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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: OldGray on April 24, 2017, 03:40:25 pm
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I have a 305 w/TH350 trans. I'm not sure but I've been told I most likely have 3.07 rear end gears.
Getting ready to do a swap for a 700R4 to improve highway driving and mpgs.
With my 29.1" tires, the info I got from the RPM calculator is below.
The guy doing the swap for me says I want to be around 2000rpm at 70mph.
With 3.55's, 70mph puts me at 2946rpm / 2008rpm w/OD.
With 3.73's 70mph puts me at 3096rpm / 2110rpm w/OD. (1960rpm at 65mph w/OD)
Based on what my mechanic said, it looks like 3.73's would work well...right?
If not, opinions please.
Thanks
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I have a 305 w/TH350 trans. I'm not sure but I've been told I most likely have 3.07 rear end gears.
Getting ready to do a swap for a 700R4 to improve highway driving and mpgs.
With my 29.1" tires, the info I got from the RPM calculator is below.
The guy doing the swap for me says I want to be around 2000rpm at 70mph.
With 3.55's, 70mph puts me at 2946rpm / 2008rpm w/OD.
With 3.73's 70mph puts me at 3096rpm / 2110rpm w/OD. (1960rpm at 65mph w/OD)
Based on what my mechanic said, it looks like 3.73's would work well...right?
If not, opinions please.
Thanks
Either one of those will work great. I recently swapped from a 350/350 combo to a 350/700r4 with 3.73 gears. Im pushing 70 around 2k and 80 around 2300rpm i believe. Well worth the swap.
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Thanks. Did your MPGs improve? I currently average around 11-12mpg hwy. :'(
I rarely drive with a heavy foot. So 3.55 or 3.73 better?
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I have an NV 4500 5spd manual in my 83 with 3:73 gears so the OD ratio is pretty much the same as the OD on the 700R4. I get 10 mpg with a pretty healthy 454.......you should get much better than that.....although 305's are not known for their fuel efficiency......or their power. Prolly be better off with a 350/700R4
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It is basically a truck that I use when I need to haul something, or tow a small open trailer (which is rarely). I would definitely drive it more if the mileage was better so that is what this is about.
The 305 was recently rebuilt before I bought it, so (unless someone gives me a 350) I probably will live with it. But who knows... maybe... someday...
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Thanks. Did your MPGs improve? I currently average around 11-12mpg hwy. :'(
I rarely drive with a heavy foot. So 3.55 or 3.73 better?
Either will work comparably. When you look up how to calibrate your speedometer (according to hear ratio) they suggest the same drive and driven gear.)
I'm waiting to get my correct driven gear and gear housing that goes on the trans itself. I know I'm getting better MPG just not sure cause my Speedo is running 10mph faster than I actually am.
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i would go for the higher ratio. this was you have the tq factor to help you get up to speed and then when cruising youll have OD to give you a much lower ratio
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Yeah, after mulling it over for a while I am now leaning towards 3.42
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i would go 3.73 or 4.10, you dont want your rpms to be too low while cruising or you might not even be able to use OD. but im not 100% on all this. i know my 94 with a 4.3 and 4l60e is all the time kicking in and out of OD while going 70. i really cant keep it at 70 or it will just lug the engine and my speed will drop, so i have to be going around 73 to be able to keep the speed up
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With the 3.42s and my tires:
1935rpm @ 70mph / 1797rpm @ 65mph
Is that not high enough?
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But what about cruising at 55? You'll be 1500 at 50 you'll be at 1400. Guess it depends on where you'll be driving. I really want to see what bd, vile and hatzie have to say to tell you the truth
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With the 3.42s and my tires:
1935rpm @ 70mph / 1797rpm @ 65mph
Is that not high enough?
Since my swap ive driven about 800 miles. I have 3.73 gears with a 28inch tire 1600 stall. 1st shifts into 2nd at 15-18MPH around 2k rpm, into 3rd at 30MPH around 2k rmp, into OD at 50-54 around 2k rpm. I cruise anywhere from 65-80 (Texas Highways) at 80mph im around 2200rpm.
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I'd love to hear as many opinions as I can get!
Again, my goal is MPG. I don't tow a lot, and when I do it might be a motorcycle on a 5x8 open trailer, so no heavy stuff really. I am not looking for hot-rod performance, but a bit more off the line (than I have) would be great. Here are the numbers (IN OD) again for comparison:
3.42's - 70mph = 1935rpm / 65mph = 1797rpm
3.55's, - 70mph = 2008rpm / 65mph = 1865rpm
3.73's -70mph = 2110rpm / 65mph = 1960rpm
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if your goal is mpgs 65 mph isnt a good speed to judge that on.
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I will have to look at my IPO codes to see what gearing I have. I do have the 700R that came with the truck and the stock 350.I don't drive it hard and have always had good luck with the mileage.
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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.
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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.
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Our trucks are as aerodynamic as a brick with wheels! ;D
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i think a brick has better aerodynamics. but wind is the limiting factor as to how fast we can go
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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.
Sent from my iPhone using Tapatalk
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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?
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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
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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.
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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.
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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.
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the 366 was the surface area of the trucks nose x 3.2 cu inē
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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
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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.