Hey fellow Big Block owners, :banghead:

Whats the best things to replace to improve fuel mileage Carb,intake , adding headders, flowmasters, . Any and all suggestions are welcome "shamu " is getting 5.6 miles to a gal. now and it needs to improve. 10 to 12 is my target love the power but my pocket book is suffering .

Best thing to replace for better gas mileage? The Suburban.

Always drive down hill with the engine turned off and the trans in neutral. :lol2:

Sorry couldn't help myself

The problem here is the weight of the vehicle. At nearly 5500 pounds, it is a heavy vehicle. It is possible to change the character of the engine by installing a different cam, but I would need to know some particulars. The idea would be to optimize the cam for the engine speed most frequently seen on the road. It sounds like you do considerably more stop and go than steady cruising. You might try keeping a steady foot, attempting to anticipate lights, other traffic and accelerating more slowly, allowing the vehicle to accelerate toi the speed you desire, rather than accelerating quickly and backing off. The carb is also capable of a different calibration, allowing slightly leaner mixtures, however, this is only in moderation since the unit is already lean for emission purposes. Frequently, the distributor is the culprit; if the vacuum advance is defective, or the centrifugal advance is sticking, low mileage is the result. Checking and adjusting the ignition timing is also advisable. Setting it up to 4 deg above the factory setting may benefit you here, but be careful of detonation, especially if you cannot hear it. BEFORE messing with anything on the carb, carefully check the distributor advance systems, (there are two) for problems. It is myopic to simply blame the carb for the problem. I can advise here if necessary. Ray

I had a similar problem with my 79 454 4x4. I modified quite a few things and managed 12 mpg on the highway. Here's a list of my mods and there value:
Crane mileage master cam, 7
New intake, can't remember brand, low end torque, 5
Rhoads type lifters, ?
headers, 9
Jacobs ignition, 10
custom exhaust, dual, 8
roller rockers, 5
new holley 750, vac sec, 7
many others,...
The important thing to consider is that you have a high torque engine, use that to you advantage and run it at low revs. I strongly considered an aftermarket overdrive add-on transmission for the highway but ended up selling the truck because I couldn't keep the body together. If yours is in good shape I'd concentrate on intake and exhaust as well as the Jacobs ignition kit. Don't worry about the weight, 12 mpg is attainable!!
Good Luck.

Being a '70s vehicle, you can do a lot more than you can with a newer one. Headers & duals would be the first thing I'd change. Not sure about the emissions laws in TX for older trucks, but if you MUST keep the CatConvt, get two Hi-flow ones from Walker or Perfection in 3" dia, then reduce down to 2.5" decent-flow mufflers.
An open breather (or flip the lid) will help. They make smaller-ratio accessory pulley sets. Those & an electric fan, will take some drag of the engine. A multi-spark ign system like Jacobs or Mallory will make sure you burn every drop of fuel used. Bug deflectors kill highway milage, but
those upper rack spoilers that force air behind the tailgate seem to help.

Do you have oversize tires? They kill milage.
Not sure if '78 still had the 3sp TH400 trans, but if it does & you ever need to rebiuld, you should seriously consider a swap to a newer Auto-overdrive unit. I assume it's 4WD - is the frt axle live full time, or can you
install lockouts?

Also try synth oil & a teflon additive like Prolong. I used it on a Caddy 425 & got 2 more city MPG. Good luck..