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Old 06-23-2016, 02:48 PM   #1
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Axle Ratio 3.92 v 4.18

Q for the gearheads on axle ratio recommendations:

My 4x4 Sprinter will live primarily along the Front Range of Colorado with frequent trips to the mountains. Do not plan to tow much or have a heavy build out with loads of batteries, tanks, generator, etc. I will eventually mount the largest tires available (265/75r16 or 265/70r17).

Should I spec it with the standard 3.92 axle ratio, or the optional 4.18 ratio?

TIA,

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Old 06-23-2016, 03:21 PM   #2
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4.18. When you put the bigger tires on, you can reprogram it for 3.92 gears, and with the larger tires your speedometer will be correct (or close, depending on overall tire height change). Also being in the mountains, you'll stay in top gear more often with the lower gears.
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Old 06-23-2016, 11:55 PM   #3
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Depends which engine and what sort of RPM's it likes. My experience, for consideration:
I have a GMC 6.6 Duramax with 5 speed Allison transmission with 3.73 gears and 285/75/R16's, compare to my Ford 6.0 Powerstroke with 5 speed transmission with 4.10 gears and 315/75/R17's.

The GMC turns ~2,200 RPM's at 75 mph, which is a little higher than necessary for that engine, which really performs well when lugged at low RPM; it red-lines ~3,500 RPMs.

The Ford turns ~2,200 RPM's at 75 mph, which is a little lower than the engine likes when pulling up even a slight hill or headwind; that engine redlines at 4,000+ RPMs and regularly winds up >3,500 RPMs when givin'er.

Both combinations perform very well in the CO mtns (two-lane highways with mountain passes at 55-65 mph), but I can't go deeper into comparison because it's applesranges.
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Old 07-27-2016, 03:34 AM   #4
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I am no expert for sure. However, i did read something on another build where the guy put the largest tire he could on the new 4x4. He reported the difference in torque or peep loss was subtle, but noticeable.

I think there are some formulas for figuring this stuff out (sorry i have no links to them). I too am considering a sprinter and want the largest tire. I'd prefer to keep the stock performance though.

If you find the answer (or just make a decision) post back your thoughts.
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Old 07-27-2016, 03:38 AM   #5
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4.18 - no question!
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Old 07-28-2016, 01:03 AM   #6
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Quote:
Originally Posted by BajaSportsmobile View Post
4.18 - no question!
Baja can you expand? I know your running larger tires than stock. Do you think using the higher gear ratio (4.18) and running the larger tires brings it back close to stock specs of the 3.92 and stock tire size with regards to MPG/RPM's?

I know my uncle had a converted van (decades ago) on 33's. He never swapped the gears. Was a bit of a dog of the line. At the same time I'm not looking to be rev'ing all over town either with a higher geared rear end .
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Old 07-28-2016, 08:33 AM   #7
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Axle Ratio 3.92 v 4.18

4.18 gear will turn the engine 6.6% faster than the 3.92 setup. Ceterus parabus.

4.18/3.92 -1 x 100= 6.6%

Cruise at 2200 rpm before the gear change would now cruise at 2345 rpm.

If you upsize your tire circumference by 6.6% no change in engine speed will occur.

31.5" tire C=Pi x d or 98.9"

33.0" tire C=Pi x d or 103 11/16"

103 11/16 / 98.9 -1 x 100= 4.8%

So the upsize in tire in this example, 4.8%, will not meet the 6.6% increase in revs the engine would see with just the gear change alone.

The engine will be turning 6.6%-4.8%= 1.8% faster at any given speed. Helpful if the new wheel and tire combination is heavier than the original setup.

If it cruised at 2200 rpm before the gear and tire change it would now cruise at 2240 rpm. Frankly unnoticeable from a wear and fuel usage point of view.




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Old 08-03-2016, 04:16 AM   #8
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Thanks Pnty!

Looks to me like the stock 4x4 tire is 235/60 16 that would be 28 inches tall. A common upgrade I've seen is either a 16 or 17 inch wheel to a 31.6 inch tall tire. According to Tire rack that's an 11.4% increase. So, looks like if one knew they were going to upgrade to the larger tire the higher gear ratio would be a good idea to maintain the torque and RPM's as close to stock as possible. Albeit, it would still be a little under geared. Slightly better MPG but maybe a tad sluggish off the line. Though it would be way better than going to the taller tire and having the lower gearing!

Wait that's not right. You said circumference, not diameter.

So using your math

Stock is 28 tall x 3.14 = 87.92
aftermarket is 31.7 tall x 3.14 = 99.22

99.22 / 87.92 - 1 x 100 = 12.85 % increase in circumference.

Still works out to be nearly the same as comparing diameter but makes considering the 4.18 even more appropriate, IMO.
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Old 03-12-2017, 11:02 PM   #9
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This options guide http://assets.mbvans.com/Mercedes-Be...y-Brochure.pdf says the 2017 4x4 is only available with 3.92. Apparently 4.18 is not available on 4x4, and I think not on any 6 cylinder. I really want 4.18. If anyone has additional info. please post. Thanks.
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