Fox Carb mpg?

Well the no first gear makes total sense as the 4cyl T5 has a 3.97 first gear (if I remember correctly) and 5.0s were 3.35 or 2.95. Overdrive is 0.79, I think, which is a little worse than a 5.0 T5.

I would suspect your carb is jetted a little rich and also probably a little big (unless you are racing - in which case you have absolutely the wrong transmission).

Don't forget a 302 cubic inch motor, revving at 6000 rpm will only move 524 cfm of air. A 550-600 cfm carb would be better suited and if only street cruising you could get away with a 500 cfm carb (keeping revs under 5500). All these should increase fuel economy. One with vacuum secondaries would help as well.

Another option would be put in some 3.08 gears and gain lower RPMs cruising on the highway. See comparison of first gear final drive ratios below, as well as OD final drive ratio and RPMs in OD for 70 mph.

3.97 1st x 3.73 = 14.83 --- 0.79 OD x 3.73 = 2.95 with an OEM size tire (25.8" diameter) would be 2695 RPM at 70 mph
3.97 1st x 3.55 = 14.09 --- 0.79 OD x 3.55 = 2.8 with an OEM size tire would be 2565 RPM at 70 mph
3.97 1st x 3.08 = 12.23 --- 0.79 OD x 3.08 = 2.43 with an OEM size tire would be 2225 RPM at 70 mph
3.97 1st x 2.73 = 10.83 --- 0.79 OD x 2.73 = 2.16 with an OEM size tire would be 1972 RPM at 70 mph

Stock 5.0 WC T5 (some other 5.0 T5s have 2.95 1st)
3.35 1st x 3.73 = 12.5 --- 0.68 OD x 3.73 = 2.54 with an OEM size tire would be 2320 RPM at 70 mph
3.35 1st x 3.55 = 11.89 --- 0.68 OD x 3.55 = 2.41 with an OEM size tire would be 2200 RPM at 70 mph
3.35 1st x 3.08 =10.31 --- 0.68 OD x 3.08 = 2.09 with an OEM size tire would be 1915 RPM at 70 mph
3.35 1st x 2.73 = 9.14 --- 0.68 OD x 2.73 = 1.86

Stock AOD 1st
2.4 1st x 4.10 = 9.8 --- 0.67 OD x 4.1 = 2.75 with an OEM size tire would be 1915 RPM at 70 mph
2.4 1st x 3.73 = 8.95 --- 0.67 OD x 3.73 = 2.5 with an OEM size tire would be 2285 RPM at 70 mph
2.4 1st x 3.27 = 7.85 --- 0.67 OD x 3.27 = 2.19 with an OEM size tire would be 2000 RPM at 70 mph
2.4 1st x 2.73 = 6.55 --- 0.67 OD x 2.73 = 1.83 with an OEM size tire would be 1673 RPM at 70 mph


Based on the info in the above chart I would guess you have 3.73 gears (as RPM for 70 mph is almost identical to what you reported).

To your question, would a change between 2300 and 2800 make a difference in fuel economy - ABSOLUTELY. If 2800 RPM is your baseline and you reduce it to 2300 RPM, that is 500 RPM reduction (about 18%) to travel same distance. So for your 30 mile trip you should use 18% less fuel (approximately) assuming your driving style is the same. So, doing some quick and crude numbers... you used 2 gallons for your 30 mile trip. With a lower gear ratio (changing to 3.08 gears) you would only need 1.6 gallons for the same trip. You would get 18.75 mpg only changing the rear gear. Overfueling as I mentioned above may be an issue as well.

I have a 90 GT with an AOD and 3.73 gears. A couple years ago I did some good highway driving (250-300 mile trips - 6000 miles as a DD in 2 months) and would get 22-23 mpg on the highway all day long (at 65 mph).

Good luck and let us know how it works.

With my edelbrock 650 on a 306 with a 4 cylinder t5 and 3.55 gears I got 26 mpg cruising at 72mph. That was during my 10 hour trip to Myrtle Beach SC from MD for Mustang Week.

The trick was a little el cheapo air fuel ratio gauge from Advanve Auto. The power valve ( for acceleration ) is the little pumper up top, it has three settings by moving the arm pivot. The two screws up front adjust the idle afr. Tuen all the way until they stop gently. Back off 3/4 and start car. Keep unscrewing both until the car starts correctly and the idle AFR is correct.. Both screws should turn the same amount of times. This needs to be tweaked anytime the seasons begin to change or you chang elevation compared to sea level. There is an adjustment for the secondaries as well by bending the rod. ( I never had to adjust them ) I hooked the gauge up to a factory o2 sensor. People say they aren't accurate and use a WB but I used one for 7 years ( daily drove most of those ) and always got good mileage and performance. The trick is to keep the gauge as close to stoich as possible at idle and cruise. You don't want it pegging rich all the way when accelerating. When off throttle it's normal to spike lean. I think the part number on my edelbrock was 1401. I had an edelbrock intake and 1 inch carb spacer that was an open square, not 4 holes.

Be prepared to get a lot of grief for an edelbrock. Tgey are routinely called " edeljunk " and " edelbrick ". I never had a problem with it in 7 years. Cleaned it with carb spray once a year. Still worked when I sold it.