MPG seems more like GPM

[425hp87chevy]

Hi, I am now working at an 85 k20 4x4 lwb. At the moment it has a 6'' suspension lift and 3'' body lift, 345/75/16 tires. Powerd by a 74 454 freshly rebuilt, 750cfm carb, headers straight pipes hooked to a sms465 np208 and a 1 ton 10.5'' ring gear rear diff with 4:10 ratio.
Amazing toy i will admitt but gas milage is making it a once a week cruzer. Id really like to be able to drive it and not cost me a furtune. Im not against changing anything on the truck. i thought about puttin a 500cfm carb on a swapping to 3:42 gears (being the lowest made for my rear diff) but i figure i wont gain enough mpg to be worth the change and i lose power besides. thought about cutting out the lift and changing rear diffs and high sped gears but i really like where the truck sits. Ive thought about a cummins swap or maybe a 5.3 with some boost or something. i just dont know what to do to get decent milage and still be able to turn the big tires nicely. So i guess im looking for suggestions on what would be my best set up and use as many of my components as possible. id love to keep the 454 but it is what it is.. Love to hear your ideas!! thanks

[Jason S]

On gasoline, my '73 gets about 10 mpg on the highway.

It is a very similar setup to yours, but with less lift and less cubic inches plus the drag of an automatic. I have propane on the truck now and it makes it a bit more economical to drive.  The HP is less, but I didn't have to invest a lot into the setup. I have the option of running propane or gasoline, and can easily remove the setup if I choose.

Compared to swapping in fuel injection or changing gears or adding an overdrive, propane is a real economical route. If you're mechanically inclined you can install a new propane setup for $1,000 to $1,500 (all new parts).  If you can find an older rig to get the parts from, you could wind up spending around $500 (including rebuild kits) or less. 

On propane, you'll burn more gallons but the cost per gallon would be quite a bit less. Essentially, it should allow the truck to be driven more for less money.  If your 454 has more squeeze (i.e., higher compression), you can tune your truck to have great horsepower and mileage.  This is in addition to the built in benefits of burning propane (no I'm not a propane dealer)...

Food for thought...

[bake74]

     With the 454 you are only going to get so much out of it no matter what you do.  If you are unhappy with the milage, something you might consider (depending on how much you want to invest compared to drive time ) is swapping out the 454 with either a fuel injected 350 or 454 and over drive trans.
     This will take time and money, but if you plan on driving your truck for the next 10/15 years, you will save money over the long run.
     The 74 I am building is getting a A833 o/d 4 speed trans I picked up just for that reason.  I wanted to be able to drive it and enjoy it.  Eventually I plan on putting a newer fuel injected motor in it too.  I also plan on keeping the truck untill I die, so I have long, long time plans for it.

[Lt.Del]

the taller 3.42 gears will not necessarily save you mpg if you drive a lot in an urban/semi urban area that calls for lots of traffic lights and stop signs.  If you mostly drive on highways and do 55mph+ it will save some over your 4.10 gears.

I have tall gears on my '79 383 (3.08 ratio) but do mostly semi urban driving, not interstate.  She will fly on the interstate and get reasonable mpg there, but city driving it really sucks the gas.  Thinking about a higher gear (ie: 3.73 or 4.10) to actually save gas believe it or not.

Perhaps the best you could do besides tuning on that big block is to lose those tall wheels, then you'd have to drop the lift some to make it look decent.  Some would say you could tune a 454 to get as good mpg's as a small block if you keep your foot out of it.  Never had a big block so i can't say.

Check all your vacuum lines to ensure you have no leak.  A vacuum leak would kill your mpg's.

[425hp87chevy]

Well the propane is not what im looking for, but thanks for the info about it. As for the taller gears, i drive in the 60 to 65 mph range most often so they should help. If also been looking into an EDELBROCK Flo-Pro 2 kit. If anyone has one of these setups id love to hear what you think of it!

[malibu795]

ezi efi imo is better buy the edelbrocks proflo 2. also exhaust. bbc love to breathe  imo min tube headers and dual 2.5 or 3" exhaust. that will wake it up alot.

[1MCP7.3/87GMC]

Fuel injection will help improve gas milage in your 454 MSD Ignition makes a system called Atomic EFI. It's a really good system it's designed like a carb so you don't have to change out your intake assuming your running a square bore 4 barrel. It's easy to install and minimal wiring it's self tuning along with self diagnosing it's actually a cool system. It's one of the cheaper systems but a good system despite the price it's around 2k. And I know it will run more efficiently but does require you to install an o2 sensor. But defiantly will pay for itself.

[Skunksmash]

A 454 4x4 with a lift and tall tires that gets gas mileage that is affordable. That'll be the day...

[Stewart G Griffin]

After all those modifications, what are the verifications that the speedo----and thus odometer are correct?


i think the first step is to actually determine what the mpg is accurately.

[tpull123]

Well I have done the Cummins swap, and also have 6" lift and 4.10 gears, so as soon as I find this vibration and get a few more miles on it,  I will let you know how economical it is.  Unfortunately i only have 33" tires at the moment, I planned my gearing around a 35" tire.

It better be darn amazing $4.25/gal diesel price!!  F@#K

The noise is ridiculous too....I did quite a bit of sound deadening and apparently have more to do.

[VileZambonie]

What carburetor do you have on it? Is it tuned? What about the ignition system? Tell us about the engine internals. Have you considered dumping the granny tranny?

[roadrage41]

The pro Flo system is the most archaic system on the market. Literally a toss up between it and a GM factory TBI system. It was designed in the early 90s and there haven't been many updates. For the money it costs they are a real pain in the rear.