In current 7.1 I was able to build two 500t survey-fighter designs that apparently have infinite range, as they never use fuel for some reason.
These two here have been on their way for years, yet never used a liter of fuel, but definitely consumed supplies and morale (maybe their desperation drives them onwards):
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The designs are nearly identical save the exchanged geo and grav sensors, and look like this:
More engine detail in case it is relevant:
The only two other ships I owned in this game used fuel normally, so it is not a general race issue. Not sure if other fighters with higher grade engines would work the same, or if it is restricted to just these super efficient ones. ...There have been profound fighter changes in this version, but I guess if it was a general issue, then somebody would have noticed by now.(on the other hand - survey fighters are rare)
If I should test it nonetheless in this game to confirm, I will if asked.