Gear change
Elder User
Joined: May 2004
Posts: 936
Likes: 1
The speedo does not change with a gear change. I switched from 3.73 to 4.10 and the dealer said that there is nothing they can reprogram for the transmission. The transmission has learned the new gearing just fine - at first it held gears too long.
Junior User
Joined: Mar 2005
Posts: 81
Likes: 0
What I meant was you have to get the speedo adjusted after tire or gear changes. (read it again after it posted, was not clear)
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Posting Guru
Joined: Dec 2004
Posts: 1,713
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So you guys are saying the speedo corrects itself when you do a gear or tire change.
Are you guys sure about this? Thanks in advance. Wrench.
Are you guys sure about this? Thanks in advance. Wrench.
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Junior User
Joined: Mar 2005
Posts: 81
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I have a TruSpeed on my truck. The guide for it gives you adjustments for GEAR or TIRE or GEAR & TIRE changes. I do not think they would have that option if gear ratio did not affect the speedometer. If you change your gear ratio, it is going to throw off the speedometer. 3.73 to 4.10 might not be much. 3.73 to 4.30, is more of a change. IMO, I would have the speedo adjusted. I have a 5 speed, so I cannot comment on the tranny learning and adjusting.
Junior User
Joined: Mar 2005
Posts: 81
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BlueKnight - Another option, since you have already changed the gears, get someone in another vehicle to run with you to find out if your speedo is off. That will tell you if you need it adjusted. Simple way to find out.
Elder User
Joined: Feb 2003
Posts: 881
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From: NJ ... the garbage state
is the true speed only for a super duty? or is it for various vehicles? the tire is definatly going to affect the speedometer. the gear change affecting the speedometer will be dependant upon where the signal is being picked up. if it picks it up at the trans, yes, it will make a difference. if it pics it up in the rear, post gear reduction, then no, it will not make a difference. if i am correct, the super duty pics it up via a sensor next to the rring gear, so it would in fact, not make a difference.
Junior User
Joined: Mar 2005
Posts: 81
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From what I know, they make them for more than just the SuperDuty. But when I ordered it for my SD, it gave me the conversions for gears and tires. I have not geared my yet, but it was accurate with the tire change.
Elder User
Joined: Oct 2004
Posts: 587
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The speedometer is calibrated from a sensor on top of the axle housing that counts the number of rotations of the pinion gear per mile. Changing the gear ratio does not alter the diameter of the ring gear, only the number of teeth on it. It still rotates the same number of revolutions per mile, so the speedometer stays accurate.
However, changing the diameter of the tires changes the number of revolutions of the ring gear per mile. Therefore, the speedometer must be recalibrated when changing tire diameters.
However, changing the diameter of the tires changes the number of revolutions of the ring gear per mile. Therefore, the speedometer must be recalibrated when changing tire diameters.