I've put a lot of money into modding it. It's all been paid back in gas savings. The car has 299,648miles on it, so as parts fail, I put better (ie. more expensive) parts on that may or may not help mpg. A rusted out exhaust led to an expensive larger sized mandrel bent replacement that I enjoy listening to.
I summed up this achievemetn on the second page of this thread with this:
"Looking back, I would not say "One" thing put it over the top...except what was mentioned earlier that pushed me over the 50mpg threshold. I've worked hard at taking out as many stop signs and lights from my commute and spending a greater percentage of it on the highway and I have modded the car to get the best possible highway mpg's, because I'm at a large disadvantage in city driving with an ATX. The ATX ZX2 is geared higher than the MTX, so it can do better on the highway, and I keep my speed down to the 50-55mph range. Aeromods are key on the highway and I have modified my car to run a lean 17:1 AFR on the highway, that's the only way to get this sort of mpg out of this car. The LRR tires, deletes, and Zero Toe have greatly helped in runing leaner than I could before. I used to run down the road at a 52-53 LOD getting instant mpg readings around the mid 60's. Now I run down the road at a 46-48 LOD seeing mid 70's on the instant reading. RE-92's are an amazing tire and I would highly recomend them to any serious Ecomodder.
So to sum it up: Route Planning, Low Speed, Aeromods, Lean, and LRR are the heavy hitters on my mod list, but the sum of all the little things help as well.
Keep in mind that it has taken two and half years to make this happen."
All of the effective mods combined cost less than $500 and I save over $1000 per year in gas costs. I don't know why everyone doesn't do this