Where is a good place to start?
 

Id love to do it all but reasonableness calls for brains in this instance. I haven't done too much to my car so far. Ive only removed the wipers, antenna, passenger mirror and done headlight sealing. I had an upper grille block but it deteriorated. It was cardboard with gorilla tape. The moisture got to it and it got soggy. The mirror and grille block are fairly small mods in my opinion. Id like to know what mod to aim my focus on next. Seems kind of noob of me but id rather be that than the newby that thinks he knows best.

Some things ive pondered and asked about were a kammback, wheel covers, undertray, rear diffuser, tire width, spats, and cowl hood. A kammback on a Mustang might not look that great plus my trunk lid opens to vertical. So that would need some extra ingenuity. Wheel covers are doable but my rims are 17" aluminum Bullitts. Id need to make a way to secure the cover without doing rim damage. Undertray and diffuser are definitely on the table. For performance and better fuel economy. Tires dont seem to be a major breaking point, at least not with keeping close to my stock tire size. Spats are doable but I havent done enough research yet. A cowl hood might get the air to transition smoother onto the windshield. Ive seen a few add spacers to the hood hinge to raise the rear of the hood. Thats very unsafe. But a cowl hood starts at $300 so id never see the return.

You have a lot of performance $$ into the car, what's your balance of "economy driving" vs. "performance driving"? Are options like a taller final drive on the table?

Try another grill block material - coroplast or plexiglass.

How about a kill switch so you can coast more often?

Power steering delete (pull the pump, drain the rack, cap the lines, get a shorter belt). Though this may be easier for me to say with a 1.8L engine over the front wheels than your 4.6L :)

Most of the money in performance was done before I was thinking about fuel sipping. But alot of it is lower weight spinning mods and mods that make the engine not strain for power. My car was also originally automatic.

I actually try to drive fairly efficient. Though im not as skilled as the hardcore guys. I do as much EOC and neutral coasting as possible.

My gears are 3.73s. The stock gear is 3.27. 3.27 just feels awful where as 3.73 feels like the gear the Mustang should have had. They do make 3.08s and 2.73 but I might as well just start in 2nd gear at that point.

Different material for the grille block for sure.

I cant do a power steering delete on my 4.6 not because of the weight but because of the hydro-boost brakes. It runs off the power steering pump not vacuum. I suppose it could be done but then id need a new manual rack and a different master cylinder and in the end it wouldnt pay for itself.

The point of the taller final drive is to reduce RPM in top gear on the highway, not at low speed. Reduced RPM at cruise will lower your losses from engine friction and accessories. It will also operate the engine in a more efficient condition - lower RPM and higher throttle for the same power, which gives reduced throttle plate losses.

Surely your car has enough 'pep' with stock gears and performance mods to get out of its own way without needing the lower final drive. I guess it's a priority thing - are you prioritizing quarter mile times, highway MPG?

How are you doing on the tyre pressure front?

Why not keep the mustang as a fun car and get yourself an inexpensive civic/corolla or something? Even the hardcore guys have a heck of a time hitting 100% over epa (would be 36 mpg in the stang). Getting a civic/corolla would get you there instantly and with a little ecodriving on top you'll easy be above 40 mpg.

I hear you DennyT. That taller gear just makes a Mustang so sluggish where as 3.73 feels like a nice compromise. 4.10s or 4.30s would go in if I wanted all out performance. I don't take it to the track at all. Its my daily driven car.

I keep them at about 36-38psi all around.

Ive thought about getting a little sipper to mod the crap out of. But I added up the cost of the car, the insurance, the gas id need to fuel it, repair costs and maintenance cost and I figured it would take a long long time for me to spend that money through the tank of my Mustang. Thats even with a $500 early 90s Metro. And I think thats where some people are fooling themselves. They never calculate the expense of the purchased vehicle and all its cost. Then figure out how long it would take in gas money, in the inefficient vehicle, to equal that car and all its costs. Its going to take a long time to spend $1000 in gas money for my Mustang. And thats why I haven't done it. BUT there is always the option of getting rid of the Stang and getting the gas sipper type car. Then id be way ahead. Sell the Stang for $9k get a car for $500-1k and be getting double my mileage plus cheaper insurance and cheaper replacement parts. But id be sad because i love my V8 fun car.