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#1 |
Registered User
Join Date: Aug 2009
Location: Texas
Posts: 58
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Here is the thing I am currently faced with:
My truck is a 71 sporting a 350 with an RV cam. I have a 700R4 tranny with a 3.08 rear. On the interstate the best mileage I can get is somewhere in the 10-11 range at 2000 revs. From what I have read from other posts is I should be getting better economy than this. My question is does this seem right to you? Chime away! Other potentially useful info: 500 cfm Edelbrock carb. 28" tall wheels. 2.5" duals running from headers |
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#2 |
Registered User
Join Date: Jan 2014
Location: Billings Montana
Posts: 183
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Re: What do you think of my MPG?
No!
I have a th350 with a 350 crate engine and a 3:08 rear gear and a Holley 650 I just broke 18 MPG in my '72 K10 daily driver. I was at 16-17 for ever then I swapped out to 2 1/2" manifolds and changed gas stations. The biggest thing I tell people is to set idle mixture. That's the next largest part of the equation next to timing. |
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#3 |
Registered User
Join Date: Aug 2009
Location: Texas
Posts: 58
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Re: What do you think of my MPG?
So Daman you think I need to do some carb tuning to get those lost miles back?
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#4 |
Registered User
Join Date: May 2015
Location: Northiron Hotrods Bisbee, ND
Posts: 27
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Re: What do you think of my MPG?
Get rid of the Edelbrock carburetor. Either use a simple 600 or so cfm Holley with vacuum secondaries, or switch it to a Quadrajet (which requires either changing the intake or using an adapter).
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#5 |
Registered User
Join Date: Jan 2014
Location: Billings Montana
Posts: 183
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Re: What do you think of my MPG?
Yes and a little timing and make sure all your plugs are gapped right
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#6 |
Registered User
Join Date: May 2015
Location: Northiron Hotrods Bisbee, ND
Posts: 27
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Re: What do you think of my MPG?
If you have a GOOD HEI distributor, with verified 12 volts to it, you should be able to jump a spark a half inch or better on a tester. R43 comes to mind for the spark plug, but don't quote me on that. Get new plugs and gap them at .45 - Mark the balancer with a white paint pen, then loosen the distributor hold down bolt (9/16), verify the location to "0" with a timing light.
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#7 |
Registered User
Join Date: Aug 2009
Location: Texas
Posts: 58
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Re: What do you think of my MPG?
Thanks for all the replies!
This gives me a few things to try |
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#8 |
Senior Member
![]() Join Date: Jan 2002
Location: Williston, ND
Posts: 2,999
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Re: What do you think of my MPG?
That doesn't sound right at all. My 496 gets 9.8mpg with a th400. I'd definitely check what others have said, timing n spark plugs n fuel mixture. Does it smell like it is running rich?
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68 Chevy CST/10, 50th Anniversary, 327/th400, 373 posi, buckets 68 GMC Shortbox Stepside, 402/th400, Tilt, Tach, Bucket Seats, 3.08 Positraction 69 Chevy Camaro SS, 383, 4 speed, positraction 70 Chevy El Camino SS396, Triple Black, 454/th400, Tach 70 Chevy Shortbox Stepside 4x4, 350/th350, Tilt, Tach, Cab Lights, Bumper Guards, Tow Hooks 70 Chevy 3/4 Longbox, 350/auto, 70k, SURVIVOR 71 Chevy Blazer, CST, 350/auto 71 GMC Sierra K2500, 454, 4 speed, 4.10 posi, 37s 71 Custom/30, Open Road, 350/th400, tilt 72 Chevy Shortbox Fleetside, 496 Stroker/th400, Tilt, Tach, Bucket Seats, 3.42 Posi 72 GMC Sierra Grande 2500. 427/th400, tilt, a/c, tool and stowage, buckets/console, 3.54 posi |
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#9 |
Registered User
Join Date: Jan 2014
Location: Billings Montana
Posts: 183
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Re: What do you think of my MPG?
One other thing is if everything is moving freely. A lot of people forget about bearings and dragging brakes.
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#10 | |
Registered User
Join Date: Feb 2011
Location: Modesto, CA
Posts: 2,188
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Re: What do you think of my MPG?
Quote:
Op- your numbers are certainly off base but don't spend a single dollar on that engine until you've confirmed it's a solid, with a compression test, verify the burn on the plugs, and timing. |
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#11 |
Registered User
Join Date: Nov 2011
Location: Williamsburg, VA
Posts: 958
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Re: What do you think of my MPG?
Your luggin the engine with a 3.08 gear and a 700r4. This definitely has a negative impact on your mileage. You need a lower rear end gear or no O.D
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#12 |
Registered User
Join Date: Aug 2010
Location: Edwards, CA
Posts: 7,503
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Re: What do you think of my MPG?
That is like 75 MPH with engine turning 1960 RPM, trans O/D of 0.7:1 through a 3.08 rear gear turning 28" tires. Somebody check my math here but that is like running a 2.15 rear gear direct.
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Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID ** Blazer SPID ** Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders ** Do you have a 1972 Plaid Pickup? ** Plaid Blazer ** Plaid Suburban Last edited by SS Tim; 05-17-2015 at 09:05 PM. |
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#13 |
Registered User
Join Date: Aug 2009
Location: Texas
Posts: 58
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Re: What do you think of my MPG?
I could be wrong on the tire height but at 75 I am doing 2000 revs.
I have Dakota digital gauges so I am positive that number is correct. NOTE: I was wrong on the carb. it is marked 1405 which is a 600 CFM Edelbrock |
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#14 |
Registered User
Join Date: Nov 2011
Location: Williamsburg, VA
Posts: 958
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Re: What do you think of my MPG?
That is way too low. Your killing your engine and MPG
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#15 |
Registered User
Join Date: Aug 2009
Location: Texas
Posts: 58
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Re: What do you think of my MPG?
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#16 |
Registered User
Join Date: Aug 2010
Location: Edwards, CA
Posts: 7,503
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Re: What do you think of my MPG?
The rear gears are too high (low numerically) to pushing that truck. since your engine is probably below its power band it lugging. Try driving it around with the O/D locked out and it will probably get better mileage at thoses speeds. Something in the 3.73 or lower (higher nummerically) is a lot more suitable for pushing a high drag vehicle at these sort of speeds.
__________________
Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID ** Blazer SPID ** Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders ** Do you have a 1972 Plaid Pickup? ** Plaid Blazer ** Plaid Suburban |
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#17 |
Registered User
Join Date: Aug 2009
Location: Texas
Posts: 58
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Re: What do you think of my MPG?
SS Tim
I have been looking into 3.42 gears. Are they still too (numerically) low? |
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#18 |
Registered User
Join Date: Jan 2011
Location: Townsend MT
Posts: 1,725
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Re: What do you think of my MPG?
Depends on the weight of your truck along with a whole bunch of other variables.
As previously said, make sure you have a solid engine and tune it right. |
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#19 |
Registered User
Join Date: Aug 2004
Location: phoenix az
Posts: 105
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Re: What do you think of my MPG?
What type of distributor are you running? You should have a vacuum advance one what you want is when your cruising for MPG total advance can be as high as 32+ degrees. When you put your foot into it then it should retard it so you don't ping. Here is a good write up on it. Nothing wrong with the carter carburetor as long as it is working correctly. If you think it needs to be rebuilt take it to someone who rebuilds them. Unless you want to covert to an 1850 Holley but you might find it to be a new can or worms doing that. I have rebuilt and tuned Holley's, Carter, and Quad-jet's over the years all have their own quarks but they work. I always suggest if you suspect the carb is old and leaking take it to someone who knows what they are doing. Also check for vacuum leaks with some carb cleaner spray it around the gaskets at the base of the carburetor and other areas where vacuume is see if engine idle changes. Make sure you only spray little at a time.
http://www.hotrod.com/how-to/additio...nition-timing/ Last edited by kell490; 05-18-2015 at 12:41 AM. |
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#20 |
Registered User
Join Date: Sep 2004
Location: Rancho Santa Margarita - SoCal
Posts: 1,432
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Re: What do you think of my MPG?
I agree 100% with Tim on this, before you go spending money on carb swaps try driving around with the O/D locked out (3rd gear, don't let it go into 4th) and you will probably start seeing better numbers. If nothing else it's an option that costs nothing but time, if that doesn't work then look into plugs/timing/carb adjustments, etc.... I would look for a 3:73 like he said, that would be your best bet and I think one of the best ratio's for a 700R4 trans. That rear end would do great with a 3 speed but the O/D is what is killing it like others have mentioned.
BTW, how does the truck run? Is it smooth? Any hesitation or sputtering? The 1405 is a decent carb when it's working well. |
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#21 |
Registered User
Join Date: Aug 2004
Location: phoenix az
Posts: 105
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Re: What do you think of my MPG?
Does your torque converter lockup I think the stock 700R's are suppose to lockup? The 700R you can add a switch to manually lock it that helps to be locked up on the highway. 2000 RPM at 65 MPH sounds about right I would not throw a new set of gears in it unless you want more low end torque or want to run bigger tires.
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#22 | |
Registered User
Join Date: Nov 2011
Location: Williamsburg, VA
Posts: 958
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Re: What do you think of my MPG?
Quote:
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#23 |
Registered User
Join Date: Aug 2009
Location: Auburn
Posts: 78
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Re: What do you think of my MPG?
Aren't most newer trucks turning that RPM (or lower) at interstate speeds?
In any case, to the OP, try the free things first. I'd start with parasitic losses in the drivetrain by checking resistance on all four wheels, all engine accessories, and the engine itself by pulling all the plus and turning the engine over by hand. While they're out, re-gap the plugs and check the timing. Verify the torque converter is locking. Once all the maintenance things are complete and before spending a dime doing mods, genuinely assess how your driving style could negatively impact MPG. This is the "imagine an egg under the accelerator pedal" part of the post... Also, while I disagree with the too-low RPM theory, it is free so absolutely try keeping it out of O/D and monitor MPG. Only after exhausting the free stuff would I start spending money and even then it'd be a modest amount on things like retuning the carb. Lastly, let us know how you find improvements. This is one on which all of us benefit. |
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#24 | |
Its a Truck Thing......
Join Date: Dec 2004
Location: Sierra Vista, Arizona
Posts: 3,159
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Re: What do you think of my MPG?
Quote:
Smitty
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#25 |
Registered User
Join Date: Nov 2000
Location: Huntsville Ontario Canada
Posts: 4,052
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Re: What do you think of my MPG?
The 1405 out of the box is rich enough to run a 454. You need to get that carb jetted leaner .
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