What kind of gas mileage do you get with your 67-72? - Page 6 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Rich72C10 · 02-09-2021, 09:22 AM

The more I think of it, I didn't go to actual MAX, since I stopped at 2.5k. It is not that the canister is completely trashed/non-operational - it is just does not keep a vacuum when I applied my pump.

As for my cruising speed. 2.5 is about 60-ish, then 70 a bit below 30k if I recall correctly. I really don't see the point trying to cruise to 75~80. 95% of all my driving has been around town - which around here there are lots of 60 MPH roads (non-interstate). To me, it is a lot more fun going the "back way" than get on I35 in my truck.

So it seems time for some more flashy light and perhaps a friend to work the peddle for me

Rich72C10 · 02-09-2021, 10:14 AM

I feed I have eaten way to many replies in this tread and have gone off topic way to much.

I have created my own thread, Timing & Gas Milage.

Hopefully you guys that have helped me so far will pivot there as I update the thread :-)

Rich72C10 · 02-27-2021, 05:03 PM

Quote:

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.

Okay, after changing out to New Pertronix Flame-Thrower II, New Flame-Thrower Coil & Wires with 12° initial timing, ~900 RPM idle (Park) and ~640 RPM idle (Drive). Oh and I am using manifold vacuum advance, not ported.

60 Mile test drive, great mixed city and highway (Tx Farm Roads up to 70 MPH).

15 MPG!

SBC 350/TH350 and 3.07 rearend.

Rich72C10 · 04-12-2021, 03:49 PM

update.

So after using over a half tank (well by the gauge) and adjusting the idle up for what will likely be summer "AC Idle" @500 in Drive - I am getting 13 MPG. This is still 80% if not more highway (re: Tx Farm Roads) driving. My 1st outing with the AC was not so happy, I had to keep putting her in neutral, so I upped it. I'd say it's 700~750 in neutral now. This half tank also wasn't me trying to get the best possible MPG. I wasn't dogging it all the time but I was far from being a grandpa too. Nothing like going WOT to beat a few yellow lights or passing some slow poke ;-)

I guess that is good?

PS: Last night on our drive I actually ended up being behind a 68 or so C10 on the Devil's Backbone Scenic Drive. He was very low to the ground and was either a dark blue or perhaps even purple - I couldn't tell as it was dark. Though I could tell he had the same LED backlights as me! I was really hoping he was going to stop at the bar or the rest stop but turned on a secondary road. I thought it would be weird if I had followed him to a house LOL

**1970cstblazer** · 04-12-2021, 06:46 PM

The 350 4 speed Blazer would avg. 15MPG. The 402 TH400 4.10 truck gets 12 MPG no matter if empty or loaded.

DT1 · 04-13-2021, 05:53 PM

My 71 has a bone stock 350/350/Quad. On flat land at 60 MPH It gets 10 MPG.
On a downhill at 60 MPH it gets 10 MPG.
Against the wind at 60 MPH it gets 10 MPG.

My 72 is also a bone stock 350/350/Quad. Same thi Name: Finished C-10 005.jpg
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Name: Finished C-10 005.jpg
Views: 261
Size: 58.0 KB

ng.

**Accelo** · 04-13-2021, 10:17 PM

When fuel mileage doens't change depending on load or speed it is indicative of under gearing.
Exactly why a modern overdrive transmission works so well in these trucks.
I was able to get a 30% improvement with a .70 overdrive ratio in a 700R4/4L60. The 6L80 have even higher ratios with double overdrive, both 5 and 6 are overdrives, with the final ratio in Sixth is 0.67.

The wide overall ratio spread of 6.04:1 allows a “steep” first gear, as well as a very “tall” overdrive top gear for low-rpm highway cruising.

The trend is for higher differential ratios and lower first gear. This tread is nothing new and has progressed since the power-glide days.

Also a another reason to switch to a 4L60. The lower first gear if good for performance.

When in doubt "switch it out".

Rick

02-09-2021, 09:22 AM	#126
Rich72C10 Mr. Cheyenne Join Date: Jun 2020 Location: Kyle, Texas Posts: 1,687	Re: What kind of gas mileage do you get with your 67-72? The more I think of it, I didn't go to actual MAX, since I stopped at 2.5k. It is not that the canister is completely trashed/non-operational - it is just does not keep a vacuum when I applied my pump. As for my cruising speed. 2.5 is about 60-ish, then 70 a bit below 30k if I recall correctly. I really don't see the point trying to cruise to 75~80. 95% of all my driving has been around town - which around here there are lots of 60 MPH roads (non-interstate). To me, it is a lot more fun going the "back way" than get on I35 in my truck. So it seems time for some more flashy light and perhaps a friend to work the peddle for me __________________ Richard *'72 Chevy C10 Cheyenne Super WHT/MED BRONZE/WHT SBC 350 \| TH350 \| LWB \| AC \| Buddy Seats Upgrades:** Tilt \| Front Sway Bar \| Radio \| Sliding Window \| Bed Spare w/Lock Bed Rails \| Full LEDs \| Courtesy Lights \| Tachometer \| Glovebox Lock & Light \| Head Light Relays \| *Upgraded Fully Super Trim

02-09-2021, 10:14 AM	#127
Rich72C10 Mr. Cheyenne Join Date: Jun 2020 Location: Kyle, Texas Posts: 1,687	Re: What kind of gas mileage do you get with your 67-72? I feed I have eaten way to many replies in this tread and have gone off topic way to much. I have created my own thread, Timing & Gas Milage. Hopefully you guys that have helped me so far will pivot there as I update the thread :-) __________________ Richard *'72 Chevy C10 Cheyenne Super WHT/MED BRONZE/WHT SBC 350 \| TH350 \| LWB \| AC \| Buddy Seats Upgrades:** Tilt \| Front Sway Bar \| Radio \| Sliding Window \| Bed Spare w/Lock Bed Rails \| Full LEDs \| Courtesy Lights \| Tachometer \| Glovebox Lock & Light \| Head Light Relays \| *Upgraded Fully Super Trim

04-12-2021, 03:49 PM	#129
Rich72C10 Mr. Cheyenne Join Date: Jun 2020 Location: Kyle, Texas Posts: 1,687	Re: What kind of gas mileage do you get with your 67-72? update. So after using over a half tank (well by the gauge) and adjusting the idle up for what will likely be summer "AC Idle" @500 in Drive - I am getting 13 MPG. This is still 80% if not more highway (re: Tx Farm Roads) driving. My 1st outing with the AC was not so happy, I had to keep putting her in neutral, so I upped it. I'd say it's 700~750 in neutral now. This half tank also wasn't me trying to get the best possible MPG. I wasn't dogging it all the time but I was far from being a grandpa too. Nothing like going WOT to beat a few yellow lights or passing some slow poke ;-) I guess that is good? PS: Last night on our drive I actually ended up being behind a 68 or so C10 on the Devil's Backbone Scenic Drive. He was very low to the ground and was either a dark blue or perhaps even purple - I couldn't tell as it was dark. Though I could tell he had the same LED backlights as me! I was really hoping he was going to stop at the bar or the rest stop but turned on a secondary road. I thought it would be weird if I had followed him to a house LOL __________________ Richard *'72 Chevy C10 Cheyenne Super WHT/MED BRONZE/WHT SBC 350 \| TH350 \| LWB \| AC \| Buddy Seats Upgrades:** Tilt \| Front Sway Bar \| Radio \| Sliding Window \| Bed Spare w/Lock Bed Rails \| Full LEDs \| Courtesy Lights \| Tachometer \| Glovebox Lock & Light \| Head Light Relays \| *Upgraded Fully Super Trim

04-12-2021, 06:46 PM	#130
1970cstblazer All stock and staying that way Join Date: Jul 2003 Location: Elkland, PA Posts: 2,076	Re: What kind of gas mileage do you get with your 67-72? The 350 4 speed Blazer would avg. 15MPG. The 402 TH400 4.10 truck gets 12 MPG no matter if empty or loaded. __________________ 1970 K/5 Blazer CST 4WD, TOTM winner December 2024! Medium Bronze poly, 93k ACT. miles, 350, 4 speed, rear positraction, 16.5" x 8.25" HD Kelsey Hayes wheel option, tilt, tach, vacuum, AM/FM, manual throttle...Dad ordered and purchased new 4/70. Currently frame off restoration finally getting close to completion.. 1970 K20, To be a clone of my Grandfather's K20, Dark Blue poly, 350, 4 speed, 16.5" X 8.25" Kelsey Hayes wheel option, tilt, tach, vacuum, AM radio. Latest project truck 1972 Cheyenne Super 20 2WD, Dark Blue/White, 90K ACT. miles, 402, TH400, 4.10 open, tilt, tach, vacuum, A/C, AM/FM, manual throttle.. A mostly original paint never rusted Texas survivor... 2017 Sierra 1500 SLT 4WD, Black, 46k miles, 5.3, 6L80E, 3.42 LS, 20" polished wheels, everything but moonroof and 6.2... 2019 Canyon SLT 4WD, White, 67k miles, 3.6, 8L45, 3.23 LS, 18" wheels

04-13-2021, 05:53 PM	#131
DT1 Registered User Join Date: Oct 2009 Location: Gardnerville Nv No Mo Cali!! Posts: 869	Re: What kind of gas mileage do you get with your 67-72? My 71 has a bone stock 350/350/Quad. On flat land at 60 MPH It gets 10 MPG. On a downhill at 60 MPH it gets 10 MPG. Against the wind at 60 MPH it gets 10 MPG. My 72 is also a bone stock 350/350/Quad. Same thing. Attached Images

04-13-2021, 10:17 PM	#132
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,356	Re: What kind of gas mileage do you get with your 67-72? When fuel mileage doens't change depending on load or speed it is indicative of under gearing. Exactly why a modern overdrive transmission works so well in these trucks. I was able to get a 30% improvement with a .70 overdrive ratio in a 700R4/4L60. The 6L80 have even higher ratios with double overdrive, both 5 and 6 are overdrives, with the final ratio in Sixth is 0.67. The wide overall ratio spread of 6.04:1 allows a “steep” first gear, as well as a very “tall” overdrive top gear for low-rpm highway cruising. The trend is for higher differential ratios and lower first gear. This tread is nothing new and has progressed since the power-glide days. Also a another reason to switch to a 4L60. The lower first gear if good for performance. When in doubt "switch it out". Rick



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