http://en.wikipedia.org/wiki/Beyer_Speed_Figure

'The Beyer Speed Figure is a system for rating the performance of Thoroughbred racehorses in North America designed in the early 1970s by Andrew Beyer, the syndicated horse racing columnist for The Washington Post. First published in book form in 1975, by 1992 the Daily Racing Form began incorporating Beyer Speed Figures in a horse's past performances and the system has evolved to where today each performance by every horse is given a Beyer number which reflects the time of the race and the inherent speed of the track over which it was run. On the Beyer scale, the top stakes horses in the United States and Canada earn numbers in the 100's, while extremely strong performances may rate as high as the 120's. In Europe, Timeform has a different set up that yields a different rating number. The popular rule of thumb for a rough equivalent of the Timeform score is to deduct 12-14 points to achieve the Beyer figure'.

On pro's and cons: http://www.drf.com/products/beyers/beyers.html

'If handicapping horses using Beyer Speed Figures were a perfect science, it would be relatively simple to scan the past performances and find the fastest runners in each and every race. Picking winners and cashing tickets would be as simple as finding horses that earned superior Beyer Speed Figures in their last or next to last start, and then merely dashing to the betting windows with your latest mortgage payment to reap the rewards. In this perfect world there would be 1-10 shots in every race, and the betting public would make money faster than following Martha Stewart's insider stock tips.

Unfortunately, factors such as track bias, troubled trips, trainer conditioning, post position, medication, pace of the race, trainer intent and track condition make the routine practice of betting horses with the highest Beyer Speed Figures both a risky and expensive proposition. It's true that finding horses that have earned the best Beyers in their most recent starts are an excellent starting point in uncovering quality entrants with raw speed. However, to foolishly complete your handicapping analysis using only this one angle would be a very costly mistake. It's amazing, however, how many novice bettors continue to fall into this common betting trap and continue to shred tickets'.

'Successful Beyer Speed Figure bettors are those who remain flexible when evaluating current numbers and are able to adjust to changing track conditions, trainer placement or intent, post position, workout tabs, pace and the addition of medication (Lasix). Beyer Speed Figures are a tremendous and reliable gauge of a horse's true ability. They're not, however, a neat, mechanical numerical system for pointing out magical winners, which many people interpret them to be. I don't believe such a numerical system exists. You must be aware of the many factors that contribute to a horse running differently today, and learn to capitalize on when the betting public is overlooking some of the obvious contenders'. - Dean Keppler'