fuel economy for 258

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   I'm getting about 10mpg out of the 258 in my '67 Rogue. I may have already ****ed myself with the mods I've done, but here they is:
'83 258
-small cam (@ .50  duration 208/208, lift .421/.421)
-offy 4 bbl dual plenum alum intake
-holley 390 cfm carb
-clifford header with dual 2.25" exhaust
-GM HEI ignition

Rebuilt 904
Stock rear with 3:08 gearing
Lowered, with 7.5" tires/dog dish hubcaps (to help with the visual)

There's probably a better combination of single exhaust, a different header/head, different/no cam, or a host of other changes, but everything's already in and I don't want to change it right now since I just finished the car about a year ago.

I'm looking for more mpg than 10, and this isn't anything near a race car. It's just a daily driver, and I like the sound of the header and the aesthetic of dual tail pipes with a six. Is it possible to up that mpg?

first steps?:
-adjust timing
-have carb professionally tuned
-swap out the intake/carb for a 2 bbl setup
-switch out exhaust for dual 2" tubes?

Thanks to everyone, maybe/probably/especially Farna, Uncle John, and Tyrod Tom!

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jdarosa Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Nut Joined: Oct/31/2014 Location: Portland, OR Status: Offline Points: 264	Post Options Post Reply Quote jdarosa Report Post Thanks(0) Quote Reply Topic: fuel economy for 258 Posted: Oct/24/2016 at 1:52pm
	I'm getting about 10mpg out of the 258 in my '67 Rogue. I may have already ****ed myself with the mods I've done, but here they is: '83 258 -small cam (@ .50 duration 208/208, lift .421/.421) -offy 4 bbl dual plenum alum intake -holley 390 cfm carb -clifford header with dual 2.25" exhaust -GM HEI ignition Rebuilt 904 Stock rear with 3:08 gearing Lowered, with 7.5" tires/dog dish hubcaps (to help with the visual) There's probably a better combination of single exhaust, a different header/head, different/no cam, or a host of other changes, but everything's already in and I don't want to change it right now since I just finished the car about a year ago. I'm looking for more mpg than 10, and this isn't anything near a race car. It's just a daily driver, and I like the sound of the header and the aesthetic of dual tail pipes with a six. Is it possible to up that mpg? first steps?: -adjust timing -have carb professionally tuned -swap out the intake/carb for a 2 bbl setup -switch out exhaust for dual 2" tubes? Thanks to everyone, maybe/probably/especially Farna, Uncle John, and Tyrod Tom!
	1968 American 440 – 232/AT 4dr

6768rogues Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/03/2007 Location: United States Status: Offline Points: 6241	Post Options Post Reply Quote 6768rogues Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 3:19pm
	That is horrible mileage. My 67 Rogue convertible 232/2bbl gets in the low 20s. Are you sure no gas is leaking out of it somewhere?
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Pdok Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Apr/03/2011 Location: Alabama Status: Offline Points: 1025	Post Options Post Reply Quote Pdok Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 4:00pm
	I got 14.8 with almost that exact configuration. I had the Clifford intake though. My cam had 485 max lift though. I think you've got tuning issues or bad vacuum leaks. Put a vacuum gage on it before you do any major changes. Lots and lots of things can cause bad mpg that are pretty easy to diagnose.
	76 Grem X 258/904,4.0 head/MPFI, Comp X250H cam, Hughes springs, Clifford header, serpentine swap.

tyrodtom Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Sep/14/2007 Location: Virginia Status: Offline Points: 6214	Post Options Post Reply Quote tyrodtom Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 4:10pm
	There has to be something very wrong. I had a 69 232, with same carb and intake, no cam, probably the same header, but smaller exhaust. Straight stick overdrive. It could get high 20s when I drove it right, but usually low 20s, high teens. I don't think I could have got it down to 10 even if I had my foot in it all the time. you're probably losing 4-5 mpg because of the automatic, but that still leaves you way low. But it wasn't just a bolt on, I had to do quite a bit of tuning with jets, power valves, cam and spark timing and spark plugs, etc. to get it where I was happy. Good grief, I get better mileage with my old J-10 360, towing a 4000 pound load. I race a 4 cylinder in a stock class with a stock cam. I can get radically different results by just monkeying around with the cam timing.
	66 American SW, 66 American 2dr, 82 J10, 70 Hornet, Pound, Va.

Fluffy73 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/21/2007 Location: Castlegar, BC Status: Offline Points: 3059	Post Options Post Reply Quote Fluffy73 Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 4:23pm
	Is your vacuum advance hooked up correctly and working? 7.5" wide tires are creating a major drag. Narrower tires will help. What converter are your running? How loose/tight is it? Lockup or not?
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tyrodtom Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Sep/14/2007 Location: Virginia Status: Offline Points: 6214	Post Options Post Reply Quote tyrodtom Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 7:17pm
	I notice you've put a new cam in. From my experience with racing I remember that most reground cams are not ground so they are centered the same. If you installed that cam without checking where the true center was, your cam timing is suspect.
	66 American SW, 66 American 2dr, 82 J10, 70 Hornet, Pound, Va.

uncljohn Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jan/03/2013 Location: Peoria AZ Status: Offline Points: 5394	Post Options Post Reply Quote uncljohn Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 9:45pm
	Years ago I built a MOPAR 225 that stock was hard pressed to get much better than 10 mpg in a full sized 2drht and automatic going back and forth to work. Why? Under carbureted, undercammed and flog the heck out of it to get it to move in freeway speed stop and go traffic. Engine too small weak and car too heavy. When finished with the engine and final drive almost the exact same as yours, freeway gas mileage was over 20mpg and commute economy over 15 and it would step out and move with out flogging it. I would hazard a guess that your tune is way off. If you are driving the car hard with a poor tune, it is still a heavy car with a small engine. A very poor fuel economy exercise in frustration. My Spirit with 9:1 compression and an Iskenderian cam with .450 lift and 202 degrees at .050, single exhaust, automatic with 4bbl carb equivalent, gets around 18 and frankly driving it like it is a small V8. There really should be no excuse for 10mpg with what you say you have. If it isn't the tune.
	70 390 5spd Donohue 74 Hornet In restoration 76 Hornet, 5.7L Mercury Marine Power 80 Fuel Injected I6 Spirit 74 232 I-6, 4bbl, 270HL Isky Cam

tyrodtom Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Sep/14/2007 Location: Virginia Status: Offline Points: 6214	Post Options Post Reply Quote tyrodtom Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 10:26pm
	The biggest mileage killer you've got is that automatic, maybe as much as 5 mpg right there, but that's not easily changed. But you've also got some other basic problem, or you enjoy the sound of your exhaust too much, and drive accordingly. When I took the automatic out of my 74 Hornet ( 904 I suppose ) and replaced it with 4 speed from a 79 Spirit, my mileage got 4 or 5 mpg better just from that. If you check a bunch of old threads on this forum about low mileage complaints with AMC sixes, you'll see one common element most share, they're automatics. Edited by tyrodtom - Oct/24/2016 at 10:29pm
	66 American SW, 66 American 2dr, 82 J10, 70 Hornet, Pound, Va.

jdarosa Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Nut Joined: Oct/31/2014 Location: Portland, OR Status: Offline Points: 264	Post Options Post Reply Quote jdarosa Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 11:17pm
	Good to hear on the automatic. I drive it like an old man, so it's not from my foot. I'll mess with the timing and get it tuned, and check for fuel leaks...I'd hope to get some mileage out of it before the big job of swapping in a 4 speed...oh lord. As far as the converter, it's non-locking and whatever the stock one would be for an 83 Concord. Thanks everyone
	1968 American 440 – 232/AT 4dr

DaemonForce Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/05/2012 Location: Olympia, WA Status: Offline Points: 1070	Post Options Post Reply Quote DaemonForce Report Post Thanks(0) Quote Reply Posted: Oct/25/2016 at 12:48am
	I'm suspecting fuel leaks, ENERGY leaks(cam + nonlocking torque converter) and possible timing issues. What carb are you using and does the power valve have a tendency to puke itself? Have you considered a mild dual pattern cam like the Crower 44242/44243 or CompCams 68-115-4? The 4spd swap shouldn't be a big job but you WILL need a proper donor and there just aren't any. I hope you have a garage.