Just noticed, you're trying to convert the literal text "Fuelt" to a number, you need to use a token to convert the variable "Fuelt" instead:
Set integer [Fuel] value to the converted value of Fuelt
Should be
Set integer [Fuel] value to the converted value of {TXT:Fuelt}
However, there is a further caveat: Because the input uses a "." as the decimal seperator, and I'm assuming you're using a "," as is normal in France(and other places), "100.00" will convert to "10000".
I recommend, in this particular case, to simply drop the section entirely:
Set Text [Fuelt] to [http://localhost:3001/Marvin/apiData.php?ship=fuel] (Lower Case)
Set Text [Fuelt] to '{TXTSUBSTR:Fuelt:0:{TXTPOS:".":Fuelt:0}}'
Set integer [Fuel] value to the converted value of {TXT:Fuelt}
This will return "100" when given "100.00", but also "9" when given "9.99".
If you don't, you'd have to use a Decimal value instead(Converting to Int doesn't seem work on floats in VoiceAttack), but then those don't currently work on systems that use a "," as the decimal separator.