Kind of depends on the engines part load efficiency and how much power is consumed by the transmission / driveline.
In example, I used two larger tractors to pull a 21 ft windrower, an implement that I guess consumes about 15-20 HP. The one was a 140 HP 4450 JD with powershift transmission. The other was a 170 HP IH 3788 with manual transmission.
I have read and been told that the JD 15 spd powershift consumes about 15 more HP than the same tractor with a manual transmission.
Pulling the light load of that PTO driven winrower, the powershift JD consumed about twice the fuel of the manual transmission IH.
There may be some light load efficiency difference between the JD 466 and the IH 466 engines, but I suspect the largest difference was the JD powershift transmission was consuming about as much power as was being used to power the implement, so it's engine had to burn more fuel to do the same job.
With both of these tractors pulling a full load, the fuel consumption difference would not have been as great, as the 15 HP consumed by the powershift would have been a much smaller portion of the total power generated.