Clamshell Fuel Mileage

Just wanted to share some success I've had with the gas mileage on my '73 Custom Cruiser and I wanted to see how it compares to other Clamshells.

When I got the car on the road this spring I religiously checked my gas mileage with every fill-up and with every part replaced. I was consistently averaging 8.3 MPG and about 150 miles per tank of gas despite all new tune-up parts, properly inflated tires, a consistent driving routine and making sure not to get into the Q-Jet secondariness. Anyway, about a month ago I sent a spare Q-Jet out for a rebuild with instructions to rebuild it based on the stock specs of the engine. I installed the new carb last week and just ran through a full tank of gas. To my surprise I got just over 200 miles on this last tank, which equated to 10.3 MPG. That includes a few full throttle blasts too. So, I'm pretty pleased and I'm thinking it's a decent number since all my driving is around town and in a 5,400 car.

I'm curious how this compares to other Clamshell owners and if my joy over the 10 MPG figure is misguided.

Chris

 

Wowzers! I couldn't imagine being happy with 10 mpg. But, I guess it's better than 8.

 

just took my 76 CEW from San Diego to San Francisco for a trade show-fully loaded with trade show stuff. Am religious about checking mileage as well.
In town normally empty I get 11-12 mpg, on this trip loaded I got on all four tank fills 230-245 miles (without empty light) with 15.1 to 15.4 mpg. That was driving on average 55-70 miles per hour. Let's hear from others

 

This is exactly what I get (around 11 mpg) with my '73 Custom Cruiser. These are large, heavy cars with huge engines, and they were just not built for gas mileage at a time when a gallon of gas was 35 cents. The oil crises of the '70s were coming, but they hadn't quite arrived yet.

My family owned a '73 Custom Cruiser back in the day, purchased new, and I learned to drive on it. (I turned 16 in 1973). I remember that the gas gauge would move so quickly from F to E that you could almost see it move as you drove.

 

hullinger, I don't have one of these cars to test with but are you looking for some advice to get some higher fuel economy? You can find some methods here: www.ecomodder.com

Some of the advice is simple like removing the roof-rack if the car has one and increasing tire pressure from 35 to 45. Others are more involved and some are driving methods, some are also not even legal in some areas.

 

Never actually "checked" mine around town it's so so. I do know I lent it out last summer to my friends wife for a trip to niagra falls, and many stops on the way back. Her aunt was up from texas there were 7 people in it. they used $135.00 in gas. I believe that's pretty damn good. It's actually great on the highway. 353.84 miles just to niagra plus all the running around that's seems pretty good to me. 75 Buick estate W/71 455 poncho power

 

Let's keep a basic truth in mind here: you can't get blood from a stone.

These cars were never designed with economy in mind. Removing the luggage rack might increase the mileage from 11.00 to 11.01 mpg, and increasing the tire pressure to 45 might add 1 or 2 mpg, but at a very high price, in my opinion. The quality of the ride would deteriorate as it would be like riding around on solid rubber tires, and the tires would wear unevenly with the centers wearing faster than the edges.

If mileage is truly a concern, the only real solution is to drive a smaller, lighter, more fuel-efficient car, not, as I said earlier, one of the heaviest passenger cars ever made with one of the largest engines ever put into a passenger car.

 

You haven't given us enough information here to determine if this is "pretty good," "pretty damn good," or anything else. We need to know how many gallons were used in going 354 miles, not how much money was spent. Or else we also need to know the average price per gallon paid.

If we guess that an average of, say, $3.75 per gallon was paid, then $135/$3.75 = 36 gallons. 354 miles/36 gallons = 9.83 mpg, which is actually a little on the low side compared to what others have been reporting but probably reasonable if there was a lot of stop-and-go, city driving.

 

I have not officially checked my '72 Kingswood Estate 3-seat with the 402, but I do know that I used exactly 1/4 tank the other day - and it was exactly 50 miles, most of which was 55 mph highway driving with a couple of stops in town. lol.

 

I had a very similar experience a few years ago when I had my '75 CEW and drove it from L.A. to S.F. I kept it under 70 MPH and had very light cargo (two pieces of luggage, one other passenger with me, but I did all the driving). I got just over 15 MPG going up and just under 17 MPG coming back home.

 

I reckon mileage gains could be had with an LS motor swap with a modern overdrive auto tranny or a six speed.

At what cost, though?

 

With new installed headers + dual exhaust setup I got 13 MPG on a 230 mile highway trip at 70 MPH with the cruise control on, for city traffic I barely get 8-10 MPG which is reasonable for a 455.

 

No point in making it harder than it has to be, or more expensive.

A 5.3 liter truck engine, used, with computer. ($500)
LSX swap harness ($450)
The 4L60E transmission from same, used. ($400)
Add an inline electric fuel pump, frame mounted. ($100)
99+ Corvette fuel filter/regulator ($75)
SBC to LS or BBC to LS engine mount adapters. ($135)
Possible having to swap the driveshaft or have it shortened. ($150-$200)

You can spend $600 for a tailshaft swap to use your stock mechanical speedometer, or make a custom gauge cluster (or use the truck one from where you pulled the engine), or find another way to work around that problem. GPS units make a good speedometer. 700R4 tailshafts CAN be swapped onto 4L60E transmissions but you still need a VSS signal for the computer.

Lots of forum posts and how-tos out there.

Reselling your big block engine and TH400 will recoup some of the cost.

Buying a whole wrecked but running 99+ GM pickup or 03+ GM van to do the conversion will save time and hassle plus you have a truck to scrap/partout when you're done.

 

Or you can just leave that money in the bank, write bigger checks for fuel, and enjoy riding around instead of wasting time under a shade tree making changes in a stock good running wagon.
Or buy a SmartCar spending way less on fuel but way more for parts when it breaks down and the next boat won't be here with the parts till next spring.

 

My Dad owned a '74 Electra Limited 4dr with the 455. He drove me back from college in South Carolina to New Jersey, (this was back in '78), and we got 12mpg on the highway with the ac on the whole way. Funny I can remember that, but not what I watched on tv last night.