137% is a high value for efficiency.

If you want +- 0.1 %

and if you are measuring the velocity as 9.2 then 0.1 % of 9.2 = 0.0092

Now if you are measuring ENERGY as part of the efficiency then the error in velocity must be half of the percentage error in efficiency.

Hence the accuracy of measurement required is 0.05% of 9.2 = +- 0.0046 m/s

There is still a lot that I am having to guess about.

ie that you have some known input of energy, you measure the speed of the plane, and from that work out the ratio of energy out / energy supplied.

If this interpretation is correct then the accuracy of +- 0.0046 m/s would give the accuracy in energy and in efficiency that you are seeking.