318 guys- you know your fuel mileage ?

[toolmanmike]

Very interesting... What vehicle is the 318 in? What am I missing here!!?

 

[JimKueneman]

1968 Coronet, 2.94 gear stock rebuilt 318 with FiTech EFI = 12 MPG
1972 Duster 2.94 gear rebuilt with Toth heads and Edelbrock RPM intake and 500 CFM carb = 13 MPG
1966 Valiant 3.21 gear rebuilt 360 with Edelbrock heads, RPM intake and 500 CFM carb = 12 MPG
1963 Dart 3.23 gear rebuilt 225, decked to 9:1 dual one barrels = 19 MPG

All are jetted using an AFR meter and are actually a touch on the lean side during cruising as I was trying go get the best MPG I could.

 

[toolmanmike]

1968 Coronet, 2.94 gear stock rebuilt 318 with FiTech EFI = 12 MPG
1972 Duster 2.94 gear rebuilt with Toth heads and Edelbrock RPM intake and 500 CFM carb = 13 MPG
1966 Valiant 3.21 gear rebuild 360 with Edelbrock heads, RPM intake and 500 CFM carb = 12 MPG
1963 Dart 3.23 gear rebuild 225, decked to 9:1 dual one barrels = 19 MPG

All are jetted using an AFR meter and are actually a touch on the lean side during cruising as I was trying go get the best MPG I could.

Wow Jim, do you drive uphill all the time? Sounds like some nice combo's though.

 

[JimKueneman]

Wow Jim, do you drive uphill all the time? Sounds like some nice combo's though.

Nope mostly open road at 65-75MPH. I never understood how you guys get that milage, to the point I have concluded most can't do math correctly . If you look at the rating for these cars what I am getting is exactly what Chrysler said they should get.

 

[toolmanmike]

Nope mostly open road at 65-75MPH. I never understood how you guys get that milage, to the point I have concluded most can't do math correctly . If you look at the rating for these cars what I am getting is exactly what Chrysler said they should get.

I think the key to mileage is to keep the cruise RPM at or below 2000. THat seems to be the magic number.

 

[JimKueneman]

I think the key to mileage is to keep the cruise RPM at or below 2000. THat seems to be the magic number.

It is directly proportional to RPM and displacement so that is a true statement.

 

[Phantomx]

I can’t imagine getting 12 mpg on the open road with a stock hiway geared 318 car. All of my 318s have typically run 15-17 mpg. Even my 71 d100 with manual trans and 2.76 gear would get 15-16 going 70-75 mph. My 70s smogger 360 in my 67 barracuda would get 16-17, and 20 when I put the o/d 833 in it. Hell, my 71 w200 with 383 and 4.10 gears would do 14 hiway at 65 mph and 10 in town.

I always had tight front ends with good alignment, and as much timing as I could put in without pinging. If they had an aftermarket carb that was easily tuned, I tuned them.
Travis..

 

[guy gadbois]

My poor Belvedere - 318, 727 and 2.9 rear gear - gets between 13 and 14 on a good day just cruising around with a tailwind; its ridiculous.
My memory is, that bitd, a stock 318 would only do 16-17 if you were careful. Apparently, from what Ive read, the minute you do a 4 bbl swap, you lose 3 or 4 mpg with no other changes. Again, from what i have read, it seems that a lot of the loss is from reversion due to the manifold runners being larger and mismatched to the smaller port size on the 318 head; I dunno, but it sorta makes sense to me. Again, assuming all Ive read is accurate, there were a few aftermarket manifolds made specifically for the 318 back when and they do not cause the problem. Apparently the worst of the lot these days is the Edelbrock Performer; naturally my car has the Performer manifold, carb and cam....

[as an aside... my 383 RoadRunner would get about 12 - 13 in the mid70s when I had it, and my foot was not light...]

 

[Phantomx]

One thing that you absolutely must have for economy is a functioning vacuum advance. I forgot to bring that up before.

I think the reason people may lose mpg when they do a 4 barrel swap is they just get a carb and plop it on, zero tuning. You shouldn’t lose mpg just by swapping to a 4 barrel unless it’s entirely wrong for the combo, or you always have your foot in it.
Travis..

 

[guy gadbois]

One thing that you absolutely must have for economy is a functioning vacuum advance. I forgot to bring that up before.

I think the reason people may lose mpg when they do a 4 barrel swap is they just get a carb and plop it on, zero tuning. You shouldn’t lose mpg just by swapping to a 4 barrel unless it’s entirely wrong for the combo, or you always have your foot in it.
Travis..

It made no sense to me why just changing intake and carburetion would cause such a drastic drop in petrol mileage either, so I began to investigate not long after I got my car as i noticed it needed a trip to the filling station about every other time I drove it. I fiddled with the carb. I had a friend who is much more talented than I mess with it. I took it to a fellow who is locally well known for Mopar stuff in particular; he adjusted on it and even welded in a bung on the exhaust for an O2 sniffer etc. Plugs look good, car runs as well as a basically stock 318 ca, and it drinks gas like a camel at an oasis. I think its as good as its gonna get with this combo. I just cruise around with it, occasionally hit the highway and rarely ever open the secondaries. The only thing that makes much sense is the two or three articles I read about reversion and poor intake design.

 

[YY1]

The "Performer" IS the intake designed with 273/318 size runners.

I've actually heard the opposite- swapping to an appropriately sized 4 barrel can increase MPG.

 

[Phantomx]

If you’re never on the highway at steady cruise, I can see it being 13-14. When I was tracking this stuff, I was driving about 50-50 in town and highway. Lots of stopping and starting means lots of accelerator pump shot. Every time you push down on the gas, you’re shooting a stream of fuel down the throat. If your new carb has a big accelerator pump, that could be a contributor.
Travis..