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#26 |
Registered User
Join Date: Dec 2012
Location: Temple City
Posts: 3,566
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Re: MPG and Manifold Vacuum
First and foremost rpm does not dictate mpg. The vehicle in all its Glory, and set up determine mpg. Of your trucks needs 50 hp to move down the road it's needs 50. Either a 230 I6 at 2700rpm or a 350 at 2200.
2 guys I knew had 4x4's on 35" tires. One had a 283, and the other a 396. Both were full bolt on's with cams. On the hwy they got the same mpg, but once on the dirt the 283 litterly got twice the mpg. I would re gear the rear to get my highway rpm to 2500 at least. Carbs unlike EFI are less tolerant to rpm/mpg formula. Carbs need air velocity to keep fuel atomized. |
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#27 |
Registered User
Join Date: Jan 2013
Location: Columbia, SC
Posts: 1,926
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Re: MPG and Manifold Vacuum
I might try the 0.86 OD gear to bring the RPMs up slightly. I might install a fuel ratio gauge to see if my fuel ratio is correct. In the end this is an exercise to squeeze a few more MPGs out of the setup, but I’m very happy with the engine performance and drivability as is.
__________________
My 65 C10 build: www.lugnutz65chevystepside.weebly.com Want to know more about T5 transmissions? My website has a T5 Info Page and a Step by Step T5 rebuild. |
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#28 | |
Registered User
Join Date: Nov 2014
Location: west coast, Wi
Posts: 534
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Re: MPG and Manifold Vacuum
Quote:
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#29 | |
Registered User
Join Date: Jan 2013
Location: Columbia, SC
Posts: 1,926
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Re: MPG and Manifold Vacuum
Quote:
Let me know if this video helps. https://youtu.be/BcgX8R2l9Ok
__________________
My 65 C10 build: www.lugnutz65chevystepside.weebly.com Want to know more about T5 transmissions? My website has a T5 Info Page and a Step by Step T5 rebuild. |
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#30 | |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,522
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Re: MPG and Manifold Vacuum
Quote:
Could save some trans gear swaps. |
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#31 |
Registered User
Join Date: Jan 2013
Location: Columbia, SC
Posts: 1,926
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Re: MPG and Manifold Vacuum
I’ve run the numbers through an RPM calculator. Gear change is cheaper since I can do the work myself. Four new tires would be expensive. Besides, I wouldn’t get the RPMs where I need them unless I used REALLY SMALL tires, which would look silly.
__________________
My 65 C10 build: www.lugnutz65chevystepside.weebly.com Want to know more about T5 transmissions? My website has a T5 Info Page and a Step by Step T5 rebuild. |
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#32 | |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,522
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Re: MPG and Manifold Vacuum
Quote:
And you only need two tires. Bought wisely maybe 50 bucks. Not much if someone needs to pay a mechanic for a swap that may not work out. |
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#33 |
Registered User
Join Date: Jan 2013
Location: Columbia, SC
Posts: 1,926
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Re: MPG and Manifold Vacuum
Deleted my earlier comments. I got confused as to who was recommending what to whom.
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__________________
My 65 C10 build: www.lugnutz65chevystepside.weebly.com Want to know more about T5 transmissions? My website has a T5 Info Page and a Step by Step T5 rebuild. Last edited by Lugnutz65; 08-16-2019 at 08:59 PM. |
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#34 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,522
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Re: MPG and Manifold Vacuum
You know, I was only replying to Marty to let him know there’s easier ways to check ratio changes.
And yes higher rpm helps. I’ve had a 74 350 that got it’s best mileage of 18 mpg turning 3400 rpm using a Holley 800 dp. Or an LS6 chevelle with 3.08’s that loafed down the road at 2800 rpm and pulled down 20 mpg. Every motor is different. |
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#35 | |
Account Suspended
Join Date: Jun 2015
Location: No longer here
Posts: 1,000
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Re: MPG and Manifold Vacuum
Quote:
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#36 |
But Found Her 25yrs Later!
![]() Join Date: Oct 2010
Location: Oregon City, Oregon
Posts: 10,507
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Re: MPG and Manifold Vacuum
I use this one:
https://www.fueleconomy.gov/feg/savemoney.jsp My daily driver car I only drive about 6000 miles a year. My truck I drive much much less.
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I lost my 65 - Found it 25 years later: http://67-72chevytrucks.com/vboard/s...d.php?t=426650 66 C20 Service Truck: http://67-72chevytrucks.com/vboard/s...d.php?t=428035 |
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#37 | |
Registered User
Join Date: Nov 2014
Location: west coast, Wi
Posts: 534
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Re: MPG and Manifold Vacuum
Quote:
3.42 gears 26.5" tires .76 OD puts rpm at 2142. time will tell if this is ok or not if not shorter tires may not be the best option a .86 od may be or a lower rear gear such as 3.73 or 4.11 I have frequented the rpm calculator site |
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#38 |
Registered User
Join Date: Oct 2013
Location: Maine
Posts: 2,343
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Re: MPG and Manifold Vacuum
2142 rpm at what speed? I think most carbed engines start running pretty good after 2000 rpm. I am amazed at newer cars that can pull 75 mph in OD at 1600 rpm's. I know my old 350's couldn't do that.
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76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10 |
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#39 | |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,522
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Re: MPG and Manifold Vacuum
Quote:
A .86 o/d would bring you up to 2400 rpm. That might be an ideal ratio for your motor. A 3.73 with a .76 o/d gives 2300 rpm and a 4.10 with a .76 gives 2500. Which way to go depends on your skills and budget. Changing o/d ratio or rear gears can be costly. A high winding 283 or 327 might like the .86 better. A 383 or bbc could handle the .76 better. Most times gearing is a lot of trade offs! For Cadillac al this is all calculated at 65 mph. |
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