Earlier is better! We strongly recommend betting at least one hour before a race. At that time there is a greater chance the bookies are accidentally offering odds that are too generous.
The odds shown are just typical odds at bookmakers observed in the past few minutes but you may well find better odds shopping around using OddsChecker.com or similar. Or even better, use a betting exchange like the Betfair Exchange.
If you bet really early on a betting exchange then you can "cash out" if the odds shorten just before racetime.
If the odds are long then you may wish to consider betting each way.
