Under new rules for 2008, the companies are allowed to choose a track for testing for four days, only using test riders and not with their regular team riders.
The rules say the tyre companies can also test through the breaks in the season where otherwise testing is banned. This is seen as something as a trade-off for pressure applied by Dorna last year in increasing each riders' tyre allocation from 31 to 40 tyres over a weekend.
The rule also enables tyre companies to test at circuits that have been resurfaced since the last visit or at circuits that are new to the calendar, such as Indianapolis this year. Brno has been resurfaced and so companies will go there before the August 17 race too.
Bridgestone has opted for Estoril as the company feels that is one of the tracks where it needs to improve.
Tohru Ubukata, manager of race tyre development at Bridgestone said: "It has a fast, long straight with a good blend of corners, fast, medium and high speed which tests all aspects of tyre performance."
Mugello is still the chosen test circuit for Ducati with on going tests throughout the season.
Michelin has chosen Jerez, where it has won all but one of the recent races, but Jean Philippe Weber told autosport.com: "We have gone for Jerez because we know it well and for the fact that you can have very cold and hot weather here over a season. It is also a balance of whether or not the circuit is actually available or not."
Teams have the possibility to test at their chosen tracks only after a race has taken place and so being another fundamental part of the decision with both tracks being second and third on the calendar.
Bridgestone can use Ducati's Niccolo Canepa or Shinichi Itoh while it has yet to be confirmed if Alex Barros will ride for Michelin.
Last year Bridgestone did use Neil Hodgson with a Superbike on MotoGP tyres at Estoril, something that was not against the rules.
autosport.com