What kind of gas mileage do you get with your 67-72?

[HO455]

Originally Posted by Lancialonnie

This is a frustrating topic for me! I had read the many posts from people getting 15-17 mpg and that was the range I wanted to be in so I could daily drive my 72 fleetside longbed.
I bought a new Blueprint Engines 383, 430 h.p. and had an early 700r4 non-electronic lock-up rebuilt and upgraded. I have 18's and haven't verified but it should be the original 3.07 gearing. Got everything installed with FITech supplying the fuel and started putting on some miles.

First oil change was at just over 600 miles and I've been logging every tank. Mostly around town with some country driving at 60-65 and I was getting just over 7 mpg! Very disappointing to say the least on mileage but wow is it fun to drive! Smooth, great acceleration, sounds great, no issues with performance. H

I purchased a complete MSD ignition system (distro, 6AL, coil) and had an X pipe installed and starting using Drive instead of OD in town. Now I am just over 9 mpg and I did a couple freeway trips at just over 12 mpg. I don't know of anything else to do to get the mileage up.

Don't get me wrong, the truck runs fantastic and it is amazing on the freeway now. With the tired 307 and no OD it was screaming loud at 65 now at 85 it is quiet and just cruises along comfortably.

I've never driven an LS motor so I don't know if I would have been as happy with that option but I'm sure it would have given better gas mileage?

You didn't mention how tall of a tire your running. But conservatively guessing a 27 inch tire you likely are running 14-1500 rpm at 55. I would bet the cam and head combination is not very efficient at that low of an rpm. You might try doing a test run where you don't use overdrive and see if you can get better mileage. The 3.07's with the OD and taller tires are likely dropping the rpm too much and the flow rate on the intake side is too low for efficiency.

[MidLifer]

A Gen III or later Vortec will give better mileage than a carbureted or TBI engine. 5.3's as stated above should do 20 on the highway. My 6.0 with 4.10 posi rear and 16" wheels with tall tires gives about 16, and my foot is kind of heavy on it. I just love the off-the-line grunt of the LQ4. If I was lighter on the foot I might squeeze another few mpg out.

[Sparky dave]

This 68 on ebay is meant to get 40 uk mpg's
https://rover.ebay.com/rover/0/0/0?m...2F293993455423

[Accelo]

369 with THM 400. 10 on the highway. 3/4T with 4:11's.
Same motor with a 700R4 mileage went to 13 on the highway. 30% improvement.
Would only do 95 in overdrive. Any more throttle and it would kick down into 3rd.
Missing one passenger mirror after the experiment. I know the transmission could have been modified to pull full throttle in overdrive. I only attempted this once and as such I didn't care.

Didn't change much in town. Just crappy mileage like always.
Rick

[ChevyRacefan]

71 vortec 350 with ramjet cam and 1.6 rockers , 2004r 3.08 rear gears 26" tall tires.
15 in town if I keep my foot out of it.
19-20 highway at 75-85mph
Should do better once I lower it and add the belly pans and front air dam. and close up much of the front grill opening .
The step bumper hanging down there is like a big parachute,

[Rich72C10]

Stock 350/350 combo (mild RV Cam), 4BBL Qjet, 3.07 rearend with 275 /60 R15 - I haven't seen 10mpg yet, one time I got close.

Though I barely have over 800 miles on the rebuild and I do have a Qjet sent off for rebuilding and tuning. I also have set in the garage / driveway with a lot of idling or very short trips for test runs and not going easy on the gas pedal. Only a few weeks ago did I figure out I needed to adjust the kick down cable for the TH350. Before that, WOT did not much of anything.

[ChevyRacefan]

Originally Posted by Rich72C10

Stock 350/350 combo (mild RV Cam), 4BBL Qjet, 3.07 rearend with 275 /60 R15 - I haven't seen 10mpg yet, one time I got close.

Though I barely have over 800 miles on the rebuild and I do have a Qjet sent off for rebuilding and tuning. I also have set in the garage / driveway with a lot of idling or very short trips for test runs and not going easy on the gas pedal. Only a few weeks ago did I figure out I needed to adjust the kick down cable for the TH350. Before that, WOT did not much of anything.

You might want to look at the timing curve, and vacuum advance.
under 10 is big block numbers. my 402 did better than that around town.
Something is very wrong. These trucks are not heavy.

[Rich72C10]

well, that makes me feel good - very & wrong - two words I really didn't want to hear/read about my truck :'(

[Rich72C10]

I wanted to wait a bit before filling up again but before my Sunday drive to Cars & Coffee I filled up. With my Sunday trip and after little drives to the stores I got 11.3 MPG - I used about 1/4 tank yesterday from sun up to sun down.. All my other MPG checks i had been doing all manner of odd MPG testing/driving I was doing. For the longest I was trying to figure out why I never had any power when I punched it (then finally figured out the kick down adjustment for a TH350).

I'd say 1/3 of driving was 50~60 MPH (stuck behind flat bed truck hauling a zillon bags of concrete), then my return trip 1/3 60~75 MPH, then the final 1/3 tooting around town for various stops to stores in town.

[i82much]

if you have a holley, i understand reducing the main jet size and increasing the PVCR to compensate can really lean out high speed cruise AFR. Lower speed cruise is tougher because you have more idle circuit in play and there is a lot going on between the ifrs and iabs. But i bet most guys with a holley could pick up a mpg or 2 just by playing with the pvcrs and main jets.

[ChevyRacefan]

Originally Posted by Rich72C10

I wanted to wait a bit before filling up again but before my Sunday drive to Cars & Coffee I filled up. With my Sunday trip and after little drives to the stores I got 11.3 MPG - I used about 1/4 tank yesterday from sun up to sun down.. All my other MPG checks i had been doing all manner of odd MPG testing/driving I was doing. For the longest I was trying to figure out why I never had any power when I punched it (then finally figured out the kick down adjustment for a TH350).

I'd say 1/3 of driving was 50~60 MPH (stuck behind flat bed truck hauling a zillon bags of concrete), then my return trip 1/3 60~75 MPH, then the final 1/3 tooting around town for various stops to stores in town.

I would look at the timing curve. and vacuum advance.
At what rpm is the mechanical advance "All in"

[Rich72C10]

I would have to figure out how to do all of that ;-) I'll be researching - more fun for me.

[Rich72C10]

My 1st test with my hand pump to the Distributor JM6701 Vacuum Advance Canister, I pump it up to 10 and it drops to zero in about 25 seconds. I assume that is not good and the Canister should hold vacuum?

[Steeveedee]

Does your brother have a variable timing light? He should be able to determine the total advance, if so, and could check the vacuum advance as well. I get 10 MPG in combined driving, mostly city. 400 engine, 400 trans, 3.54 rear gear and a big camper shell with a bunch of stuff packed into it.

[Rich72C10]

My brother is 5 hours away from me and the truck - he has passed it on to me :-) However, I got myself a Equus 5568 Digital timing light sometime back. I only used it to insure my timing hadn't moved and to set my idle. I just called him about the Vacuum Advance Canister not holding vacuum and he said, ya you need to fix that pronto..... no to figure out what the kind of a Vacuum Advance Canister replacement this Distributor takes (it was bought/installed by the previous owner).

Originally Posted by i82much

if you have a holley, i understand reducing the main jet size and increasing the PVCR to compensate can really lean out high speed cruise AFR. Lower speed cruise is tougher because you have more idle circuit in play and there is a lot going on between the ifrs and iabs. But i bet most guys with a holley could pick up a mpg or 2 just by playing with the pvcrs and main jets.

@i82much - I have a 78 Truck Qjet on now, installed by the previous owner. I have a 72 C10 QJet out for complete rebuild & performance turning. But months away before it is returned due to backlog at https://quadrajetpower.com/

Originally Posted by Steeveedee

Does your brother have a variable timing light? He should be able to determine the total advance, if so, and could check the vacuum advance as well. I get 10 MPG in combined driving, mostly city. 400 engine, 400 trans, 3.54 rear gear and a big camper shell with a bunch of stuff packed into it.

I see how to do total timing now after watching a YouTube - that is easy enough to do. I want to say my timing is set to 12, though for the life of me I cannot remember if I am remembering the number correct at the moment. I think I need to sort out that that leaky Vacuum Advance Canister but I know that does not come into play for when you are timing with the light, since you disconnect the Vacuum Advance Canister during timing.

[Steeveedee]

There are a lot of different vacuum advance cans. They have different vacuum levels for actuation and different amounts of advance offered. I don't know how to steer you to the exact right one. On points ignition I've always used the one that gives the most vacuum advance, which in the Standard Ignition parts book was a VC-24A, IIRC. For HEI, I'd need to investigate. All I can tell you is that I have a Mallory HEI and don't know max advance or what the vacuum advance pulls in, either. I'll have to get one of those variable timing lights myself, as mine doesn't have that feature.

Here's a link. I'm going to read it myself.

https://www.crankshaftcoalition.com/...ance_Specs.pdf

[72c20customcamper]

I get 12 highway or around town . But then I do 70+ with 4:11 gears 265/75r16s

[Rich72C10]

Originally Posted by Steeveedee

There are a lot of different vacuum advance cans. They have different vacuum levels for actuation and different amounts of advance offered. I don't know how to steer you to the exact right one. On points ignition I've always used the one that gives the most vacuum advance, which in the Standard Ignition parts book was a VC-24A, IIRC. For HEI, I'd need to investigate. All I can tell you is that I have a Mallory HEI and don't know max advance or what the vacuum advance pulls in, either. I'll have to get one of those variable timing lights myself, as mine doesn't have that feature.

Here's a link. I'm going to read it myself.

https://www.crankshaftcoalition.com/...ance_Specs.pdf

I called the manufacturer a bit ago, it appears to be some propriety china canister. They are nice enough, I pay shipping and he sends me a new can. He warned me it is not easy to replace. Lovely... So I have added to my "need to buy list" a PerTronix Ignitor III D7100700 with their coil. Their canister looks to be a more common canister. I like the looks of it to, similar to mine with the female connectors for the spark wires.

Okay, I am set at 12 degrees and I am getting 17hg vacuum after she is good and warmed up off the manifold (I removed the A/C vacuum reservoir line that is tied to the tree and connect my Bosch vacuum gauge there).

Connect up the vacuum up to the distributor canister, I rev up and hold at 2.5k. At this RPM I am getting around 40 degrees. It is as steady as I can keep my hand on the hot throttle and not drop my light, it stays steady there.

Not sure if I did that right or those are good/bad numbers...

[Rich72C10]

Rats, at mechanical (no vacuum on Distributor) I am at 23° at 2.5K RPMs

[Steeveedee]

OK, either you have too-strong springs on the advance weights, or the weights are stopping at the limits. The limits and/or weights can be modified. Is it an HEI? The VAC isn't that hard to remove and replace. Is it one of those electronic ignition distributors that look like stock?

[Rich72C10]

I am just repeating what George at topstreetperformance.com told me about the VAC. This unit is their low end HEI Ready-To-Run - there are no adjustment to this thing. Below is all they list, you want to change some things, you need to go pro.

But to be honest, I am looking else were and even to a Progression Ignition

TSP’s Ready to Run distributor is designed for easy installation. With the included ignition module, all you'll need to complete your set-up is a coil. Using simple design elements throughout, it's a great way to get an easy-to-install ignition at a affordable price. With an aluminum housing, steel internal construction, and a high-quality ignition module, this distributor will get the job done.

• Compact design with clip-on cap
• On-board ignition control module
• Fits small block 262-400 and big block Mark IV 396-454

[custom10nut]

Ok, I’ll add a second response

Do you mean “Gas Smileage” ?

[Steeveedee]

OK, a whole new proprietary brand distributor. I ran a points distributor in my truck up until I put a Buick engine back in my '68 Skylark when the 305 SBC wore out. I then transplanted the Mallory HEI into my truck. I personally stay away from the aftermarket stuff that I've never heard of. I'm not going to be much help on that distributor, TBH.

[Rich72C10]

Ya, I am sewing on it - assuming that thing is actually good it is likely over kill for my stock engine. On the other end I am looking at a PerTronix Ignitor III D7100700 - far more reasonable and more out there in the wild.

[ChevyRacefan]

Originally Posted by Rich72C10

Rats, at mechanical (no vacuum on Distributor) I am at 23° at 2.5K RPMs

The dead vacuum advance can, and the 23* total , is a big part of your mpg issue.
What is the initial timing set at?
AND what is your cruise rpm. is it lower or higher than the all in (timing) rpm of 2500?