Which Speedometer Gears
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Joined: Jan 2004
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Which Speedometer Gears
2004 F150 Heritage 4.2 V6 M5ODM2 Transmission
Changed the differential from 3.08:1 to 4.10:1 to improve towing (and it did) and now need to change the output to the speedometer. This a 1/3 up and 1/4 down in real speed vs indicated speed i.e. 45 mph real is 60 mph indicated (1/3 of 45 is 15. 45+15=60 or 1/4 of 60 is 15 and 60-15=45).
I assume that I need a speedometer driven gear with 25% more teeth. The logic being that I am slowing the speedometer down so I use the 1/4 down side of the equation.) Is that correct?
Will I need to change the drive gear in the tailshaft?
Thanks for the help.
Changed the differential from 3.08:1 to 4.10:1 to improve towing (and it did) and now need to change the output to the speedometer. This a 1/3 up and 1/4 down in real speed vs indicated speed i.e. 45 mph real is 60 mph indicated (1/3 of 45 is 15. 45+15=60 or 1/4 of 60 is 15 and 60-15=45).
I assume that I need a speedometer driven gear with 25% more teeth. The logic being that I am slowing the speedometer down so I use the 1/4 down side of the equation.) Is that correct?
Will I need to change the drive gear in the tailshaft?
Thanks for the help.
Lead Driver
Joined: Sep 2006
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A check of your owner manual shows the truck never had an option for 4:10 rear.
Thus maybe no speedo gear for that ratio.
A possible problem is the transmission is not very strong.
A low gear like that puts a lot of Torque strain on the gears in the transmission and maybe why the factory would not offer a 4:10 in that build.
You may have to find a solution from the aftermarket or some other vehicle that uses the same basic trans with a wider speedo gear selection..
Good luck.
Thus maybe no speedo gear for that ratio.
A possible problem is the transmission is not very strong.
A low gear like that puts a lot of Torque strain on the gears in the transmission and maybe why the factory would not offer a 4:10 in that build.
You may have to find a solution from the aftermarket or some other vehicle that uses the same basic trans with a wider speedo gear selection..
Good luck.
Mountain Pass
Joined: Apr 2017
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I'm not sure about your transmission, but my 2001 gets it's speed from the sensor and ring in the differential and not a gear in the transmission. Does your sensor even have a gear at all? I think you need to recalibrate the speedo setting with a programmer or have the dealer do it.
As to the other comment I think the torque increase is actually on the axle shafts and not the drive train before the rear end gear reduction. The torque should stay the same before the differential.
As to the other comment I think the torque increase is actually on the axle shafts and not the drive train before the rear end gear reduction. The torque should stay the same before the differential.
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
There hasn't been a cable driven speedo in an F150 since about 1992, if your speedo is off you need to recalibrate it electronically but I suspect it isn't off any more than it was before the gear change since speed input is sourced from the VSS/ABS tone ring in rear axle and gear ratio doesn't affect it.
Mountain Pass
Joined: Apr 2017
Posts: 170
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There hasn't been a cable driven speedo in an F150 since about 1992, if your speedo is off you need to recalibrate it electronically but I suspect it isn't off any more than it was before the gear change since speed input is sourced from the VSS/ABS tone ring in rear axle and gear ratio doesn't affect it.
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Several subjects to cover here.
M5OD-R2 Tansmission is rated for 330 FPT
4.2 V6 is rated for 260 FPT
So the transmission is only operating at about 78% of its rated capacity. Also note that rated capacity is normally about 1/2 to 2/3s of tested mean failure so expected failure within one standard deviation would be somewhere near 500 FPT. The only issue is the transmission can be slammed harder with the 4.10 than the 3.08 differential but I don't slam equipment of any type.
The speed input source on this truck is not the sensor ring on the differential, that ring is for the rear wheel ABS, but one off the transmission tail shaft, or so Ford tells me. Ford also tells me they are locked out from speed changes of any type and to get an after market computer controller.
M5OD-R2 Tansmission is rated for 330 FPT
4.2 V6 is rated for 260 FPT
So the transmission is only operating at about 78% of its rated capacity. Also note that rated capacity is normally about 1/2 to 2/3s of tested mean failure so expected failure within one standard deviation would be somewhere near 500 FPT. The only issue is the transmission can be slammed harder with the 4.10 than the 3.08 differential but I don't slam equipment of any type.
The speed input source on this truck is not the sensor ring on the differential, that ring is for the rear wheel ABS, but one off the transmission tail shaft, or so Ford tells me. Ford also tells me they are locked out from speed changes of any type and to get an after market computer controller.
Lead Driver
Joined: Sep 2006
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Just to comment, the extra load on the transmission from towing and the lower gear is the combined weight of the truck and tow load.
The motor rpm moves up higher in its torque band than stock.
The tow weight is on the end of the whole thing.
The trans is in the middle.
It has to pass the higher engine torque as well as move the total weight so its not so simple.
The lower gear does mechanically multiply the torque but also makes the engine run higher rpm for the same road speeds as well as move the total weight.
I assumed initially you were dealing with a speedo gear but in an 03 the PCM sends the data to the dash for speedo readout.
You might make a cover to put over the speedo with new calibration marks as a last resort.
Good luck.
The motor rpm moves up higher in its torque band than stock.
The tow weight is on the end of the whole thing.
The trans is in the middle.
It has to pass the higher engine torque as well as move the total weight so its not so simple.
The lower gear does mechanically multiply the torque but also makes the engine run higher rpm for the same road speeds as well as move the total weight.
I assumed initially you were dealing with a speedo gear but in an 03 the PCM sends the data to the dash for speedo readout.
You might make a cover to put over the speedo with new calibration marks as a last resort.
Good luck.
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@Bluegrass 7
You are absolutely correct, it is the load and the strength of materials doing the towing. Right now I think the weakest link in the system is the clutch. I put in the 4.10 to do exactly as you mentioned to have more RPM and to get into the higher part of the torque curve.
I have 40K on the system (both with the original 3.08 and the 4.10) and have had no issues. I alway try to be gentle on the system and never slam the clutch or gears. Grew up with a 4 Spd 57 Chevy 1/2 ton with bad syncros and had to learn early on to double clutch and match engine RPM to gear and vehicle speed to shift without grinding gears. Those skills and habits have served me well.
You are absolutely correct, it is the load and the strength of materials doing the towing. Right now I think the weakest link in the system is the clutch. I put in the 4.10 to do exactly as you mentioned to have more RPM and to get into the higher part of the torque curve.
I have 40K on the system (both with the original 3.08 and the 4.10) and have had no issues. I alway try to be gentle on the system and never slam the clutch or gears. Grew up with a 4 Spd 57 Chevy 1/2 ton with bad syncros and had to learn early on to double clutch and match engine RPM to gear and vehicle speed to shift without grinding gears. Those skills and habits have served me well.
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