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6" Lift w/ 33's

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Old 07-10-2018, 06:45 PM
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Quick math says 4.88's would be good and feel kinda sporty. 2,300rpm @ 70

Edit: Checked the specs on a Kumho Road Venture in a 35x12.5x17" size. 608 rev/mile
General Grabber x3, same size: 594
Firestone Destination M/T2, same size, 603. Let's use the Kumho as a base.

608 rev/mile
60mph = 1 mile/minute
60mph = 608 RPM at the tire
x 4.88:1 gear ratio: 2,967.04 rpm driveshaft.
x0.7:1 OD ratio: 2,076 RPM @ the crank at 60mph
x (7/6) to get 70mph: 2423 @ 70 mph

If this was a 5.4, that would be about 2,070 rpm @70mph to get the same piston velocity. That's just a shade higher than what I see with my 3.55:1 gears and stock tires. It's not bad, but the top of 2nd gear is 80mph. If this is going to be more of a fun vehicle than a highway vehicle, you might want to look for a bigger number than the 4.88's. I cant say I have a problem with the gears in my truck, but I could see the appeal of 3.73 or 4.10's for towing or playing.

Short version: 4.88's would be better for your combo than 4.56's.

Last edited by Eric Kleven; 07-11-2018 at 03:53 PM.
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Old 07-11-2018, 02:16 PM
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Old 07-11-2018, 03:53 PM
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Originally Posted by swill92 View Post
Can you point me to some literature/articles/websites where I can learn about piston velocity and/or how to calculate what the right ratio for a 4.6L would be?
Check the edit I did to my post above.
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Old 07-11-2018, 04:54 PM
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Thanks for the help! I don't wheel, it will be a daily with city, highway driving mostly. It will also be used for camping/over landing some as well. I think with a 4.6L, the 4.88's make sense as long as I'm not driving across states.

Downside is I do plan trips from OR to UT and CA and other areas, so highway would suck with the 4.88s right?? I'm planning on running Mickey Thompson Baja ATZ P3s after I lift.
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Old 07-11-2018, 04:55 PM
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Those tires will be 305/70R18.
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Old 07-11-2018, 06:06 PM
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Highway won't be bad at all. 2,400rpm on the 4.6 is pretty close to the piston speed of a 5.4 at 1,900-2,000. These engines don't have a problem with the RPM. The relationship between your exhaust and ears might be a different matter, however.
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Old 07-11-2018, 07:47 PM
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Alright well I'm not sure if you're a genius or what, but I'm sold on the 4.88's! Thanks for all the info, really appreciate it! Are you an engineer, or how do you know this stuff?
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Old 07-12-2018, 11:30 AM
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I had 33s when I first installed my 6" lift a month ago and they looked too small. 35s look just right. Im running the stock LS 3.55 gears but want to switch to 4.10. I have the 5.4 engine
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Old 07-12-2018, 11:32 AM
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Originally Posted by swill92 View Post
Alright well I'm not sure if you're a genius or what, but I'm sold on the 4.88's! Thanks for all the info, really appreciate it! Are you an engineer, or how do you know this stuff?
From what Ive seen 4.88 is overkill unless youre going to run 40 inch tires or something similar in size. The formula i use is New Gear Ratio = [(new tire size/original tire size) x Original gear ratio]
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Old 07-12-2018, 12:48 PM
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Originally Posted by redraiderford View Post
From what Ive seen 4.88 is overkill unless youre going to run 40 inch tires or something similar in size. The formula i use is New Gear Ratio = [(new tire size/original tire size) x Original gear ratio]
With the shorter stroke on the 4.6 he'll want the 4.88's to make up for the loss of torque compared to the 5.4.

You'd be right on for the 5.4. 4.30's or 4.56's would work pretty good there.

Last edited by Eric Kleven; 07-12-2018 at 12:57 PM.
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