What kind of mileage can I expect on a 78 T/A with a 400?
Yes, I know to not expect it to be very high, but I am curious what
other people are getting with similar cars.
Driving it around town, I am getting 8.5 mpg. Going just 85 miles
cost me $25.
Is this at all normal or should I expect better?
I know it is obviously cool as it is early spring and the choke is on
more often, which will lower the gas mileage, but 8.5 mpg is like
driving a motorhome or something.
It was getting 13 mpg last year.
I'm not driving it hard. I'm not sure what rear end gears it has, but
it has posi-traction... perhaps only one gear ratio was available with
I figured it would be ~12 mpg because the weather is cooler now, not
I'll have to check the igntion system over & see if it's OK.
The HEI advances literally weld themselves together with rust due to
moisture and the coil firing through the shaft and the advance
My 79 TA wouldn't turn a tire on dry pavement when I got it. The advance
was stuck, rusted all up. The cat was plugged solid. I bought it because
it was in amazing shape and looked almost new, and only had 18K on it.
Just whacking the converter off and polishing and lubing the advance cam
woke it up quite a bit. The vac advance was fine, it just couldn't move
the advance mech, as it was frozen solid. When it was all good, the
milage went from 12 to almost 17. When I put the cam, heads, intake,
etc, on it, it went down to 14, but it ran great.
The advance problem was never permanently solved until I put an external
coil on it.
Those 'flyweights-and-spring'(centrifugal weights, I guess is more
nearly proper) make a tremendous difference, don't they? A point I'll
remember--never happened to me, but an acquaintance was having problems w/a
customer's 87 Silverado just 2 weeks ago. This guy is known widely as the
best carb man in our upstate. Truck wouldn't idle slow enough--& even at
high idle, when you put it in gear OR merely switched on ac, it'd cut right
off. Off and on, over several days, he'd replaced module, rotor, and pickup
coil, and cap was new & looked good. After being ready to give up & return
it to cust., unfixed, he decided to try something I'd heard of: the dist.'s
main shaft becoming magnetized & throwing the outputs from the pickup
coil/module to kilter. Bingo! He grinned when his screwdriver was sucked
up to the shaft. He changed dist. with one lying on his bench & it solved
the problem. He & I both saw it for our first time. s
I forgot all about that bit! My friend had an older Suburban that didn't
run right and after the dealer put all the stuff on it for a huge price,
without fixing it, I looked at it, and the shaft was very strongly
magnetized. We took it out and whacked it a couple of times, and I ran
it through a bulk tape eraser about 10 times and it finally lost 99% of
it's pull. The service manager looked at us like we were nuts, but the
computer/ignition guy comes over and says, "Yeah, that can do it". They
ended up eating all the labor and most of the parts cost, and gave most
of his money back.
I always wondered how the shaft could stay so strongly magnetized in a
hot motor like it did. One more reason to move the coil away from it.
Thanks for the good suggestions. Checking the vaccuum advance is a
I never thought to look at the centrifugal weights. Hopefully if this
is the problem I can buy new ones at an auto parts store?
I don't know, I took the whole distributor apart, and cleaned and
polished everything and was ok. I ended up buying a new distributor
after two cleanup sessions, and moved my coil to the fire wall, and it
never happened again.
I've got a '78 W72 400 manual-trans T/A. On a good highway run, I get
18-19 US mpg. Relatively short drives in winter weather (below
freezing) gets me down to 15 mpg.
I'm in Canada where right now it's a dollar a litre, or something like
$65 to fill up. Fuel economy is pretty important to me. Mostly I use
the car for getting out into the country, so a lot of rural driving.
I expect consumption to be about 14 litres/100km in general, which
translates to about 17 MP(US)G.
That's pretty bad.
Now I've replaced the 3.42:1 rear gears with 3.08:1 for nicer highway
cruising. And I run stock-sized H-rated 225/70-15 tires; I expect
that fat mushy 265/60 white-letter items would add a lot of rolling
resistance. (The odometer is accurate: the speedo drive gear was
replaced, and I've checked it against kilometre posts.)
My Quadrajet is a rebuilt stock item, with the stock very-lean
metering rods (about the second-leanest available according to Doug
Roe's table of metering rods). The cam is a Crane firstname.lastname@example.org which
makes it a bit larger than the stock W72 item. All emissions items
are still there, including working heat riser valve on the driver's
side manifold. Exhaust is dual, with dual worn-out catalytic
converters. I find I can't run the timing to the max 18 degree
advance specified because it pings a lot on my default 87 octane, so I
have it backed off to 16 degrees. The advance weights and springs are
stock GM, as is the HEI module.
Car doesn't have A/C, but still weighs in at something like 3850 lbs
curb weight. And when the T-roofs are out, aerodynamics go right
away. But it's fun! (At the strip, the car runs about 15.2 @ 93 MPH;
the 60-foot time is a miserable 2.4 with the 2.43:1 first gear and
3.08:1 rear and 27.4" tall tires. Oh, and I don't haul out the spare
tire or cool the motor off or anything like that.)
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