Hey all I've noticed that lately my '72 U code Supreme is getting less mpg than normal. Up until recently I was able to run back and forth to work (about 50 miles round trip) and burn about 1/4 tank. Now I'm using almost 3/4 of a tank for the same trip. It's running fine with no issues, no leaks and I've made no adjustments. It has an HEI and I also have an inline filter between the pump and the carb, so even though it's running good, maybe I need a new filter? It's been about 2 years and about 6K miles since I put the filter on, and I always go to the same gas station and only run 93 octane.

Any ideas?

 

I assume you have confirmed that it is the fuel being used, and not a gauge freaking out suddenly. I think you've got a leak. Since you would have said if it is dripping in your garage, I am going to guess a "leaks under pressure only" leak at or after the fuel pump. Recommend backing her out on driveway and looking at fuel pump and lines. That's a hell of a leak, though, to use that much fuel.

 

I checked the pump and the lines...no leaks,, and no smell of gas while idling or while cruising. I'll double check how many gallons she takes at next fill up, but in the meantime, what about the filter?

 

The fuel filter has zero influence on fuel economy. I suppose if it was clogged to the point the engine won’t run would drastically improve the mpg, but only on a tow truck.

Assuming the fuel gauge is accurate, the problem should be easy to find. That’s a bunch of gas for such a short trip.

Fill the tank, record the odometer reading. Make a couple round trips to your work, fill it back up. Divide miles driven by the gallons needed to fill it back up. Then, assuming your speedometer is accurate, you will know what the fuel economy actually is.

Btw, fuel gauges aren’t linear. I can fill the tank on my car, and drive a 100 miles and the gauge reads 3/4. Another 125-150, it’s empty.

Im guessing your going to find a leak. For the economy to nose dive like that with no noticeable change in the way the car drive the gas has to be going somewhere.

Which brings up another possibility, pull the oil dipstick. Make sure the oil doesn’t smell like gas, or is now reading overfilled. if so, the fuel pump diaphragm has split, and its dumping fuel into the oil pan.

 

X2 on checking your oil.

 

I'll check the oil, but If the diaphragm has split and its dumping fuel into the oil pan, then what's happening to the excess fuel that the pan can't hold? Also, I've done the math and I'm averaging roughly 8-10 mpg, whereas before I was around 12-14. As I said, she's running like a champ, and even from a holeshot at WOT, I can wind 'er up to 100 with no issues. Seems to me if it was a fuel delivery issue, she would stumble and hesitate under a pull like that, right?

 

Have you pulled the spark plugs to check their condition? Using that much gas they should be black. Any smoke out the tailpipes? Black?... Running rich. Blue?...torn fuel pump diaphragm.

 

Will check the plugs but no smoke from the pipes.

 

Have you tracked this over several tankfuls or has this just occurred? Is it possible that someone siphoned your gas?

 

Im going to take a different tack here. Holeshots and runs to 100, maybe a little more throttle here n there, i think your just more used to the car and dont mind wringing it out abit. I get 10 ish w mine 455 n qjet burnouts, wot, general hooligan driving ( on a closed course w a professional driver im sure i could do better but thats not really why i like to drive.

 

My last fill up was the last time I wrung it out, but it was just one time. This fill up I've behaved myself and I keep it around 65 on the highway at a steady cruise. Been doing some research and maybe it's the "winter gas?"

 

Nope...cuz I check the gauge every time I get in and no noticeable difference.

 

I'm thinking that last week's nice weather has made your right foot heavier.

 

I’m in the winter gas/winter air/ foot discipline camp (despite a few days off from winter)

 

Checked the oil..it's good..no smell of fuel.

 

Checked the plugs...they're good 👍 ..not a hint of ⚫ or unburnt fuel.

 

check the road for black marks from the tires on the way to work!

 

Winter gas is suspect, i noticed my last fill on my daily the pump had a sticker on it that said from 11-1 to 3-1 this pump dispenses oxygenated fuel.

w no significant changes ie driving habits temps routes my mpg has increased by about 7.5%

 

"Winter blends" contain more "aromatics" to vaporize at lower temps. Everyone's gas mileage goes down during colder months.

 

When you checked the plugs did you inspect the wires? I've always had problems with GM vehicles burning the ends of the plug wires. My work truck I wrapped the ends of the wires with asbestos paper because I couldn't find insulators locally. The rubber would burn to a point where it was arcing against the exhaust manifold.

 

check your timing maybe its retarded

 

It's the crap gas they are putting out now. I'm in Calif. with winter gas, and it is crap. And we don't need winter gas here.

Gene

 

It's not so much a winter thing as it is an emissions thing. The EPA mandates oxygenated fuel in many large metropolitan areas, and I'm sure California uses it nearly everywhere.

 

Don't confuse summer and winter gas blends with oxygenated (ie, ethanol-laced) gasoline. There have been winter and summer blends of gasoline forever, long before there was ethanol added. The aromatics are changed depending on the seasonal temperature to make the gas easier to atomize in the cold or less likely to vapor lock in the heat.

The oxygenated fuel thing came in before computer-controlled cars. It was intended as a way to "trick" the carb into running leaner in the winter when the choke was closed more frequently. Anyone who understands how carburetors work (which pretty much eliminates anyone in government) would understand that this also defeats the purpose of having a choke in the first place, which makes the car run crappy. Of course, computer controls and O2 sensors completely negate the effect of oxygenated fuel because the ECU changes the A/F ratio to accommodate the leaner fuel. Again, don't expect anyone in government to understand this. Of course, the oxygenated fuel thing morphed from an emission control tool to an agricultural subsidy, so it isn't going away anytime soon.

 

Yeah, good point. I had Retro Ranger's post about the oxygenated fuel sign in my head when typing that, and remembered posts discussing lower MPG using that compared to "regular" gasoline.

 

From post #19

Avgas is also "blended" for lower ambient temps....to vaporize at colder temps.

 

. X2, if you were using your vacuum advance before make sure it’s still working.

 

It may be the gas causing the issue. I've noticed all my cars get worse milage in the winter. I have been running non ethanol gas now for about a month, and despite the colder temperature and winter weather, my milage has gone up significantly. I was getting about 13mpg with my 455 Cutlass on 10 percent ethanol mix cruising conservatively at 60 mph. Under approximately the same conditions, I just got 16 mpg using ethanol free gas. Another thing I noticed is my fuel gauge would peg out at the hot indicator light when full and drop like a stone after about 100 miles. After the first tank of ethanol free gas, the guage read just slightly above the F when completely topped off and there was not drop off, just a steady accurate reading. I've heard ethanol can gum up the fuel system. I'm wondering if real gas can do the opposite to the fuel gauge contact.

 

Colder air= denser= more oxygen =more fuel demand =more power