Fox Carb mpg?

[Smurfstang88]

Ok so i want to get a general idea of what mpg i should be getting with my current set up.
Car is carbed with a 650 edlebrock avs2, 355(i think) gears, with a 4CYL t-5. I hate this trans for alot of reasons but its what im stuck with for now.. anyways the car feels awfully geared id say similar to a 4.10 fox but not quite the same. It has basically no first gear, it can idle its way up a mountain in first nearly . And at 65mph it sits at 24-2500rpm and 2700 at 70. So i drove about 60 miles on 10GALLONS and really didnt drive it that hard and it was just about empty(fuel gauge is dead btw). Avg of around 7mpg. But about half of that drive i had a older edlebrock on it since this one needed to be cleaned so i cant be sure what the real mpg this current setup is getting. So i filled it up and cruised home about 30miles at 60-65 (2200-2500rpm)and filled it up again and calculated 15mpg. Is this all i can expect to get from this setup? I have heard of others with this carb getting 20-28mpg. And does the difference of say 2300rpm vs 2800 really have such a huge impact on mpg? Thanks for any input!

 

[KRUISR]

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.

 

[90sickfox]

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.

 

[MustangIIMatt]

With an Edelbrock 650 Thunder AVS I would get 12-15mpg in town (car was geared too steeply and was running too small a tire for highway use at that time).

Same carburetor on a 400cid small block Chevrolet in a '75 Chevy longbed farm truck would give me 14mpg highway all day.

Both were set up using a vacuum gauge.

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.

You won't hear one bad word from me on Edelbrock's carburetors. I've had a 600, 650, and 750 on various engines, and they were damned-near EFI-like in fuel economy, drivability, and reliability until E15 gas came along. Even then, my ONLY issue with them was that the E15 would evaporate and leave nasty deposits behind when my Mustang II didn't get driven often enough. That's neither Edelbrock's fault nor is it exclusive to Edelbrock. The Edelbrock 650 I have is back on the Mustang II's 302 until the 4.6 is ready to drop in, I sold the MSD EFI system that caused never-ending headaches and put the carb back!

@Smurfstang88 ,This guy knows his stuff if you want to learn more about them:


View: https://www.youtube.com/watch?v=1WNpdy9-zdE

 

[manicmechanic007]

Your Updated Carter AFB is fine for mpg. The only better is an Autolite for fuel mileage. You need to keep the CO around 2 or 3 percent for that old car and you are probably closer to 8%. Have a manual IM test done to see. Get a printout. Lower the float level and swap jets & metering rods if possible. Working at the Ford dealer in the eighties, we used to be able to get about 19mpg out of those old Lincolns with the 460 4bbl autolite just by lowering the float level to the minimum setting. Most of those new Edelbrock carbs come jetted for a well built 350 chev with 11 to ones running at sea level. Used to have to get hotrods to pass IM around here. Used to do stupid stuff like soldering up jets and putting a small wire thru the jet to cut some flow. You may have few options with some newer edelbrocks as they have no jets and just washers sweged into the carb body. Buy a track kit and play with the carb if possible. Make sure you are running enough timing. Enjoy

 

[Smurfstang88]

Thank you all for your reply’s, especially the gear chart is useful, but im still pretty sure the gears are 3.55s because thats what they were supposed to put in, but ill have to check again just to know for sure. Also the link to that guys youtube is appreciated, I understand a little more than i did before but mostly the accelerator pump adjustment, i had no idea which hole to put the rod in, no but seriously mine was set to what the guy refers to as “hot rod mode” meaning more gas and i need to set it to “cruiser mode” to get better mpg. Once i drive it again I’ll update this if i saw any difference. Also i did adjust the idle air screws to as close as i could get it With a tach because i dont have a vacuum gauge yet but ill be getting one. One thing that wasnt clear to me is how to figure out if i need to change the metering rods. Anyways, it seems theres not a ton of options for better mpg as the issue is clearly i have to get a different trans/and or different rear gears if im gonna ever see 18+mpg..

 

[Smurfstang88]

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.

Wow! That sounds great! I was wondering do you remember what rpm u are getting at 70?

 

[90sickfox]

Wow! That sounds great! I was wondering do you remember what rpm u are getting at 70?

I can't remember. 72mph was the sweet spot for that car.

 

[7991LXnSHO]

An E7 long block and headers, Edelbrock 302 EGR intake and Holley -1850 2 carb (with added electric choke and quick change spring kit) and an SROD got 21+ at 55-65 mph. In town was at least as good as my EFI car.