Over the last decade, the use of chirped fibre Bragg gratings (CFBGs) in detonation velocity experiments has been steadily increasing. In this paper, we show how CFBG design parameters-chirp-rate, reflectivity and apodisation-affect linearity in detonation velocity tests. It is found that the optimal CFBG detonation velocity probe should have a high chirp-rate, a low reflectivity and no apodisation. As a further demonstration of these findings, we measure detonation velocity with a 24 cm optimised CFBG; the longest CFBG test of this kind so far.