Using standardized mean difference is the most reasonable solution to your case.

I assume when seconds are being used as the unit of measure in an assessment of speed, you are talking about the time to complete a fixed distance. Thus, when the unit is seconds, a smaller number means a faster speed whereas when the units are meters per second, a larger number means a faster speed. You can deal with this in the calculation of a standardized mean difference (smd) by reversing the sign of a smd calculated for a study using seconds as the unit to assess speed.

As Gordon L Warren said: just reverse the sign of one of the types of outcomes to achieve the same direction of the effect. The standardized mean difference takes care of the rest of differences between the two measures.

Alternatively, if indeed speed is measured by the amount of seconds it takes to complete a fixed distance, calculating speed as m/s shouldn't be too difficult either.

Good luck with your meta-analysis. If you would like to perform meta-analysis in Excel, check Meta-Essentials at www.meta-essentials.com or http://onlinelibrary.wiley.com/doi/10.1002/jrsm.1260/full

Thank you for your answers, Robert Suurmond should I reverse the sign of the mean values in m/s?

Dear Alaa Abou Khzam you should instead reverse the sign of the standardized mean difference directly. In other words, first calculate the effect size and than simply add a minus sign to one of the groups of studies.

Alternatively, you could also switch the treatment and control groups in the calculation of the SMD. M1-M2/SDp or M2-M1/SDp. Should give you the same results, but requires more manual labor and is more prone to errors.

Good luck!