do those figures tend to substantiate your 1.5x?
No it does not; it is only a 1.15 difference. I assume you have a recently calibrated ASI and an accurate flowmeter.
The ASI is calibrated annually, as required under G-reg LAMS ... but do note I was referring to TAS not IAS.
Flowmeter is the fuel flow function on the JPI EDM-800, which I've had installed for four years. Across this time, it has been consistently accurate to better than 1% on total fuel used vs fuel required to refill tanks (other than a period when some low-life was stealing fuel from the aeroplane - for which he was fined £60, some justice for what could easily have resulted in a manslaughter charge - but that, as they say, is another story!)