New Guy w/ an 84 Mustang GT
 

Hi all, Jeff here. From Northern Iowa with a 1984 Mustang GT. I am currently swapping out the tired carburated 5.0 with an '87 EFI 5.0. It has a T-5 and 3.08's
So far my best was 18mpg with the carbed motor. I am hoping for low 20's right out of the box with EFI. I have added a late 80's ranger front air dam in hopes to help with MPG.
Right now it has solid rims on it with 14" tires. It wont be running those forever but they work for now.
Thats all I can think of for now more to come..

Welcome! It seems like the Mustang community is really growing here.

What is your commute like? Any other goals?

I don't daily drive this car but do go on long trips with it to bigger shows and what not. My goal is to drive to Denver this April for my sisters wedding and get 25+. I will be hauling a small motorcycle trailer to carry all the 1 year olds stuff. Long term goal. Do power tour and get 30 mpg average over the whole trip

Putting this in here so I can remember later on:
1984 EPA rating:City-13 Comb-15 HWY-20
Now the new motor is a 1987 5.0 H.O with a 1988 harness and computer
1988 EPA rating:City-14 Comb-17 HWY-23
My car has a 3.08 rear gear and the 1988 manual should come with either 2.73 or 3.08. I am just writing them both off as 3.08
My car currently has 225/70r14 and the stock tire on a 88 is 225/60-15
The 14" tire is 26.40" and the 1988 is 25.62"'
So if I stay with that size tire the operating RPM would naturally be lower. I can even go bigger if need be. (14" tires are hard to come by in this size)
Another thing I have going for me is the stock gear ratio of the t-5. I got this off of the interwebs so might not be 100% correct.
T-5s by year
84 1st 2.95 2nd 1.94 3rd 1.34 4th 1.00 5th .625

85-89 1st 3.35 2nd 1.93 3rd 1.29 4th 1.00 5th .675
5th gear has a lower ratio in the 84 versus the 88. Thus improving over a stock 88.
To recap the rear ends are the same, I am running close to 1" taller tire on the rear slowing rpms at speed, and my transmission is .050 lower than a stock 1988 Mustang GT. So given the EPA "rated" the 1988 at 23 HWY I believe with those 2 things I should be better than 23. Whether thats 25 MPG we will see.

If anyone smarter than me can figure out what I should be getting, that would be awesome.

Welcome! I have a mustang as well (kinda runs in my family.) I have a Sn-95 model, so completely different aero wise from the fox bodys. I know 85 is the first year Ford put that fuel injection, I dont know how much better they upgraded that system in 87.. but if they didnt make any changes I know I would stay with a carburetor-ed engine. Much more options/ after market things you can do to a carbed 302, than you can with that year fuel injection, I dont think ford intended MPG at all with that fuel injection system. thats just my 2 cents.

the t-5 is great! If you have 3.08s do still have the 8.8 rear end? do plan on changing the rear gears? If you want to get serious ($$$) id swap out for a good 9 inch, probably could weigh less.

Aero wise, its really fun to mess with a stang, even though I have a newer-ish model im trying to go for that older trans am road racing look, and the aero mods I have done so far are making it happen haha!

In 1984-85 ford had cfi which really wasn't that good. I believe in 86 they went to a one year only efi. 87 was the first "good" year. 1988 was still speed density which is what I have.

The 3.08 are in the factory 7.5" rear. When I upgrade it will be to the 8.8" out of a sn-95 and 5 lug. Will probably run the stock 3.27 for a while.

As far as mods go, I really want them to look good over all. Subtle yet clean.

Ok cool, didnt know that about the EFI! I feel you about the appearance, with a fox body id be more picky with what body mods I do. What wheels do plan on putting on? maybe some dish wheels? And I think a front air would help this car out alot. Maybe some rear window louvers?

Myself, would def do rear window Louvres in a fox.

I personally love the louvers. My wife on the other hand does not. If I find one with the right price I might buy one. Another thing I have been thinking about are those lexan light covers. I don't like the way they look. But if they prove to be an improvement they are something I could remove at shows or when ever I want

Another fox body...welcome.

Good to see some four eyed pride around here, welcome! I know you have a 1984 but just to clarify 1986 was the first year for speed density FI, it was also the first year for factory 8.8 diff. and the World Class T5. And the first year for the third brakelight, so if you are spotting old Mustangs and see one with 4 headlights and 3 brakelights, it's an '86 lol. If I can recommend one thing that you will actually be able to feel a difference when driving and that will also help with your FE, it's an aluminum driveshaft. The factory ones are big heavy lumps, and reducing that much rotational mass is something that you can notice. Finding used parts for these cars is another plus to modding one, they have been so scienced out by the performance community over the years, and so many people have built them up that there are a TON of used parts floating around for them out there.

Get your self a wide band O2 gauge and carb tuning set.
Up to $250 for the gauge and $40 to $100 for a rod and jet set would be much cheaper than going EFI and would deliver much of the same MPGs.
Plus with older/cheaper EFI you cant lean burn, older EFI tries to stay as close to possible to 14.7:1. With a carb tuning kit and O2 gauge, depending on conditions you can get it to do what ever you want.

so hows ur pony working out?

ALso, increase the psi in the tires....maybe to 40psi for starter.
ANd you gotta get a gauge....Scangauge II
see the first link in my signature

the 9 inch is considerably heavier and way over kill. if you decide to put a 8.8 in make sure its a 31 spline other wise ther isnt gonna be a big difference in strength to justify the weight cause the 7.5 and 28 spline 8.8 use the same axels. secondly a 7.5 can handle some serouse power i had one behind a turboed 2.3 in a ranger that put down 467hp at the wheels and was never babied oly reason i went with an 8.8 was got a good deal on it

Still no real update. I have had it running and on the road. I have to re-run my fuel lines to clear exhaust. Hope to put some real miles on it soon.

good luck.
glad to have you!

1.Centerforce or similar style clutch. reduce slippage increase mpg...
2.& 3. elec. fans and waterpump.
4. If you can live with the rack n pinion alone remove hydraulic pump.
5. side mirror deletes
5. 6. 7. Front spoiler and or air dam, lets just say ground effects kit.
8. If you can find it get a SVO biwing it compliments the aero, and does provide some down force.
9. SVO wheels were also nice for aerodynamics but your running 4 bolts not 5.
there was a GT wheel that looked like SVO wheel also mabe that works for ya.
FYI. upgrade to 5 lugs isnt bad job and not very expensive. the rear disks feel nice too.
10. forgot underdrive pulley.
Also with some exceptions 88% of power adders increase engine efficiency, and make wasting gas seem more fun too.

I wouldn't suggest added downforce. But spliiter/dam would pretty effective and cheap.