In all actuality, it is 75mm. The blade measures just under 75mm (2.95") and the intake inlet measures the same. It doesn't matter what size TB you put in front of the intake (75, 78 or 80mm), the intake inlet size is 75mm. It will only be able to flow as much as the inlet will allow. Believe me when I can tell you from experience, ALL 3 bolt truck DBW throttle bodies, stock and aftermarket, neck down to 75mm on the backside to be able to not cause problems going into the inlet. This is a very overlooked fact people don't consider.