Gear change
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Junior User
Joined: Oct 2001
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From: Nebraska
Gear change
If you change ring and pinion gears on a 1994 f150, does the speedometer need recalibrated? I've heard that the sensor reads off the ring gear and the speedometer will be accurate regardless of what gearing you change to. Does anybody know for sure?
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Gear change
[font size="1" color="#FF0000"]LAST EDITED ON 30-Oct-01 AT 02:42 PM (EST)[/font][p]The sensor does read off the ring gear like you have heard. One revolution is one revolution no matter what gears you put in. What would cause the need for speedo cal is the changing of wheel/tire diameter. Hope this helps.
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Junior User
Joined: Oct 2001
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From: Nebraska
Gear change
I've got 32" tires. I have had the speedometer recalibrated for that already. Now I'd like to change gears. I read somewhere the OD ratio of an E40D is .67 does anyone know if this is correct?
Postmaster
Joined: Jun 1999
Posts: 2,719
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From: Lee's Summit MO
Gear change
Since you allready had the speedo recalibrated, then no I DO NOT think you would need to have it recalibrated for different gears.
1993 F-150
1989 F-150 4x4
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Junior User
Joined: Oct 2001
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Gear change
there is some formula to figure out if your speedometer is off or not, and by how much. if i find it ill post it. with my truck, when i put my 33" tires, the speedometer was off by 12%, then i switched from 3.08's to 3.73's and the speedometer is only off by 6% (roughly 5 mph or so, depending on how fast youre going).
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From: Buena Vista
Gear change
Here is a formula for Speedo corrections.
You must use the actual size of the tire not what it says on the sidewall!
Take the new tire diameter and multiply that by your indicated speed on the Speedo than divide that by the old tire diameter.
Then what you have is your actual speed. EXAMPLE=
31.9 x 65 mph over 28.9 = 69.3 mph; as you can see it is 4 mph off, but if you are going 25 mph, it will be not off as much.
Does any one want any more formulas; I have one for determining at what speed (mph) you will be running at a certain rpm?
And I have one for determining RPM's at a certain speed.
And one for determining the gear ratio in your truck, if you don’t know.
And one for if you put bigger tires on, it will tell you what your gear ratio was reduced to.
And one for if you put bigger tires on, it will tell you the ratio you need to get back to your stock one.
And one for determining your crawl ratio.
You must use the actual size of the tire not what it says on the sidewall!
Take the new tire diameter and multiply that by your indicated speed on the Speedo than divide that by the old tire diameter.
Then what you have is your actual speed. EXAMPLE=
31.9 x 65 mph over 28.9 = 69.3 mph; as you can see it is 4 mph off, but if you are going 25 mph, it will be not off as much.
Does any one want any more formulas; I have one for determining at what speed (mph) you will be running at a certain rpm?
And I have one for determining RPM's at a certain speed.
And one for determining the gear ratio in your truck, if you don’t know.
And one for if you put bigger tires on, it will tell you what your gear ratio was reduced to.
And one for if you put bigger tires on, it will tell you the ratio you need to get back to your stock one.
And one for determining your crawl ratio.
