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Old 02-12-2012, 10:33 AM   #1
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Towing gear ratio assistance

I reciently upgraded my truck 07. 6.0 PSD w/tow boss package. The truck has 22.5 big rig rims with 255x70 22.5 tires that are 36.6 in tall.
My question is, i am considering going to 4.88 gears as my TH is 16000 loaded. Is the 4.88 gear to steep with that size tire?
What are your taughts or recommendations?
Anyone running this combo?
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Old 02-12-2012, 12:37 PM   #2
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You've made a serious mod, IMHO, you should ask a pro your questions.
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Old 02-12-2012, 01:47 PM   #3
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Well, first item: have you tried to tow it yet with the new wheels? What ratio is in it now--4.30?
I had an '07 dually w/4.10s and it certainly would have needed 4.30s and probably 4.88s to pull 16000 with that size wheels, if it would even been able then.
And expect fuel mileage to be the pits, + extra wear and tear on everything that rotates.
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Old 02-12-2012, 06:56 PM   #4
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No I haven't tried to tow it yet. It has the 4.30 gears on there. I tried the gear ratio calculator and based on the tire size difference its telling me to go to a 5.07 gear which on the f350 would be 5.13 gear that available in that ring and pinion size. which i think is a bit to steep, so i figured 4.88 would be more appropiate with that tire size change works out to be a 4.24 gearing. Calculations were all based on going from a 31 to 36.6in tall tire.
Just tring to see if any one has gone this route and what they think? Good or bad.
I'm currently in Afghanistan so i will have to try all this when I get back to good old USA.
Just doing research now while I have the time, so when i return i know which route to go.

Also I got the truck with those wheels
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Old 02-12-2012, 07:12 PM   #5
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Can't help with your question, but thanks for your service!
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Old 02-12-2012, 07:57 PM   #6
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Thanks anyway
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Old 02-16-2012, 12:59 AM   #7
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add .1 (point 1) to the stock gearing for every inch bigger in tire diameter that you added ...that will get you REAL close to the gear vs tire diameter you had stock
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Old 02-16-2012, 07:33 PM   #8
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Look at the specs for the truck.
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Old 02-20-2012, 08:19 AM   #9
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don't think there will be any specs for that situation from Ford...
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Old 02-21-2012, 01:41 AM   #10
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OK. lets try another approach. My previous truck was an 03 F350 PSD 6.0 W/4.10 and a 33.05" tall tire. It towed the RV fine except for some hills comming thru I10 in florida when i picked the RV up. I was a 10 hr trip back to NC. Had no issues towing other than being over weight. So upgraded to a truck with the tow boss package.
Well based on my previous truck w/4.10 and a 33.05 tire I used the gear calculator.
33.05" tire w/ 4.10 going to 36.6 will require 4.54 gears
33.05 tire w/4.30 going to 36.6 will require 4.74 gears
I'm thinking 4.88 wold now be to much. So now I'm think 4.63 gear ratio this will put me at about a 4.19 gear ratio.
What do you think? Good or bad?
It is also a daily driver will be towing the TH about 6 times for the year on the east coast.
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Old 02-21-2012, 09:10 AM   #11
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Quote:
Originally Posted by warrstang331 View Post
I reciently upgraded my truck 07. 6.0 PSD w/tow boss package. The truck has 22.5 big rig rims with 255x70 22.5 tires that are 36.6 in tall.
The tire calculators give you a ball park number. Do the math yourself to get exact numbers.


All tire and gear and speedo calculations are based on percentage change in tire revolutions per mile (revs/mile)

Your stock tires were LT245/75R17 with 664 revs/mile. Michelin XZE 255/70R22.5 tires have 563 revs/mile.
Bob Dickman Tire Center - Motorhome Tires Tires by Size

664 minus 563 = 101, divided by 664 = 15.2% longer legs.

4.30 ratio + 15.2% = 4.95 ratio = math answer to required gear ratio to get you back to stock "feel" of power and torque. But the bigger tires add both mechanical and aerodynamic drag to your truck, so you need slightly shorter ratio than 4.95. So instead of choosing 4.88 as your new gear ratio, go the other way, up to 5.13 if available.

A quick glance at Randy's Ring and Pinion shows they have a Yukon ring and pinion set for your Dana 80 rear axle in 5.13 ratio.
High performance Yukon Ring & Pinion gear set for Dana 80 in a 5.13 ratio - YG D80-513

That should get you back to very close to your stock feel of power and torque with your tall tires. But if you want even more grunt up the mountain pass, then they also have a 5.38 ratio. And Randy's sells a coupla of other brands of ring gear and pinion sets, including the expensive Dana Spicer brand.

Dana has made more than one Dana 80 rear axle for Ford trucks over the years. So be certain the person you talk to understands that you have a "Super" Dana 80 in an F-350 DRW that has the TowBoss pkg. The ring gear and pinion set might be different than in an ordinary F-350 dually.
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Old 02-22-2012, 01:07 AM   #12
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SmokeyWren thanks for that bit of info.
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Old 03-05-2012, 04:59 PM   #13
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In my opinion depending on if your a driver that runs fast or is more like 65 mph driver your going to want 4.88 gears . given the fact your hooked to a trailer that puts you in need of A F-550, more gear is better . I ran a F-450 years ago with a ZF 5 speed that had 4.88 gears and 32.5" tall tire , top speed with a 7.3 motor was right around 90 mph at 3200 RPM. Yes it was turning hard but engine speed wasnt a problem , your 6.0 has more power but still has taller tire that will give you more ground speed. pulling heavy I think the 4.88 will be just right , and I would not suggest anything less then 4.56 .
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