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Old 03-20-2014, 06:43 PM   #1
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4.10 OR 3.73

I am starting a 4x4 conversion on a ford van and I am going from the stock 245/75/16 to 285/75/17 tires do I need to go to 4.10 ratio to keep it about same rpm as it is with the 3.73 ratio it has in it now. It has a 5.4 engine so even a little lower wouldn't hurt my feelings
Thanks
Jim
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Old 03-20-2014, 07:07 PM   #2
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Re: 4.10 OR 3.73

4.10 would keep it nearly the same, right around 1850 at 65mph. I also have a 5.4l and 285/75/17's and am hoping to switch to 4.56 bringing RPM to about 2050 @ 65mph (I currently have 3.55 that have me constantly jumping out of OD) to help out with power when towing. If the donor axles already have 4.10's I would run it for a bit to see how it works for you, if you have to spend the money for gears anyway I would go straight to 4.56.
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Old 03-20-2014, 07:51 PM   #3
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Re: 4.10 OR 3.73

X2 on 4.56 for that size tire.
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Old 03-20-2014, 09:38 PM   #4
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Re: 4.10 OR 3.73

I have 4.10 gears with LT275/70R18 (33.4" tall) tires currently with the 5.4l in my van. While it is good on flat ground I definitely would go with the 4.56 next time. Much better at climbing hills and towing. The van does fine now but a little extra oomph would be nice. The 5.4 is a strong engine and has no problems towing, you just slow down a bit more on hills. Either way 4.10 or 4.56 is what you should look at. Only keep the 3.73's if you are staying with the stock tire size.
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Old 03-21-2014, 03:44 AM   #5
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Re: 4.10 OR 3.73

Will I be O.K. running the 285s with 3 Inches of lift over the factory 2wd height (rub-turning radius)? What kind of mileage are you getting with 4.56s and a 5.4? My van is 97 so it is still e4od will that make a difference? It will tow very little and almost never on trips so fuel mileage is priority one but I don't want to have to keep my foot in it to run 75.
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Old 03-21-2014, 01:22 PM   #6
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Re: 4.10 OR 3.73

Ratios are initially designed for ~245/70 17 correct? 265/70, 285/70 are viable options with no change in gearing required. The larger the diameter from stock, the slower the engine will turn, but that doesn't mean it's not working harder, taking more fuel, to keep it going that speed. You DO have a mechanical advantage with a smaller diameter. It is easier to get the vehicle moving, to maintain speed, AND to stop.

My van has 4.10's, my tires are 255/80 17 BFG at almost 33 exactly. it searches a bit on hills for a lower gear. Anything bigger than 33's i'd go 4.56....
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Old 03-22-2014, 03:10 PM   #7
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Re: 4.10 OR 3.73

I have a 5.4 on 35's with 4.88's and its perfect. Wouldn't mind more power but the gears help.
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Old 03-22-2014, 09:45 PM   #8
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Re: 4.10 OR 3.73

Quote:
Originally Posted by 4x4sporty
I am starting a 4x4 conversion on a ford van and I am going from the stock 245/75/16 to 285/75/17 tires do I need to go to 4.10 ratio to keep it about same rpm as it is with the 3.73 ratio it has in it now. It has a 5.4 engine so even a little lower wouldn't hurt my feelings
Thanks
Jim
Standard 245/75R16 are 30.47 inches nominal in diameter.
Proposed 285/75R17 are 33.83 inches nominal in diameter.

To keep same RPM, 3.73 X 33.83/30.47 = 4.14

So it appears 4.10 gears are closes. If you want to get a little lower then 4.30 should be available.

Note that if you don't change gears, the equivalent would be:

3.73 X 30.47/33.83 = 3.36

Ford does run some vans and pickups with 3.31 gears (going from memory), but for a 4X4 heavy van that seems on the high side to me. Still, if fuel economy is the number one priority as you've stated, then leaving it with 3.73 might give you better MPG results.
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Old 03-23-2014, 08:19 AM   #9
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Re: 4.10 OR 3.73

Thanks. A formula is just what I needed. I have found a low miles set of 4.10s. 4.30s are hard to find that aren't dually.
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Old 03-28-2014, 11:44 AM   #10
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Re: 4.10 OR 3.73

I have the same specs. I have the 5.4L with the 3.73. Added 285s and the van was sluggish. Put in the 4.10s and then had the speedo checked and the Speedo calibration shop. Actual speed at 60mph was 59.7.

The best things are that the van shifts when it is supposed to, accelerates, and my Ford warranty is not invalidated.
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