MPG and Manifold Vacuum

[Ziegelsteinfaust]

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.

[Lugnutz65]

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.

[Marty68]

Originally Posted by Lugnutz65

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.

how hard is it to change od ratio from .76 to .86..?? I have the trans ready to install. I however am concerned .76 may put my 283 in too low an rpm and be a gas hog

[Lugnutz65]

Originally Posted by Marty68

how hard is it to change od ratio from .76 to .86..?? I have the trans ready to install. I however am concerned .76 may put my 283 in too low an rpm and be a gas hog

Changing 5th gear is not difficult, but you must do the math first or you may end up with a different result than you planned on. I made this video to explain how the different gear ratios are calculated inside a T5. If you are moving a 5th gear set from one S10 T5 to another S10 T5, then the ratio of the gear set should not change. Going from a V8 gear set to an S10 changes a lot!
Let me know if this video helps.
https://youtu.be/BcgX8R2l9Ok

[geezer#99]

Originally Posted by Marty68

how hard is it to change od ratio from .76 to .86..?? I have the trans ready to install. I however am concerned .76 may put my 283 in too low an rpm and be a gas hog

Maybe you should try the poor man’s under/overdrive with taller or shorter tires.
Could save some trans gear swaps.

[Lugnutz65]

Originally Posted by geezer#99

Maybe you should try the poor man’s under/overdrive with taller or shorter tires.
Could save some trans gear swaps.

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.

[geezer#99]

Originally Posted by Lugnutz65

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.

True enough, but not everybody can easily swap gears.
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.

[Lugnutz65]

Deleted my earlier comments. I got confused as to who was recommending what to whom.

[geezer#99]

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.

[YoungPup1977]

Originally Posted by Lugnutz65

Changing 5th gear is not difficult, but you must do the math first or you may end up with a different result than you planned on. I made this video to explain how the different gear ratios are calculated inside a T5. If you are moving a 5th gear set from one S10 T5 to another S10 T5, then the ratio of the gear set should not change. Going from a V8 gear set to an S10 changes a lot!
Let me know if this video helps.
https://youtu.be/BcgX8R2l9Ok

Have you considered developing a cost calculator, which would include the cost of gears, installation of gears VS the MPG savings. How long would it take for you to receive return on the investment of saving 2 miles per gallon of gas ? 2 miles per gallon is your target correct or is it more ? By knowing the return, would make the decision of even thinking about such changes of regearing the trans and/or rear end gears. Doing the math of return on investment would be my first steps. Enjoy !!

[LostMy65]

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.

[Marty68]

Originally Posted by geezer#99

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.

this is why I asked, I may end up with too LOW of an rmp with 3.42 gears .76 OD
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

[cadillac_al]

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.

[geezer#99]

Originally Posted by Marty68

this is why I asked, I may end up with too LOW of an rmp with 3.42 gears .76 OD
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

Experimenting with shorter tires isn’t really an option for you. At 26.5 you’re at the bottom end already.
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.