speedo correction
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speedo correction
Had my gears changed from 3.55s to 4.10s and running 285/75/16 tires (32.8). Stock tires were 255/70/16. Took it to the dealer to correct the speedo and they said they can only correct it to 3.73 gearing and 265/75/16 tires . This was the highest they can change it to due to specs and standards. Basically my speedo is still off by 5 mph @ 70 mph, has anybody got any input on this? Thanx Rich 
Last edited by karenj; Jun 14, 2004 at 02:01 PM.
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I don't think you should need any speedo correction for the gear change, just the tire size difference. The axle speed is read from the Vehicle Speed Sensor in the axle housing and takes it's reading from the tone ring attatched to the ring gear, which spins in direct relation to the axles, if the VSS system in your truck is at all like the system in '92-97 trucks.
If your speedo is only off by 5mph at 70mph, that's only 7.14% and the speedo discrepency is less (in MPH, % stays the same) at lower speeds. At 35mph it would be off by about 2.5mph. If you drove your truck for 100,000 miles with the odometer off by 7.14%, it would only be off by 7,140 miles. That's not much. I wouldn't worry about it, just keep the difference in mind when cops are around...
If your speedo is only off by 5mph at 70mph, that's only 7.14% and the speedo discrepency is less (in MPH, % stays the same) at lower speeds. At 35mph it would be off by about 2.5mph. If you drove your truck for 100,000 miles with the odometer off by 7.14%, it would only be off by 7,140 miles. That's not much. I wouldn't worry about it, just keep the difference in mind when cops are around...
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If you want to have it back to the right speed, you might look into a device by Superlift called Truspeed, it intercepts the signal from the tone ring and readjusts it before it gets to your speedo and computer. I know they make these for the 92-96 trucks, you'll have to check into the availability for your year vehicle.
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Thanx everyone for the replies. Socal ever since I went from 3.55 to 4.10, speedo was slower now it's faster.Let me make it little clearer 
before I was running 3.55 w/ 285/75/16 and truck was going faster than speedo was showing, now it's 4.10 w/ same tire and truck is going slower than speedo shows. It's not bad, but the only thing is I will have more miles on my truck than what it should show. Thanx Rich
before I was running 3.55 w/ 285/75/16 and truck was going faster than speedo was showing, now it's 4.10 w/ same tire and truck is going slower than speedo shows. It's not bad, but the only thing is I will have more miles on my truck than what it should show. Thanx Rich
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Depends on the location of your speed sensor, I don't know where it is on the 03's but if it is after the rear end gears (like on the wheel or on the axle behind the ring gear) than the gearing has no effect on the speedometer. If it's before the gears, like in the transmission or transfer case, than it will have an effect on the speedo.
The 92-96 trucks like I have, has the sensor on the rear end. It is not affected by gear change but it is on the tire size change (for the speedo). Head on up to the 03 forum and find out where the speed sensor is (try searching for "speed sensor" or "speed sensor location" first).
The 92-96 trucks like I have, has the sensor on the rear end. It is not affected by gear change but it is on the tire size change (for the speedo). Head on up to the 03 forum and find out where the speed sensor is (try searching for "speed sensor" or "speed sensor location" first).
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Mustang thanx for the reply, my VSS is in the axle and it takes it's reading from the tone ring on the ring gear, just like Socal decribed. Due to the fact that the ring gear is turning faster now, the VSS is picking that up, therfore speedo is fast. Since I got my 4.10s, when I took off for the first time, my speedo went up fast. I guess what I'm trying to find out is that alot of people on this site that have re-geared said they took their truck to the dealer to have their speedo recalibrated and from what I gathered it was all good. Mine was done to the highest specs and or standards to FORD and not to my current specs within my truck, 4.10s and 285/75/16 tires.
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Rich, since you said that your sensor reads off the tone ring on the ring gear, your speedo is automatically corrected for the gear change by virtue of the speed sensor reading the actual ring gear speed. No need for further correction on account of the gear change.
Tire size change is a different story, the speedo will need correction for that if you desire to correct it.
How were you measuring your actual speed versus speedometer speed? You may be thinking that your speedometer is reading slower than you are going because of the extra rpms the engine is spinning and thus the extra engine noise, you think you're going fast but your really not.
Tire size change is a different story, the speedo will need correction for that if you desire to correct it.
How were you measuring your actual speed versus speedometer speed? You may be thinking that your speedometer is reading slower than you are going because of the extra rpms the engine is spinning and thus the extra engine noise, you think you're going fast but your really not.
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Thanx Socal, that makes sence. I had the local PD radar me. But what gets me is I went from being too fast on the speedo to too slow on the speedo after the 4.10s, speedo reading faster than what I'm going. Maybe too compulsive with wanting my stuff just right 
. Rich
. Rich
Last edited by karenj; Jun 16, 2004 at 08:40 AM.
