How bad is the gas mileage on these yaghts?

[73 T&C]

Quick question to the hatted wonders on this site.....

I consistently get 8-9 miles city on my 73 T&C with spikes of 6 and 12 depending on the mix of driving types and right foot aggression. Highway is 14-15 if I stay at or below 70. At 80 we get down to 12 MPG.

In my 78 NYB, I am about to burn through the 1st half tank (according to the gauge which read full+ at filling) with working Lean Burn and admittedly aggressive but not crazy driving in all city traffic. I’m showing 80 miles on the trip odometer....That’s about 8+ MPG...

Is this Normal???

 

[LeBaron1973]

Sounds about right for such a heavy car.

 

[Davea Lux]

Your '73 is a low compression smog motor that was badly de-tuned for EPA standards. You can try advancing the timing which might gain you another 1 mpg, but something like 10mpg on a aggregate basis is about the best you are going to get without some serious internal engine work.

Dave

 

[rapidtrans]

Sounds about right.
Drove a brand new 74 NY to Florida averaging 15 mpg at 80 mph. Around town 10 mpg is doing very well.

 

[Xavi3r]

i suppose it is fine.
I get 12 mpg with mt 7.7l 76 cadillac hearse ( near 3 tons ) at 90 kph and " new " carb ( rebuilt carb bought 10 years ago but the car did 2000 km in 10 years i think.. )

 

[live4theking]

I wouldn't expect much more than that with the crappy gas.

 

[LeBaron1973]

Your '73 is a low compression smog motor that was badly de-tuned for EPA standards. You can try advancing the timing which might gain you another 1 mpg, but something like 10mpg on a aggregate basis is about the best you are going to get without some serious internal engine work.

Dave

Dave, are you suggesting that fuel economy can be improved by engine work, or just engine performance?

It would be interesting to learn your thoughts of what is possible and what would need to be done.

 

[Pops62]

In other words you measure in gallons per instead of miles per but when your driving your driving in style

 

[Davea Lux]

Dave, are you suggesting that fuel economy can be improved by engine work, or just engine performance?

It would be interesting to learn your thoughts of what is possible and what would need to be done.

The big disadvantage of the '73 and many later smog motors was the low compression ratios. This meant that the engines were a lot less fuel efficient because when fuel is compressed less, it burns slower and produces less power. This keeps hydrocarbons down and carbon monoxide down but does nothing for efficiency. Going to a higher compression ratio increases both power and efficiency. So if you someday decide to rebuild your engine, put in pistons that are at 9.5 to 1 compression, this way you can still run the engine on pump gas. A second modification would be to go to a throttle body type injection system. This will give you much better performance and also much better fuel economy. You already should have electronic ignition on you '73, so I would probably leave that alone. The relatively low horsepower of the '73 engine probably means you are running a 3.23 gear ratio in your T&C, this is a good ratio for mixed highway and urban driving. If most of your miles are highway miles, you can consider a 2.94 ratio after the other mods are finished. I would expect that the higher compression ratio pistons and the throttle body injection would gain you something like 5mpg over what your are currently getting. You would need to pencil out how many miles it would take to make all this pay for itself before deciding if it is worthwhile to proceed. The fuel savings are a good motivator but you have to be driving the car significant miles for it to be cost effective.

Dave

 

[LeBaron1973]

Fair enough, Randy said you'd need to drive a rebuilt engine around 7000 miles to bed it in I guess, so we decided to leave it alone. Besides, with gas here in NZ around $10 a gallon that's lots of $$$$.


Thx for the remarks.

 

[bigmoparjeff]

Believe it or not. My parents got 18 MPG highway out of our 78 NYB with the Lean Burn 440. That's probably running 55-60 mph.

Jeff

 

[Fratzog]

Unless this is your dd any concern about gas mileage seems misplaced. I'd enjoy it for what it is. A cool old gas guzzling cruiser. My .02

 

[polara71]

Unless this is your dd any concern about gas mileage seems misplaced. I'd enjoy it for what it is. A cool old gas guzzling cruiser. My .02

Kinda beat me too it Fred.

 

[CBODY67]

I'd worry MORE about the highway mpg as that's when the fuel metering is on the main system and should be in the engines more-efficient rpm range AND a reasonably steady speed.

"City" mileage is very variable. Driving style can be important, but you don't need to drive "easy" as such, either. Use moderate acceleration to keep the carb metering rods out of the "full power" orientation. Idle time at red lights counts for "zero" mpg, too! Learn to time the green lights to just before you get there, if possible. A rolling start will use less fuel to accelerate away from. Which can be important with a 4500lb+ vehicle.

Comparing our '66 Newport 383 2bbl to our then-new '72 Newport 400 2bbl (8.2 CR vs 9.2 on the '66), the '72 would not match the '66s prior gas mileage, but it was not more than 2mpg less, as I recall. 15mpg on the highway in the blocky-front '73 seems about right on the highway. You might advance the timing a few degrees, but as I recall, the factory timing was pretty much where it needed to be anyway. The specs on the '72 were "+"/"-" 2.5 degrees from the basic setting, as I recall.

If you want to do a mileage check, fill the tank and head for the Interstate. Set the cruise at "speed limit" or appropriate for traffic and put some miles on it. Like 75 or so, or more. Then turn around and head back to the same station and pump. Fill it 'til the pump clicks off. Note THAT amount, then fill it again until the pump clicks off, which might be another .4 gallon. Use that filling method for the initial fill and also the re-fill. Until you put some consistency into the mileage checks, things can vary.

From what I've seen, idle mixture adjustments as long as they are reasonably correct, don't usually affect fuel economy at low speed. After about 1000rpm, the carb should be on the transition and main metering system, I believe. Having it at "lean best idle" makes it better-running, though.
.
As I recall, E0 fuel should get about 6% better fuel economy than E10 does. E0 costs more, so economic feasibility might not be there. The car was designed when E0 was all we had Unleaded and low-lead fuel was just coming online in later '72.

CBODY67

 

[cbarge]

Really? If you want MPG,buy a Prius.
I really don't care about fuel economy
Personally,I need to calculate my MPG for long trips regardless if i get 9,10 14 or 18 MPG.
It is nice to know how far I can drive on a tank and when to start looking for a gas station.
With a 24 gallon tank,at say 16 MPG,driving range is between 250 300 miles depending on climate,weather,driving habits,etc.
When the gauge hits 1/4 tank I have roughly 50 miles to stop for gas.
But I do make sure the engine is tuned to the max for efficiency.
As of now,the BoaB is at 14 MPG. The 66 300 is at 18 MPG
Both are 383 4bbl cars both with dual exhaust.
The BoaB has an bigger cam and carb along with 2.94 gears.
The 300 is pretty much stock including the 2.76 gears.

 

[commando1]

In my 78 NYB, I am about to burn through the 1st half tank (according to the gauge which read full+ at filling) with working Lean Burn and admittedly aggressive but not crazy driving in all city traffic. I’m showing 80 miles on the trip odometer....That’s about 8+ MPG...

Yep. Same here. Well, maybe 9. But you'll get over it.

 

[Mr C]

Believe it or not. My parents got 18 MPG highway out of our 78 NYB with the Lean Burn 440. That's probably running 55-60 mph.

Jeff

I do believe it because that checks with my experience. My 77 T&C has gotten a record of 18.9 mpg on a road trip since I've had it.

 

[DocMcNeedy]

 

[73 T&C]

Unless this is your dd any concern about gas mileage seems misplaced. I'd enjoy it for what it is. A cool old gas guzzling cruiser. My .02

Fratzog,

Not worried at all. My question was completely academic.

I’m just wanting to gauge if this Lean Burn system has any real value as to burning less fuel.

I get it... 5500 lbs, 440 cu in, crap gas with as much as 10% ethanol with less BTU’s than gas and low compression low efficiency equals the 8-10 MPG I’m seeing in the wagon. Thermodynamics at play.... I get it.

I’m just wondering if it’s any better in the formals.

As to the cost and my caring... I also drive a 12 cylinder gas hog that I absolutely love. So no, the rate of consumption is actually part of the fun and goes on to amaze my friends. Luckily... I don’t have to drive very far on a daily basis. And they are all my daily drivers!

The good news is that the upward pressure on crude seems to have gone away with today’s closing of 65.30 a bbl... and... Venezuela and Russia are still completely screwed!!!

Yay!!! More for us.

Javier

 

[73 T&C]

View attachment 193142

I’m making a poster of this for my garage!!! ROFLMFAO