NEW GEAR RATIO FOR 39.5" TIRE? - Ford Powerstroke Diesel Forum
User Tag List

LinkBack Thread Tools Display Modes
post #1 of 2 Old 01-08-2012, 06:41 PM Thread Starter
Compression Ignition Addict
JD_MILLS's Avatar
Join Date: Apr 2011
Posts: 505
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Thanks: 5
Thanked 2 Times in 2 Posts
Feedback Score: 1 reviews

I'm currently a 39.50x13.5 r20 and really need to change gear ratio's still got plenty of power but i know they're killing my mileage. any ideas on what ratio i should use? i don't do any type of towing with it or off-roading. truck is strictly for the street.

JD_MILLS is offline  
Sponsored Links
post #2 of 2 Old 01-10-2012, 10:59 AM
Compression Ignition Addict
Join Date: Jan 2012
Location: Fallon, Nv
Posts: 2,403
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Thanks: 54
Thanked 11 Times in 10 Posts
Feedback Score: 0 reviews
Math 101: your gear ratio should be the same change rate as your tire change to keep things equal. So 39.5/31.5= 1.2539. This rate of change is now multiplied by your rear gear to find your "new" gear, 3.73 x 1.2539= 4.67. While you're speedo won't be 100% correct you'll be closer with a 4.56 gear than a 4.88 gear. In gearhead speak, go 4.56 if you daily drive and care about mpg, 4.88 if those don't matter.

By the numbers, your truck is performing as if it had 31.5s and a 3.00:1 rear gear. YIKES!

Karl and his 2003 6.0

Barefoot Accelerator Pedal
TRICO Force wiper blades
Yellow Military Aircraft Valve Stem Caps
Offensive "Stick Family" Decal
Big Ol'Honkin 1 1/4" Shank Receiver Hitch

WHAT!?! I was trying to list stuff that everyone else
hasn't already listed for their truck!

Karls03 is offline  
Sponsored Links

Quick Reply

Register Now

In order to be able to post messages on the Ford Powerstroke Diesel Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome