Ok this is about to drive me crazy. I have a 67f100 with a brand new 302 and c4 trans and I have 2.75 gears in the rear. I am shooting for gas mileage here where I can actually drive it oh I also have a holley 600 4 bbl and headers, the headers will probably be coming off and either stock manifolds going on or shorty headers. Anyway back to the question how much gas mileage should I be able to get out of this, because I really don't think I am getting what I should. I think I should at least be able to get 15mpg or am I wrong? Thanks in advance.

 

I don't have any experience with the 302 but my old 67 with the big 300 six got 14 or so as a rule. It was much lower geared than yours as well. I would think with those gears and your setup you should be doing much better than 15 mpg.
jor

 

I think my carb is flooding big time. Do you think 600 is too much for a 302 or just something wrong with the carb, because it is only 2 years old.

 

I don't think 600cfm is too big for the 302. I've got a mustang that has a 289 bored .040 with a 600 holley. It runs just fine.

 

If you are running an otherwise bone stock 302 I think the 600 cfm may be a little much. My guess is properly tuned you could be getting around 18. As an example my 70 with a 302 and 3.00 gearset gets around 21. Sometimes it hits 22. Ofcourse it is a 3 on the tree and in my opinion this is not typical. Mine is slightly modified but I still run my stock 2 barrel. Planning on a 4 barrel swap later.

 

Hey 18 mpg would be awesome my uncle's 84 4 cyl ranger only gets 20 with a 3.08 rear gear. We must be doing something terribly wrong. I never have had too much knowledge on a carb. I guess I better find somebody who does. Thanks for the input.

 

Someone who is knowledgeable on carbs could probably finesse your 600 to work well on your 302. Talk to some other folks about this I am not the final authority on carbs by no means. That carb flooding is killing your mileage though. Something else you could look at to improve mileage make sure your vacuum advance is hooked up.

 

tallica2002, Your setup is way off for mpgs. First off the 302 doesn't have the gonads to pull that much weight turning a 2.75 rear gear. Try a 3.25 or 3.5 and you should see your mpgs go up.

If the Nascar Busch series runs 390 cfm carbs on their race cars, why do you need 600 on your daily driver??

The C4 is a good choice behind the 302, you have the headers on leave them, but try to reduce your carb size to a 350-400 cfm and increase your rear ratio to something the 302 can pull closer to it's power band, you are lugging it now.

I ran 2.75 behind my FE and might do it again when I get something else to tow with. I am much happier with the 3.00 gear that is in there now. I am running a 390 cfm holly 4 barrel and hitting around 14 on the hiway above 70 mph.

John

 

The only problem I have with the 390cfm carb is the cost. The 600 was so much cheaper. Can a 600 be rejetted to something like a 390. And the longtube are definitely going I can't stand them. I might go with some shorties or something like that but I'm not keeping the long tubes. I have a set of 3.25 gears I might just put them in there, but what rpm should I be trying to achieve at 65 because my tires are on the smaller side they are 295 50 15 Thanks guys.

 

I put a 600 cfm Edelbrock carb on my '72 302 about a year ago and it runs great. Definately not too much carb for my 302 with a performance cam and headers. I can start it up on a cold Alberta winter day and drive it almost immediately...with no warm up. Excellent performance and driveability over the old 2 barrel.

As far as the gearing goes; I just installed a 4 speed OD tranny behind the 302 and I have not yet changed out the 3.25 rear end for something numerically higher. I have been driving it around for about a month like this and the gearing isn't that far off. I think if I change it out to a 3.50 rear it should be just about right. BTW with the .8 OD gear that works out to the equivalent of a 1:1 high gear and 2.8 rear.

 

tallica2002, I have a bud that is a Holly guru and he got me a 390cfm Holly that was run on a Pinto I think for a little over the cost of a kit. It has taken some tweeking and we are zeroing in on it. It performs nearly as good as the Edelbrock 1406 that we took off.

fordxxx, Take a look at your tailpipes to see if they are sooty.

John

 

I can answer that question right off the bat. Yes and it smokes and it shouldn't be smoking it is a brand new motor.

 

Says to me, the carb is too big.

John

 

I like your engine/tranny/rear end combo - but you are paying about a 10% penility for running an automatic over a stick. This gets worse as the tranny wears, so I would make sure the C4 is in good condition.

Your driving makes a lot of difference, but if you are holding it to 65 MPH and not charging up the hills at full throttle I would expect 20 MPG or better. Your truck is too heavy to get much better - but 20 mpg is reasonable.

If you are running a stock ignition I would upgrade that. I prefer the Petronics system at about $100, but any transistorized ignition will do your job. You will get better starting and drivability, plus better mileage.

To my thinking you are a bit over carbed - I would want something around 450 CFM. The real indication is the dark tailpipe - you want it gray and dry. I would call Holly and talk to one of their techs as you may be able to just change the jets in the carb. Make sure they know you are at a higher altitude. On a stock 302 I changed the jets from the size 58 it came with to a size 52 and gained about 4 mpg.

That and a good tune-up - make sure your plugs/wires/coil are of good quality and that the timing is right and the mechanical and vacuum advance are working.

This should get you into the 20 mpg range (highway). If it does not then I would look at your rear end gears - you want somewhere between 1800 and 2200 RPM at 65 MPH.

One last thought - the newer 302s were set up for pollution controls and were never as strong performance wise as the older ones. If all else fails I would be looking at the cam and compression ratio of the engine. I had an 85 302 in a high-top conversion van that was always a dog - poor performance and even worse mileage.

 

My '70 had a 302 w/ a stock 2 bbl, mild cam, and stock exhaust. Rear ratio is 3.25. It got 15 in heavy city traffic; 18 on the highway. I think 600 cfm is pushing it for a stock 302, but you should be able to re-jet it to work OK. For economy, I'd use a 450 and try to never crack the secondaries. Watch your tire size, too. It takes a lot of power to turn some big ol' monster mudders. For what it's worth- I was much happier with an electronic distributor.