can measure speed.
It turns out that you can indeed track both distance and time as follows:

/scoreboard objectives add travelDistance stat.aviateOneCm

This scoreboard objective keeps track of how many centimeters you have flown. Now you need to keep track of the time. This can either be using redstone or a scoreboard timer. I leave it up to you to design that, since it is out of the scope of this question.

Now you need an additional objective to write the speed to.

/scoreboard objectives add speed dummy Speed

Following commands should be repeated every second, using your timer:

/execute @a ~ ~ ~ scoreboard players operation @s speed = @s travelDistance
/scoreboard players set @a travelDistance 0

If you repeatedly execute these last two commands, then you get the distance in centimeters that the player travels within the specified timeinterval. That's exactly the definition of speed!

Make sure that you display the Speed scoreboard in the sidebar and you're set!

This is an interesting problem and I think I got a solution for you.

Speed is the difference in position per time interval. This means that if you can measure travel distance and time, you can measure speed.
It turns out that you can indeed track both distance and time as follows:

/scoreboard objectives add travelDistance stat.aviateOneCm

This scoreboard objective keeps track of how many centimeters you have flown. Now you need to keep track of the time. This can either be using redstone or a scoreboard timer. I leave it up to you to design that, since it is out of the scope of this question.

Now you need an additional objective to write the speed to.

/scoreboard objectives add speed dummy Speed

Following commands should be repeated every second, using your timer:

/execute @a ~ ~ ~ scoreboard players operation @s speed = @s travelDistance
/scoreboard players set @a travelDistance 0

If you repeatedly execute these last two commands, then you get the distance in centimeters that the player travels within the specified timeinterval. That's exactly the definition of speed!

Make sure that you display the Speed scoreboard in the sidebar and you're set!

For the sake of completion(and because it was suggested), here is how you convert your value to a different unit. I am making the following assumptions:

• There exists a scoreboard, containing constants. I name it Constants
• This scoreboard has a constant called timeUnit with value 3 600
• This scoreboard has a constant called distanceUnit with value 100 000
• You want to convert to a speed in Km/h
• Your initial value is in cm/s

If you convert to a different unit, you have to do divisions and multiplications. It is important that you do divisions after multiplications, because scoreboards don't work with floating point values. To convert from cm/s to km/h, you have to use following commands:

/scoreboard players operation @a speed *= timeUnit Constants
/scoreboard players operation @a speed /= distanceUnit Constants

These commands should be pasted right after the other commands that got repeated. If you want a different unit, then you have to alter the values of the constants.

This is an interesting problem and I think I got a solution for you.

Speed is the difference in position per time interval. This means that if you can measure travel distance and time, you can measure speed.
It turns out that you can indeed track both distance and time as follows:

/scoreboard objectives add travelDistance stat.flyOneCm

This scoreboard objective keeps track of how many centimeters you have flown. Now you need to keep track of the time. This can either be using redstone or a scoreboard timer. I leave it up to you to design that, since it is out of the scope of this question.

Now you need an additional objective to write the speed to.

/scoreboard objectives add speed dummy Speed

Following commands should be repeated every second, using your timer:

/execute @a ~ ~ ~ scoreboard players operation @s speed = @s travelDistance
/scoreboard players set @a travelDistance 0

If you repeatedly execute these last two commands, then you get the distance in centimeters that the player travels within the specified timeinterval. That's exactly the definition of speed!

Make sure that you display the Speed scoreboard in the sidebar and you're set!

This is an interesting problem and I think I got a solution for you.

Speed is the difference in position per time interval. This means that if you can measure travel distance and time, you can measure speed.
It turns out that you can indeed track both distance and time as follows:

/scoreboard objectives add travelDistance stat.aviateOneCm

This scoreboard objective keeps track of how many centimeters you have flown. Now you need to keep track of the time. This can either be using redstone or a scoreboard timer. I leave it up to you to design that, since it is out of the scope of this question.

Now you need an additional objective to write the speed to.

/scoreboard objectives add speed dummy Speed

Following commands should be repeated every second, using your timer:

/execute @a ~ ~ ~ scoreboard players operation @s speed = @s travelDistance
/scoreboard players set @a travelDistance 0

If you repeatedly execute these last two commands, then you get the distance in centimeters that the player travels within the specified timeinterval. That's exactly the definition of speed!

Make sure that you display the Speed scoreboard in the sidebar and you're set!