Be sure that the information you give findpeaks, especially for a one-sided Fourier transform, are the same frequency and amplitude values that you give to the function you use to plot the same information.
EDIT — The max function — with two outputs — will also work. It will only detect the first maximum, so if you use it, you will need to subtract the mean of your signal before using the fft function. This removes the possibility of the maximum being the value of the d-c or constant offset:
FTsignal = fft(signal - mean(signal))/length(signal);
then:
[maxpeak, maxpeakindes] = max(abs(FTsignal)*2);
will give you the data you want.
