Odometer/speedometer Calculation
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New User
Joined: Feb 2004
Posts: 17
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From: SO-CAL
Odometer/speedometer Calculation
I am ridding on 38's on a f-350 4x4 my speedometer is obviously off. cant afford a module to correct this yet but was wondering if anyone knew a calculation that would tell me a correct odometer reading. my speedometer is about 10 miles per hour off
Senior User
Joined: Feb 2004
Posts: 494
Likes: 14
From: Somewhere west of Arizona
To make a correction without a comp mod, you'll have to always calculate a percentage from original. I'll need to know your original tire size to give you that. Of course, that assumes your speedo was correct before the tire change. What was your original tire size? Check the door post. As your comp will always be off, your performance will suffer too. I don't think that you can ever get a full correction with 38's. I think that the comp can only be corrected to 35's max. Somebody will correct me if I'm wrong on this.
Elder User
Joined: Feb 2004
Posts: 946
Likes: 4
From: Orange
I posted this under "RPMs at 60", but here is a link for actual speed--it uses RPM, gear ratio, and transmission ratio to calculate actual speed.
http://www.csgnetwork.com/rearendgearcalc.html
Here is one for tire size changes, too.
http://www.csgnetwork.com/speedocalibcalc.html
I hope that you find these helpful.
http://www.csgnetwork.com/rearendgearcalc.html
Here is one for tire size changes, too.
http://www.csgnetwork.com/speedocalibcalc.html
I hope that you find these helpful.
Senior User
Joined: Mar 2002
Posts: 158
Likes: 0
From: mid michigan
Do what Bigred did. You'll be pretty close. Go with the smallest 3 digit number in (###) next to the tire size choice. My dealer wasn't aware of this, but did it and was happy to know. It's been requested of him often. he just was picking one and seeing what happened.
