14.5 MPG 350/700r/3.73's at 70 to 75 MPH - Should I be happy with that?

[stomper]

Did a 160 mi trip this weekend over a mountain pass and into Eastern Washington. AC on a little while. Do I have room for improvement or should I be happy with 14.5 MPG.

Details

'71 short fleet, lowered 4/7.5. Only payload was; me, the dog, a cooler, and emergency tools.

Stockish unknown miles '69 350 (.030 over) 140-150 psi compression.
Rebuilt Q-jet, Edelbrock SP2P intake manifold, tall open element air cleaner.
2" Ramhorns and 2-1/2" exhaust.
15 in vacuum on stock gauge at smooth idle.
Running 2200 - 2500 rpm at 75.
All new ignition. Summit HEI with one lighter advance spring - 14 degrees base timing. Vacuum advance can left as delivered from Summit.
Tune seems good (snappy and doesn't ping) but I feel the grunt falls off above 60-65 MPH.

Swap meet 700r. Lock-up doesn't want to stay locked-up after a few miles on the highway and rarely above 60 mph. I have a new solenoid to go in but that's another story.

Craigslist 3.73 posi - counted the teeth to confirm ratio.

40 series 20" tires - 29" tall. New and aligned. Didn't check air pressure.

Speed GPS confirmed.

87 octane 7-11 gas.

After reading some posts I'm thinking a couple more MPG could be expected. What do you think?

[CC69Rat]

Naa .. I'd be happy with that.

10% Ethanol ?

Not sure if you (or others) might benifit but:

1GL Ethanol = 12,000 btu
1GL Gasoline = 14,000 btu

So, if it was the crapanol and 87 I would say you're about right. These things are not the most aerodynamic trucks on the planet. 3:55 would probably help

Does it have a cam in it now or just stock through the valve train, etc. ? Do you know which heads are on it? 041 ? just thinking if it has been bored, it has probably been gone through at some point.

[notsolo]

No expert here, but that sounds pretty good if you were going 70-75 mph even part of the way. At 55 you might get 17 or so. With newer set up 5.4 L and 4L60 E you might get 17 going 70. Is your avatar the way the truck looks now...Pretty sweet....More photo's ..Please.

[VA72C10]

The lockup might net you a little better result on mpg's. Also you were going over a pass you said so lots of downshifting to get over inclines id assume...

I was able to average 17-18mpg in an 80z28 with a 355 and 700r4 with 373's and similar size tires to what most run on these trucks....but that camaro obviously was more aerodynamic.....and when I first had it I spent more time playing with the throttle and only was getting 15mpg (sometimes a lot less than that ). Plus I found over time I got a couple extra mpg when going 60-65 vs 70-75....
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[CC69Rat]

Yeah, .. I think with the 3:73, slow 'er down a little and it should pick a mpg or two.

[VA72C10]

Seems every engine has a sweet spot for fuel economy and it can be at slightly different rpm's depending on the setup...


For example....I had a 2000 tdi beetle that got around 35 mpg in the city and 45 -50mpg on the highway....but I took trips from richmond to tidewater fairly often and experimented on cruise control speeds and if I set it on 65 exactly I could get upto 70mpg on a trip up and back. Anything over and it dropped substantially to 50 or lower.
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[GASoline71]

I wish I got 14.5 mpg! I's say that's pretty decent... and here in WA State... about 95% of all gas stations are E10.

Gary

[74 stepside]

Sounds good to me. Hell my 03 Durango with a small v8, fuel injection and overdrive only gets 15 mpg on the freeway. So a classic truck getting what you got is great.

[68Stepbed]

That's about right for E10 fuel. Try it again with non-ethanol 89 octane and you should get alot better. There's no reason to run any higher than 89 in that stock engine.

My old combo, which was a mild cammed 350/TH350/3.73 would average about 14mpg with 93 octane fuel. When I awitched to the 3.07 gear, I could get about 16mpg. Of course, that's with non-ethanol 93 fuel.

[stomper]

Originally Posted by CC69Rat

10% Ethanol ?

Does it have a cam in it now or just stock through the valve train, etc. ? Do you know which heads are on it? 041 ? just thinking if it has been bored, it has probably been gone through at some point.

Wow thanks for all the quick replys guys.

I didn't pay attention to the ethanol question at the pump but I will next time!

Cam is unknown but it seems relatively stock. Valvetrain components are all stock stamped steel rockers etc.

Heads I forget the number? I recall when I looked it up they're nothing special. 76cc with 1.94/1.50 valves. I just had them off and apart for head gaskets and stem seals and everything looked ok and not messed with. Exhaust had hardened seats though so if they were didn't come with that they have been rebuilt at some point.

Think the SP2P intake is helping me or hurting me?

[stomper]

Originally Posted by notsolo

Is your avatar the way the truck looks now...Pretty sweet....More photo's ..Please.

Thanks! Yea that's the truck. The low resolution avatar makes it look better!

I started a build thread but since I got it driving and posted some pix of my 20's I have been having so much fun driving and working out the bugs I haven't posted much.

There's a few pix in this thread: http://67-72chevytrucks.com/vboard/s...=518829&page=3

[CC69Rat]

SP2P meaning Single Plane / Dual Plane ? Which intake is it?

My GMC with a basically stock 350, cast intake and a 1406 Eddy. 3OTT, Nervously driving it home at about 60 mph in downtown Atlanta in the pouring rain got about 21-22mpg. I was shocked.

Yes, 89 octane no ethanol will bump it up to 16+ I would bet. So the combination of slowing down a little bit (not saying Papaw slow) and maybe go to a 45 series tire out back (?) 1" more dia in the tire = more distance traveled per rotation of the tire = 3:60 or so rear ratio ..

what type of plugs and wires are you running?

[stomper]

Originally Posted by CC69Rat

SP2P meaning Single Plane / Dual Plane ? Which intake is it?

what type of plugs and wires are you running?

SP2P is a gas crunch time intake like this one: http://67-72chevytrucks.com/vboard/s...d.php?t=517472 Dual plane, small runners, designed for torque i.e. economy.

Plugs are Autolite cheapies (#45 #25? I forget the #?). gapped to .045. (I know....) Honestly I had AC's in but half of them had cracked porcelain and I was p.o.'d and didn't buy another set. Wires are new pre-cut 7mm from ORilleys.

[brianthelion02]

My 71 was getting about 12 at those speeds,totally stock with 3.08 gears.Try it again at 65 mph,I think you will be surprised.
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[CC69Rat]

I'd think the intake would be fine.
HEI plug gap is dead on IMO. .042 - .045

It may have been the fact you're going across a mtn pass at 70+ mph.. maybe?

[WIDESIDE72]

14.5 mpg sounds about right. What rpm's were you turning( I'm curious because I have almost the exact same set up in a 72 swb and i am curious what I am in for. I was figuring about 15 mpg would be optimistic. I am running and edelbrock performer and a q jet, unknown "rv" cam, 700r4, 3:73's. I actually pulled a 3:07 rear out because its not going to be a daily driver and I want to be able to snap my friends neck a little when I take them for joy rides!
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[Wrenchbender Ret]

My 70 steper with Goodwrench 350, 700r4, & Q-jet gets 16-18 on the highway. It has a 3.08 rear end. I think you would be better off with a 3.08 if you do a lot of highway driving with light loads. You can always pull it down a notch when it gets under heavy loads. You can still "snap your friends neck" with the low 1st gear in a 700R4.

[67ChevyRedneck]

I have a 350/edelbrock/700R/3.73. Going 70-75mph I get about what you got, 15mpg. If I slow it down to 55-60 I can push it to 16, maybe a hair more.

Around town is 10-12mpg.

You definitely want to get that lock up taken care of, it's real easy to burn up a 700R. I know from experience...

I installed the kits from Bowtie Overdrives. I used their TV cable kit. As long as you have the correct bracket I think as long as the cable is set up correctly you should be OK with the q-jet. I checked my shift pressures with a cheap gauge I bought off ebay to make sure it everything was good.

I also used the BTO lock up kit. It comes with a brake switch that unlocks the TC when you hit the brakes. This is also very important. I had a kit before my BTO kit that only locked up the TC. I had a dumbass pull out in front of me and had to lock it down, and in doing so, the truck stalled because the TC did not unlock. I missed the idiot but the truck shut off on me.

[WIDESIDE72]

Originally Posted by Wrenchbender Ret

My 70 steper with Goodwrench 350, 700r4, & Q-jet gets 16-18 on the highway. It has a 3.08 rear end. I think you would be better off with a 3.08 if you do a lot of highway driving with light loads. You can always pull it down a notch when it gets under heavy loads. You can still "snap your friends neck" with the low 1st gear in a 700R4.

Mine is not a daily driver, and 3.73's still accelerate harder, no matter what!

[lolo33]

this post is very interesting for me.

my 350 is equipped with edelbrock performance heads - manifold edelbrock RPM and Holley carburetor - automatic transmission TH350 3 gears. the problem is, that the engine runs 2500trs/mn for a speed of 55 miles !!! that's very short.
the consumtion is very high with... 28l for 100km (less that 9 miles per gallon).
so I would like to change :

1/ Carburator to an Edelbrock
2/ final transmission ?

can you explain me what is the best to get 14.5miles/gallon ??? It's difficult in france because we have to import from US so i need to order the right parts you know
many thanks
Laurent

[cdowns]

i always got 15-16 at hiway speeds with very heavy loads goin long distances to swapmeet and fleamarkets at hiway speeds of 80 and higher// i had 4:10's a 2004R/350 combo

all synthetic lubrication engine tranny rear and axel bearings help ALOT

[WIDESIDE72]

Originally Posted by lolo33

this post is very interesting for me.

my 350 is equipped with edelbrock performance heads - manifold edelbrock RPM and Holley carburetor - automatic transmission TH350 3 gears. the problem is, that the engine runs 2500trs/mn for a speed of 55 miles !!! that's very short.
the consumtion is very high with... 28l for 100km (less that 9 miles per gallon).
so I would like to change :

1/ Carburator to an Edelbrock
2/ final transmission ?

IMO, the best approach in your situation would be either a 700r4 or 200 4r package from an aftermarket vendor. It's less hassle than trying to piece something together since you aren't in the US. I have used both in various GM vehicles and actually prefer the 200 4r since it doesnt have as low of a first gear bad is qn easier swap for a TH 350. Doe some research on this forum and the web in general and you should have no trouble deciding. Your Wallet will love you for the fuel savings as well as your engine for the lower rpm's! It will be like having a while different truck. Most people say it is the best single improvement that they made.

can you explain me what is the best to get 14.5miles/gallon ??? It's difficult in france because we have to import from US so i need to order the right parts you know
many thanks
Laurent

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[67ChevyRedneck]

Originally Posted by lolo33

this post is very interesting for me.

my 350 is equipped with edelbrock performance heads - manifold edelbrock RPM and Holley carburetor - automatic transmission TH350 3 gears. the problem is, that the engine runs 2500trs/mn for a speed of 55 miles !!! that's very short.
the consumtion is very high with... 28l for 100km (less that 9 miles per gallon).
so I would like to change :

1/ Carburator to an Edelbrock
2/ final transmission ?

can you explain me what is the best to get 14.5miles/gallon ??? It's difficult in france because we have to import from US so i need to order the right parts you know many thanks
Laurent

If that's the case, your best bet would be to buy a COMPLETE kit from a reputable company like bowtie overdrives. Get the TV cable kit, Lock up kit, 700R or 200R and torque converter, and you can even get a properly shortened driveshaft. However, this route will not be cheap, at all. I was able to do my 700R swap for about $1,200 for all the parts and a rebuilt 700R, but I used junkyard parts and have easy availability to get parts.

[Daves70]

67 short step, 400 smallblock with AFR 195 heads,RPM intake, 780 holley and .512 lift cam.I have a 700R4 tranny with lockup on a switch and 3.42 gears. I get around 12 mpg in the city and recently on a long trip I averaged 17.3 mpg.I think it would do a little better with a smaller carb.So these trucks can get OK mileage.A new truck doesn't do much better and won't even come close to the power.I'm building a daily driver 70 longbox now and I'm hoping for 18 mpg with a 350.

[nightmare]

I've got a '69 longbox with a 350/700R4/3.73 posi, with 28" tires, and get about 14mpg. I think that I might get a little more with a smaller carb, something like a 600CFM. I've got a 750CFM Edelbrock Q-Jet on there now, but it needs new brass bushings which I can't find. I've already rebuilt it with the normal kit, but it needs more.