Rear gear ratio question
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From: Wentzville, MO
Rear gear ratio question
One of my trucks has a 351C that I’m putting a AOD into and currently has a 9” Ford open rear end from a 71 Mustang with 2.75 gear ratio.Apparently the PO found that 5 5 1/2 bolt pattern axles fit the Mustang housing. My other truck has a Y Block 292 with a C4 and a 3.50 9” out of a pickup. I was wondering what you guys thought about swapping the two pumpkins? It looks as though the 3.50 will work great in the 351C/AOD but I’m not sure what the ole Y Block will do with that tall a gear ratio. So anyone got any opinions?
Edit I forgot to add my tire size on the Y Block truck. They’re 275 60R 15s which should make the engine run around 21 to 22 hundred RPM at 65 MPH.
Edit I forgot to add my tire size on the Y Block truck. They’re 275 60R 15s which should make the engine run around 21 to 22 hundred RPM at 65 MPH.
Last edited by bjmayberry2; Jan 7, 2020 at 12:38 PM. Reason: Added tire size.
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From: Upper Left Coast
One of my trucks has a 351C that I’m putting a AOD into and currently has a 9” Ford open rear end from a 71 Mustang with 2.75 gear ratio.Apparently the PO found that 5 5 1/2 bolt pattern axles fit the Mustang housing. My other truck has a Y Block 292 with a C4 and a 3.50 9” out of a pickup. I was wondering what you guys thought about swapping the two pumpkins? It looks as though the 3.50 will work great in the 351C/AOD but I’m not sure what the ole Y Block will do with that tall a gear ratio. So anyone got any opinions?
Edit I forgot to add my tire size on the Y Block truck. They’re 275 60R 15s which should make the engine run around 21 to 22 hundred RPM at 65 MPH.
Edit I forgot to add my tire size on the Y Block truck. They’re 275 60R 15s which should make the engine run around 21 to 22 hundred RPM at 65 MPH.
I am going to do nearly the "same" thing with my 272 + E4OD and nearly 40" dia tires. I calculated around 2100 RPM at 60mph using the 5.83:1 ratio 2-speed axle I have with the E4OD 0.71:1 OD ratio.
Your tires will produce around 722 rear wheel RPM per mile (meaning the rear wheel RPM will be 722 RPM @ 60 mph)
so 1:1 ratio eng RPM will be 722 x 2.75 = 1985 RPM (no slip ) but you will have torque convertor slip with the C-4 so your cruising RPM will be somewhat higher depending on which way the wind is blowing.
That could be a tad tall for any vintage V-8 gas engines........ But I think the 272/292's might be up to the task as long as you're lightly loaded.
With the 3.5 ratio is your RPM around 2500 @60mph?
If you used an AOD (with a Bentsen Y-block adapter) you would get an even lower cruising RPM (around1700) with the 3.50:1 ratio in OD (locked-TC)
That might still be ok, lightly loaded at "level cruise"
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Lead Driver
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From: Wentzville, MO
Unfortunately I bought the Flat-O-Matic adapter and it is a full bell housing adapter. Soo no changing over to a Overdrive transmission this go around. I’d still like to drive the truck on the interstate at 65-75 MPH and get decent MPG. With the 9” if the tall gear ratio won’t work it’s not that hard to change the pumpkin out for different gears.
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Lead Driver
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From: Wentzville, MO
I’m hoping it will be better starting off than Kevin’s BBW since I will have an automatic. I’d really like to be close to 20 MPG.
Sorry BIGWIN56F100 (Kevin) must have texted me. He said he has the same rear gear but with a 3 speed. Said it was like starting out in 2nd gear. But could go all day at interstate speeds.
Sorry BIGWIN56F100 (Kevin) must have texted me. He said he has the same rear gear but with a 3 speed. Said it was like starting out in 2nd gear. But could go all day at interstate speeds.
Last edited by bjmayberry2; Jan 11, 2020 at 04:19 PM. Reason: Added Sorry
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Please report back your real world results! Very interested!
If I drove my old original 1959 F250 80 mph (albeit it had bias ply) I could almost feel the Grim Reaper crawl in and sit on the bench seat next to me...
BMC
If I drove my old original 1959 F250 80 mph (albeit it had bias ply) I could almost feel the Grim Reaper crawl in and sit on the bench seat next to me...
BMC
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