The aerodynamics in that area are REALLY tricky, but most of the disruption is through the airflow along the side of the vehicle, since the tires create a low pressure area right behind them, and beneath the car is a lower pressure area than along the side. When you add the mudflaps, you isolate the low pressure area behind the tires, create a higher pressure area in front of the mudflap that causes turbulent flow between tire and mudflap, and create a new low pressure area behind the mudflap which will reduce drag on the underside of the body behind the mudflap. The low pressure area behind the tire is already fairly turbulent behind the tire without the mudflap, so in the end, it is very little net gain in drag. It is a difficult area to explain or even visualize due to all the odd surfaces, and the math is terrifying.
In my opinion, it is worth it to keep the side of your car clean, because that cuts down on damage and rust. What good is .25mpg (really optimistic figure) if it ends up costing you more to keep it clean and reduces the service life of your sheet metal or paint.
This is very useful. Had to read through it slow to understand it, but I have a better grasp of it now. If I find a pair of mudguards, I'm putting them on her.
Reasons you should are listed above very well by other members.
For me, I go back to what an engineering friend of mine said. "Big things are made of lots of little things."
So, my goal is to make going to my low paid profession as cheap as possible. Mud flaps may add a tiny amount of drag and an equally tiny amount of weight. Perhaps insignificant to anybody else, but to me its one of those "little things" that is making up the "big thing." So, caulking in the headlights, removing the radio antenna, adding tiny pieces of underpanelling where there had been none before are all tiny things but in the bigger picture add up to a lot in terms of FE.
Great discussion though, I like the technical jargon offered by FIND on mudflaps. I should pick his brain about what other steps I can take with my VX to improve my FE any more.
You have a good take on this as well Benfrogg. I think it's a situation that sort of fits in the "whatever is best for you" category. Yeah, FIND really had some nice insight and explanation, and I'll bet he can find another couple of ways to help you as well!
I had mud guards on my VX, and I had an average of 55-ish mpg with the car, with a tank high of 77. I don't think they hurt anything. I think they do help keep the rocker panels from rusting out under the doors. Mine had muds installed when the car was purchased in '94, and it was rust-free, despite being an Indiana car. To this day, it's the only rust-free EG hatch I've seen in this area.
I can only really help so much with aero... Aero isn't really a field I spend a lot of time on, I just remember some things. It is a lot easier to just use software to figure those things out anyways, but fluid dynamics isn't a horribly bad field to get the basics of.
Benfrogg, that quote from your engineer friend, "Big things are made of lots of little things." Yeah, that is something worth remembering in almost any situation.