You have to pass visual inspection here in NC with your '75, so they will be checking for a catalytic converter.
Catalytic converter
Le Mans Master
Joined: Jul 2000
Posts: 6,664
Likes: 162
From: columbia city in
if your converter is plugged up then yes your mpg would go down.if its the orginal cat replace it .your bad mpg may have more to do with a lot of other things .what is your rpm at 60mph?if the rpm is high thats one problem.hows your driving habits?theres a lot to improving mpg.what mpg are you getting now?
Race Director
Joined: Sep 1999
Posts: 11,360
Likes: 382
From: Plano TX
Case in point: I once had a '83 Cadillac Deville. It still had the original pancake style cat on it and the best I got on the highway was 22mpg. Replaced it with a modern free-flow cat and had noticeable better power and repeatedly got 26 mpg highway.
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
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Pro
Joined: Feb 2000
Posts: 741
Likes: 46
From: Hebron IN
Just go all the way and get rid of that poser fake dual exhaust system.
Some headers with true duals, jetting, carb tweaking and timing advance kit will make a power difference you won't believe. As an added bonus mileage will go up as long as you're not having fun with the added power and it'll sound like a real car.
If you have emissions testing just get some new high flow cats. If not leave them off and also ditch the smog pump for a few more HP, less weight and another boost in gas mileage.
I wonder what the trade off has been over the last 40 years of low emissions vs. millions of gallons of wasted fuel?
Some headers with true duals, jetting, carb tweaking and timing advance kit will make a power difference you won't believe. As an added bonus mileage will go up as long as you're not having fun with the added power and it'll sound like a real car.
If you have emissions testing just get some new high flow cats. If not leave them off and also ditch the smog pump for a few more HP, less weight and another boost in gas mileage.
I wonder what the trade off has been over the last 40 years of low emissions vs. millions of gallons of wasted fuel?
