700R4 swap - gearing for mpg?

[OldGray]

if your goal is mpgs 65 mph isnt a good speed to judge that on.

I guess 60mph is more of a sweet spot for mpgs?

Most of my driving is on 65-70mph highways, and I am usually right within them.  Not much city driving.

[Irish_Alley]

i know for my impala it was right at 60 and i was able to verify it with the instant mpg  thingy they have. i think it has to deal with how "thick" the air is. the faster you go the more drag you will encounter.

[OldGray]

Our trucks are as aerodynamic as a brick with wheels!   

[Irish_Alley]

i think a brick has better aerodynamics. but wind is the limiting factor as to how fast we can go

[1967KaiserM715]

You want to make sure your within a reasonable power band at highway rpm-meaning it will take a certain hp rating to maintain x speed on y weighted vehicle with z aerodynamics. With a 305, I would edge my 55/60mph closer to the 2,000rpm range, a 350 I would probably let dip closer to 1,500 around the 55mph range.

Newer Chevy trucks with the 6.0 will cruise 55 at 1500 all day-but your talking significantly more power per revolution as well.


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[srozell]

Is there a calculator somewhere that would help us determine how many RPM's a certain HP engine will need to move a giant brick through the air at 60 mph?

[84c10tony]

Your mechanic is correct, 3:73's are the sweet spot for the gearing with a little get up and go for the truck, mix this with Rich Weyands suggestion, a Comp Cam 12-300-4, in a 305 or 350, it would have a lot more torque for more get up and go and better gas mileage. Even more with better flowing intake and exhaust.
The 3:42's might be ok but may slightly bog down with a 305 compared to 3:73's unless you have the higher torque cam. Keeping the system in balance with a 305 requires a bit more thinking as our heavier trucks are not aero smooth, low factory hp and low torque engine with tall gears (2:73-3:08 etc are ok for light cars).
A few years ago, we ran a lot of numbers for the combos, 27-30" tires, 700r4, 305 or 350, rv cam (caming for torque etc.) and have found the online x y and z converters are good, punch in the variables.
 My math is long forgotten but I did remember; a 3:73 ratio, 28.9"-30" tire, 700r4, 240-300 hp, 300-360ish ft lb trq. 305-350 engine (with the strong torque cam), intake, exhaust and a Well tuned Quad and hot coil hei should put me in the 20-22+mpg range at 65ish, after 55 the aerodynamics need help.
I have seen 18-22 mpg at 55-62mph, at 65 ish, approx 10% less mpg, at 70 ish approx 20% less. These aprrox's were with  27"-ish highway tires at max air pressure minus 5 and 10% for testing mpg, truck is 3477 lbs unladen.
When funds permit, it will be similar to Rich Weyands excellent suggested set up, (350 ci, 12-300-4 cam, headers, Edel Performer (regular, not Rpm) or sim.
Prefer 28.9" or 29.5" tires, the rest will be an experiment after leaving CA, Like adding another oem snorkel for dual inlets, electric fans and fun stuff. Good luck

[Irish_Alley]

your theory is correct srozell on lower RPMs can equal better MPGs IF the motor is set up for it. take my cummins, thats in my v3500 with the 4l80e and 4.10 for example. im able to achieve 16 mpg with the cummins, before the motor swap it had a 350 tbi i was only able to get 12.

dodge pairs their cummins usually with 3.55 some 1st gens got the 4.10, but they dropped the ratio to 3.55 to get better MPGs and people with the same setup as me claim 20+ MPGs. im almost certain this is because our peak Tq power (about 440 ft lbs) is about 1800 rpms and with the 3.55 thats right at 65 with my 4.10s that puts me at 2k.

[84c10tony]

Found this equation at Motor Trend with Freiburger in episode #8 hot rod garage, nitrous fogger install on the crusher Camaro which he put 3:73's in for a 700hp 7000rpm engine in the 1/4 mile..lmao

MPH x gear ratio x 336, divided by tire diameter = RPM and he recommends adding 5%

Not quite the MPG calc I was looking for, but I will look further.

Oh, and what Irish said..."If the motor is set up for it", makes a world of difference, my original stock 155hp/240lb trq 305 liked 55 mph...barely.

[srozell]

MPH x gear ratio x 336, divided by tire diameter = RPM and he recommends adding 5%

Interesting. I wonder where the 336 comes from, and the gear ratio must account for the gear selected as well as any reduction in the axle as well.

[Irish_Alley]

the 366 was the surface area of the trucks nose x 3.2 cu in²

[Irish_Alley]

that was BS by the way. on a serious note, the 366 was the common number that i found when finding a formula. i cant remember now as it was 8 years ago that i sat up one night just crunching numbers that i found from other people. i would find their tire size, rpms at a certain speed. i would plug and play the common ratios till it fit. after a few test subject and comparing to my own information on my truck i was able to come up with a formula. i dont know if im the original person who found it, but i did do some book work on it
http://forum.73-87chevytrucks.com/smforum/index.php?topic=10879.0


also this formula is for a 1:1 trans. automatics you have to fudge the numbers a little due to slippage and tire size can be off

[srozell]

lol, that was funny.

So I'm married to a math instructor at a local university. I was telling her what I was trying to work out and she made it sound really simple. I'd tell you what she said, but .. well.... she has awesome breasts and I kept getting distracted.. maybe I'll have her email me instead.

You probably aren't far off the mark when you refer to the surface area of the front of the truck. The whole wind-drag thing is about the only  part of the equation that isn't simple geometry.... for my wife.