Axle ratios?
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Elder User
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From: Eastern Ohio
Axle ratios?
Was wondering if anyone could help me figure out what my ratio is 3.73 gears, 35x12.50x17 on 05 f-250. Keep hearing that I should have more power and better fuel mileage? Any idea of what my ratio would be with these tires and how much my speedo is off? Thanks
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From: Milky Way Galaxy
Let me see if I can sort this out:
Going from a 31.5" tire to a 35" tire w/3.73 gears would give you a new effective ratio of 3.36.
Examples of MPH changes:Speedometer vs. Actual Speed
45=50
55=61.1
65=72.2
75=83.3
More power. . .No. The truck may actually feel a bit more sluggish off of the line with the new effective gear ratio and bigger/heavier tires. More MPG. . .Maybe. But only a very slight positive change, and probably not worth the loss in low-end grunt.
To get back to close to an equivalent gear ratio you would need to put in at least 4.10's (which would still be below equivalent) or 4.30's (which would be above equivalent).
Originally Posted by 50calmike
Was wondering if anyone could help me figure out what my ratio is 3.73 gears, 35x12.50x17 on 05 f-250.
Any idea of what my ratio would be with these tires and how much my speedo is off? Thanks
Any idea of what my ratio would be with these tires and how much my speedo is off? Thanks
Examples of MPH changes:Speedometer vs. Actual Speed
45=50
55=61.1
65=72.2
75=83.3
Originally Posted by 50calmike
Keep hearing that I should have more power and better fuel mileage?
To get back to close to an equivalent gear ratio you would need to put in at least 4.10's (which would still be below equivalent) or 4.30's (which would be above equivalent).
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Elder User
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From: Eastern Ohio
EnviroCon, You're info is greatly appreciated. I guess my post wasn't worded correctly, I meant that the truck was under powered because of the bigger tires not that I should have more power. Anyway thanks for the input.
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From: Milky Way Galaxy
Glad I could help. But I would highly recommend that you get the speedo corrected. There are are several systems that use that info to decide how to operate and without the correct info they may be causing even more of a feel of power loss, among other problems. Here's an article from Ford Doctor's site that talks about it:
http://www.forddoctorsdts.com/articles/article-05-21.php
There are 3 ways to recalibrate the speedo:
1. A tuner that has the ability to change the tire size in the PCM. I'm not sure which ones, since I don't use a tuner.
2. A friendly dealer that doesn't mind doing it. Usually they'll just charge you the hourly shop rate.
3. A Superlift TruSpeed speedometer recalibrator. This is what I use on my dually. I prefer this one because it allows you to adjust the signal yourself with a GPS, so you can usually get the speedo closer to your actual speed than the PCM re-programming. And the install is not that difficult.
http://www.forddoctorsdts.com/articles/article-05-21.php
There are 3 ways to recalibrate the speedo:
1. A tuner that has the ability to change the tire size in the PCM. I'm not sure which ones, since I don't use a tuner.
2. A friendly dealer that doesn't mind doing it. Usually they'll just charge you the hourly shop rate.
3. A Superlift TruSpeed speedometer recalibrator. This is what I use on my dually. I prefer this one because it allows you to adjust the signal yourself with a GPS, so you can usually get the speedo closer to your actual speed than the PCM re-programming. And the install is not that difficult.
Posting Guru
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From: Republic of Texas
This one does . . .
An SCT Xcal2 will allow tire programing as ell as gears. If that is an option you want to pursue. 
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From: SoCal
I have the similar situation I keep hearing people talk about. Specifically, 31 inch tires and I want to install 35". I have the 6.0 diesel and wondered if changing the gear takes care of the speedo recalc, or does the speedo still need a change? I'm thinking since I tow, I should change the gears to 4:30 to be closer, or above what I have now. Has anyone towed a trailer WITHOUT changing the gears? Is the power loss substantial?
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From: Milky Way Galaxy
Originally Posted by drewworm
I have the 6.0 diesel and wondered if changing the gear takes care of the speedo recalc, or does the speedo still need a change?
Originally Posted by drewworm
I'm thinking since I tow, I should change the gears to 4:30 to be closer, or above what I have now. Has anyone towed a trailer WITHOUT changing the gears? Is the power loss substantial?
Posting Guru
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From: Republic of Texas
Diy . . .
What is the cost and time involved to DIY on a gear change out? Given they are sending out 3.73's w/ factory tires @ 20" (like mine) you start w/ incorrect gears from the get go. 
Dealer doesn't seem to get it and doesn't order an upgrade to the lower 4.10's w/ the 20" tires.
What's up w/ THAT?
Postmaster
Joined: Jun 2004
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From: Milky Way Galaxy
Originally Posted by Lonewolftx
What is the cost and time involved to DIY on a gear change out?
Postmaster
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From: Arizona
Originally Posted by Lonewolftx
Dealer doesn't seem to get it and doesn't order an upgrade to the lower 4.10's w/ the 20" tires.
What's up w/ THAT?
What's up w/ THAT?
Hope this helps
Senior User
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From: SoCal
I agree with the other posts that setting the gears needs a special, qualified touch. However, can't the DIY guy pull the axles and separate the pumpkin and take it to the guy with the right touch in order to save a little money and time.
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Joined: Dec 2003
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From: Arizona
drewworm,
Maybe, depends on the guy doing the work. And if you are going to pull the axles and take the complete rear axle housing out of your PSD (axle housing is 1 piece) then take it to your local R&P shop, then re-install the axle assembly in your SD, is the time and energy worth it to you? And if it is a 4x4 you have got to do both axles at the same time. Defenitely a personal choice and i would want a big cost savings from my local R&P mechanic to justify this process.
Maybe, depends on the guy doing the work. And if you are going to pull the axles and take the complete rear axle housing out of your PSD (axle housing is 1 piece) then take it to your local R&P shop, then re-install the axle assembly in your SD, is the time and energy worth it to you? And if it is a 4x4 you have got to do both axles at the same time. Defenitely a personal choice and i would want a big cost savings from my local R&P mechanic to justify this process.