MPG question

[Mr. Machanic]

Yes, this is probably one of those stupid questions similar to "what's it worth".
So forgive me now...thank you.
I'm just looking for an approx. baseline.

About what would one expect for mpg.
78 Big10. 6200lbs. (6400lbs with me in it)
350ci with 350 auto.
Original QJ. HEI on rotor.
New plugs.
GPS shows 5 mph faster than speedo.
I don't have a tach, yet.

Currently getting 10 mpg on 87.

Thank you.
And Happy New Year to all.

[Mr Diesel]

10 is about right I think. 12 is about max with that setup. Everyone exaggerates their mpg...

[Mr. Machanic]

Thank you Mr. Diesel.

Honestly, I was hoping that 15mpg would be obtainable.
But I think that would need a different tranny and rear end.

[zieg85]

15 is achievable IMO.  Several factors have to line up.  Easy driving is the main thing.  Optimal tune is the other.  I find most issues are with inadequate timing due to advance not working the way it should. 
 

[Mr. Machanic]

Zieg85,
Thank you.
I will keep timing in mind.
I am making a list over winter of things to address when summer hits.

[Shifty]

With TBI, I can get 14 if I stick to speed limits, and don't have to many stop and goes, but reality is around 11-12.

[Mr. Machanic]

Shifty,
Thanks for the reply.

What's interesting is the 3 books I've read about the Q-jets, makes it sound like they were good on fuel economy. And they probably were when fuel wasn't corn injected.

And as a kid in my mid-teens, I remember dad had a 71 or 72 GMC surburban 454 with a tow package. I don't remember the tranny, other than auto and I don't know the carb. But it would vapor lock. We pulled a 31' Airstream Land Yacht. I remember him griping every time we filled up. And if memory serves, it was around 8 mpg, maybe 9. But that thing never even knew the trailer was back there. Regardless of the grades, we could maintain 70 with no problem. Pass semis like they were standing still. Hmmmm, maybe that explains 8mpg. LoL.
Then my grandma&grandpa liked the setup so much he bought a Chevy pickup with a 350 auto and pulled a 25' Airstream to Fla for the winter.
His gas milage was no better/worse but the truck struggled to pull the load. And semi passed him like he was setting still.
Both were the orange w/ white panels.
Ahh, the good ol' times.

[JohnnyPopper]

The Burb with a tow package 454/correct gear ratio is hard to beat when it comes to pulling a load. I had a 73 3+3 with that motor and pulled a 5th wheel toy box. Between 25-3500 rpm it was like I wasn't pulling anything.

Wonder if EGR comes into play with regards to gas mileage? I can't say I understand the concept fully but when I reviewed the functionality recently it struck me that mileage comes into play.

I think what was said about timing and vacuum advance is more interesting. The rest too, but if those are not optimized you're leaving money on the table, or in this case, out your tailpipe.

[Mr. Machanic]

Mine does not have an EGR valve.
PCV is as fancy as this one gets.

The Big 10 was built to "side step" the emission regulations that had just come out.

There is zero emission krap on this truck.
That was one of the reasons I bought it.
Only 4 vacuum lines. Advance, intake damper, choke pull off, brakes.

[Shifty]

A co-worker has one very similar.

[Mr. Machanic]

Shifty,
Might you be able to ask him if he knows his mileage?
And if it's higher that 12ish, has he modified i?
Just curious.
Thanks

[zieg85]

On my 85 C20 454 4 speed I use to get 15.6 mpg on a regular basis driving it easy.  Speed limits were 55 and I rarely exceeded it by 10%.  Pulling my car trailer it dropped to 12 so I know what a 350 is capable of.

[Shifty]

Shifty,
Might you be able to ask him if he knows his mileage?
And if it's higher that 12ish, has he modified i?
Just curious.
Thanks

In town, he gets about 12.  He's had a variety of engines in it, currently a nicely built 327 with a small cam, headers, intake, etc, and a Edelbrock 4bbl.

[JohnnyPopper]

So 5 days through the mountains, deserts, and multiple altitudes, I got 11 mpg using 91 octane. Usually single digits.

Will advance the timing and see what happens.

[Captkaos]

Q-jets are better on mileage because they have 2 small primaries that the vehicle can run on.
I would not expect more than 12-13mpg with a 350 and a TH350 trans with no overdrive and expect it to be less with higer numeric gearing and shorter tires

My 91 Crew Cab gots 13mpg in the city, 17mpg on the highway.  It is a 1 ton with a 350TBI and 4L80E.
My 87 R10 got 17mpg in the city, 22.5 on the highway.  It is a 1/2T with 305TBI/700R4.  When I LS swapped with 4L80E and twin turbos it now gets 13.5/18mpg

There is a thread on the board where people were posting they mileage: http://forum.73-87chevytrucks.com/smforum/index.php?topic=22414.0