As Hot Water, stated, a GP38 vs SD38 has the same engines so the same fuel consumption. In actual practice, the SD might use a little more horsepower for traction motor cooling but I doubt the variation would be more than 1 or 2%.

As far as the original question - the same train whether powered by an SD40 or SD38 traveling at the same speed would require either engine to develop the same amount of horsepower. So if it took a SD38 at full throttle, the SD40 would be in Run 5 or 6 to develop the same 2000 horsepower. For that particular example, my notes say a SD/GP38(-2) in Run 8 consumes 122 gph, while a GP/SD40(-2) in Run 5-6 consumes 79-108 gph, a decided advantage to the 40. They both consume 7 gph at normal idle.

There are some differences between a SD and GP. The SD typically weighs 40-55% more than the same model GP. The SD will therefor develop proportionally more starting tractive effort, and with six traction motors can develop higher continuous tractive effort at a proportionally lower minimum continuous speed. By the time you get to around 15 mph, most of the SD's advantage disappears and it actually has a lower tonnage rating since you have to lug around the extra weight which equals about half a loaded car. At 10 mph, anything with over 500 hp per traction motor will derate to that rating. So at 10 mph or slower a GP40 has the same horsepower as a GP38, an SD45 same as a SD40, etc. This is all keeping only EMDs up to the Dash-2 era in consideration.

The original reason for the original A-1-A or C-C six axle units was to spread the weight for light rail, then the six-motor C-C was to create more tractive effort at lower speeds considering the limits of the early traction motors. Railroads soon discovered by ballasting them to the same weight as a four-axle unit resulted in greater available tractive effort at low speed or bad rail conditions. The current reasons for six axle freight road engines is that they simply need the weight for adhesion at high tractive effort, carry the weight of the extra "comforts" and safety protection for the cab area, and to carry enough fuel to make the next fueling stop.