Can I use the the measured time between the first count and the last count in a fixed time window in order to increase the accuracy of a quadrature velocity measurement?

I am measuring velocity using a linear encoder. Resolution is 40microns, sample frequency is 1MHz and the real velocity is oscillatory in nature with an amplitude of 200 to 400 mm/s with a freq of about 1Hz. We use a 5ms fixed time window to measure velocity. The quanization error for this is 8mm/s. Can I get rid of the quatization error by somehow measuring the exact time from the first pulse to the last pulse in the measurement window?