Realizing all engines are different, would still love to learn more about your setup. Things like carb jetting, power valve, initial/full dist. advance (including springs used), ignition source/type (i.e. MSD 6AL), trans OD ratio and rear end gearing, etc. I have a similar motor with a 750 Mighty Demon and I get 10mpg on a good day.

IMO the range is the important thing. If you can't afford to put has in it then get something else but who wants to stop every 100 miles.... or less?

Here's what I have the Roush 427SR (351w) tuned with:
- 93 Oct fuel*, TKO600 5spd, 8.8" Rear End with 3.73:1 ratio.
- Annually I check the Carb Bolt down and torque to spec, 60-80in-lbs(5-7ft-lbs). When i first got the car 3 bolts were extremely loose and vacuum had leak when cold.
- Annually check Disty Cap & Rotor, replace them every 2 (maybe 3) years. MSD HiOutput/6AL puts a lot of wear & tear on them.
- Annually I run a can of Seafoam through a tank full... living in NC I can drive it year round so no need to winterize it but still do this on all my classic sports cars. I have added 2200miles on it since April 2020 when I bought the car.
- Have a MSD Hi-output Coil, MSD 6AL box, use cooler NGK BKR6E-11 Plugs , gapped at 0.045" (between .042"-045" works best).. Note: Roush specs using Autolite AR3923, heat range 3 plugs so the NGK's I use are equivalent with their heat range of number 6.
- Have a Mallory #8578 Pro-Billet Disty, Mechanical advance .. with 2 Silver (lighter) springs.
- Idle Set at 920rpm and Timing @ 15.5 BTDC ... at 4000 rpm, Timing @ 32.5 BTDC (Roush spec is 31-34 BTDC, must be LESS than 35 BTDC).
- Have a 770 Holley QF (80770-1) ... don't know exactly the jetting, nor power valve as it came from Roush as a crate engine ... sorry here...
- Currently have BOTH A/F Carb Mixture screws set at 1-1/2 turns OUT (I don't have 4 corner A/F mixture screws... just the 2 front ones). Here I spent a lot of time tweaking with 1/8 turn increments to find the sweat spot... when I bought the car in April 2020, the car came with A/F turned 1-3/4 out (on passenger side and 1-5/8 out on the driver side) so I reset them to starting at 1-1/2turns out as the documentation suggests and ended up here anyway.
.... When I bought the car , it's Idle timing was @ 10.5 BTDC... Disty Cap & Rotor were shot, Autolite plugs gapped .052"-.056" .. and I know I had vacuum leaks with the Carb Bolts looser than spec when cold. I was getting 10-11mpg at best....

NOTE: * I use regular 93 Octane even if it has some ethanol , so I tuned the engine with this type of gas..

Hope this helps and feel free to PM me for more discussion...

Thank you, Mark. Surprised that with the two light springs your total timing is not all-in well before 4,000rpm. Agree that MSD caps and rotors are an annual maintenance item.

Thanks... I think Roush says RPM should be above 3500 or 3700rpm .. so I just run it around 4000rpm .. to assure Full Advance ... :cool::cool:

You want all the timing in by 3000 rpm, but you should check the total timing at 3500 rpm. Often times magnetic trigger distributors back up a degree or two by 3500 rpm. I don't know why, but they do -- it's just one of those things in life that is hard to figure out. Kind of like why Superman could stop bullets with his chest but would still duck down when the bad guy threw the empty gun at him.:cool:

I'm late to the game here but, wanted to chime in and get some opinions:
I'm running a '65 K-code 289, 4-Speed Toploader and a 4.10 rearend. I'm getting about 10 mpg. My car is running 4000 rpm at 70 (peak torque @4500 rpm) and 4400 rpm at 80 mph. Isky Racing Cam. Super fun and loud! Anyone think I should drop to a 3.73 or even a 3.55 since my 4th gear is 1:1 ?

The answer to your question is another question. Do you think you want a different rear ratio?

A 3.73 will bring your 80 mph rpm to approx. 4,050, not a big reduction. However, the 3.55 may be a good option.

@HTM101 I will have to consider a 3.55. I love the sound of this engine at 4000-5000 rpm. I think I did enough due diligence with putting in the ISKY Racing Cam and other components to handle this. The 4-Speed Toploader was a must for me with it's 1:1 4th gear. I was mainly concerned with engine stress over mileage. Like I said, I hope I did enough to enjoy it at these higher rpms? The 4.10 rear, from what I've read about it is very desirable. I will just keep up maintenance and upgrades in the future. It's just too glorious sounding at 80mph!

I have a big block with a wide ratio Toploader. My rear ratio is 3.70 and is adequate for where and how I drive. 60 mph is 3,000 rpm, 80 is 4,000.

I rarely drive on an interstate and took that into consideration when I installed the Toploader. However, if I were on an interstate a lot more, I'd keep the 3.70 rear and install a 5-speed.

I average 7 mpg.

[IMG]http://www.clubcobra.com/forums/pict...ictureid=20464[/IMG]

I have a 482 cid FE, 3.54 rear, and TKO600 with .64 5th gear. In the city I get 8 mpg on 93 octane pump gas, and I get 12 mpg on the highway. It purrs along at 2,000 rpm at 70 mph.

I'm 35 cubic inches smaller, but otherwise I'm exactly the same on all the other numbers. And I really enjoy when I'm out on a remote highway and have the road to myself and hit the "quiet spot" in my revs, at about 2200 RPM, and just cruise at 75 MPH. It actually restores mental health.:cool:

That’s very encouraging. It implies mine is tuned ok since I know yours is.