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Discussion Starter #1
Hey all,

I have a '68 XR-7 with a J-Code 302. The car has an aftermarket Weiand intake with a Holley 4160 4-bbl carb (rebuilt and tuned for the car). The engine also has a Pertronix Ignitor II module in the distributor and the engine has been recently tuned up with new plugs, wires, PCV valve. I only do city driving and I'm averaging about 8mpg which seems really low.

Is this normal or do I have a problem?


Thanks for any advice.
 

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What kind of rpm's are you running? Was wondering what gear ratio you have? also are you buying good gas or the cheap stuff? That really makes a difference. You didn't mention it but did it get a new air filter?
 

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Thats about what I get with my ride. 13mpg highway, and about 5-8 in town depending on my mood. That with a lightly build 2v 289 short with bolt ons, c4, and 2.79 gears.
 

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Discussion Starter #6
Yeah, the car has a new air filter. It's got a C4 with 3.00 rear gears. The car was professionally tuned (carb and distributor). I've been driving fairly conservatively lately to see what the mileage is like. I typically use the cheapest grade gas (91) - do you think that has something to do with it? I'm not sure where else to look at this point. The only real mods were the intake manifold, carb, and 16" wheels.
 

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Wow, 91 is the cheap stuff there?!? Here in Chicago it's usually either 87 or 89. Then again it's like $4.50 a gallon for 87 here, so...
 

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Does your car have single or dual exhaust? Bad muffler(s) maybe?
 

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The 1969 XR7 in my signature gets gets about the same mileage. LOL
 

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You may want to check the jet sizes on that 4160 - have they been replaced/altered? Also if you are running 10% ethynol that may be as good as your gonna get mileage wise? You'll get better mileage w/regular even at a lower octane.
 

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Discussion Starter #13
Sorry, I screwed up on the gas - I use the cheap stuff (87 octane). My car does have dual exhaust (Flowmasters). When the car was tuned they said they adjusted the jet size down on the carb. I did check the vacuum at idle (when the transmission is in drive) and it's registering about 16. When it's in park, the vacuum goes up to about 21.

From the posts, it sounds like 8 isn't too far off from what others are getting.
 

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Is the vacuum advance working in the distributor? How rich is the idle circuit in the carb? Idle mixture screw adjustment can have a big affect on gas mileage. Adjust the idle mixture as lean as possible without causing idle quality problems. Check for possible brake drag, low tire pressure etc.

Bill
 

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And quit using the 87 octane! Back when these cars were built I think the poorest gas you could get was 89. You will see better mileage with better gas. I run 91 in my bike. Runs like crap on lesser octane also have found the difference in mileage is worth the added cost.
 

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From the posts, it sounds like 8 isn't too far off from what others are getting.
Some of those posters are referring to their big block cars. A small block with your transmission and gear ratio should be getting signicantly better than 8 mpg. My 351C 4V with 3.25:1 and AOD gets no less than 11, even when I don't use the overdrive and am doing some spirited driving around town. All the previous small blocks I've had got between 10 & 13 mpg around town and between 14 & 19 on the highway without an overdrive transmission.

Others may be satisfied with poor mileage but that doesn't mean there is nothing you can do to fix it. If your engine isn't pinging under load, there is no reason to use higher octane fuel. 91 octane isn't going to get you any recordable increase in mileage. Are your brakes dragging? Is your gas tank, fuel lines or carb leaking? Are you carrying 500lbs worth of junk in your trunk?
 

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My H code 351C w/ Edelbrock 4bbl intake and 600 cfm carb I get a lucky 8-10 in town. But I do drive it like I stole it most of the time.
 

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Also, I would wonder if you are traveling on flat land...I live in San Francisco where the nearest block is about 30 degrees, and my Cougar gets about 12 MPG in town. But that's great compared to my '73 Marquis, which gets 6 MPG around town!
 

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Wow am I doing good on gas mileage!! I have a Stock 351W with the stock 2V, FMX tranny, and 2.75 rear gears I believe, calculated almost 16mpg on 93 octane around town!
With the rear gears I barely get off idle around town. (35-40 mph @ idle in 3RD gear) :)
 

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My first car did fairly well on gas. It had a 289 in it. I am curious what I will end up with when I get this one back together. It has a 390 2V with 2.75 rear. I think I will be happy with 11 or 12 miles to the gallon. Figure that is all I get with my truck right now!
 
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