Speedo off, Odo not?
Thread Starter
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Junior User
Joined: Aug 2009
Posts: 51
Likes: 2
From: Pacific NW (Washington)
Speedo off, Odo not?
Hey everyone, I got a new instrument cluster from the 'yard a couple weeks ago that had a tach and trip meter to upgrade my cluster. Well, my old gauge read pretty much dead-on, but when I got the new one put in it seems that my speedometer reads fast. I didnt realize this until I was following my dad home while he was pulling our 5th wheel and my speed said around 65-70 area, and when I asked him he said he was doing 55-60 most all of the way home. So I was thinking maybe it had a different toothed gear inside or something cause I know you can change that to calibrate it, but my odometer seems pretty accurate and I thought the replacement gear would affect both? I'm just not sure where to start before I tear apart something I cant put back together 
and oh, I have pulled off the cable and reattached it just to make sure its on all the way.
(its an '85 F250 auto)
Thanks for suggestions guys!
and oh, I have pulled off the cable and reattached it just to make sure its on all the way.(its an '85 F250 auto)
Thanks for suggestions guys!
Fleet Mechanic
Joined: May 2009
Posts: 1,460
Likes: 6
From: Bay Area, Unfortunately
I would first calibrate it by using a known accurate speedo on the freeway. Have a friend or your dad, if his speedo is known good, drive at 35, next to you. see what your speedo says. Bring it up to 45, see what your speedo says, and then at 55, 65 and maybe even 75. Your speedo might be fine. If it needs adjustment, I would try to do it with the gear swap at the transmission or transfer case.
Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
Do not change the gear in the transmission if and only if the Odometer is accurate.
Changing the Speedometer cable gear will affect both.
The speedometer head itself will need to be calibrated. There is a spring on the inside that needs to be adjusted to bring the needle speed back down to where it needs to be. But do this only if the Odometer is Accurate. The spring is on the needle shaft, and removing the needle and faceplate from the speedometer head may be needed to gain access to the spring. Turning the spring one way speeds the needle up, and turning it the other way slows it down. It's going to be hard to adjust the speedo yourself, as it's going to be trial and error.
There are speedometer shops that will calibrate and repair speedometer heads, for a fee out there too.
Of course if both the speedo, and odometer is off, then changing the speedometer cable gear is the correct thing to do.
Changing the Speedometer cable gear will affect both.
The speedometer head itself will need to be calibrated. There is a spring on the inside that needs to be adjusted to bring the needle speed back down to where it needs to be. But do this only if the Odometer is Accurate. The spring is on the needle shaft, and removing the needle and faceplate from the speedometer head may be needed to gain access to the spring. Turning the spring one way speeds the needle up, and turning it the other way slows it down. It's going to be hard to adjust the speedo yourself, as it's going to be trial and error.
There are speedometer shops that will calibrate and repair speedometer heads, for a fee out there too.
Of course if both the speedo, and odometer is off, then changing the speedometer cable gear is the correct thing to do.
Thread Starter
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Junior User
Joined: Aug 2009
Posts: 51
Likes: 2
From: Pacific NW (Washington)
thats what I thought, and since the odometer is pretty accurate, and the speedometer is off by quite a bit I didnt want to change the gears and cause the odometer to be inaccurate.
so basically I need to adjust the spring, drive it to see how close it is, adjust it, drive it, adjust it, drive it, etc, etc until its right?
so basically I need to adjust the spring, drive it to see how close it is, adjust it, drive it, adjust it, drive it, etc, etc until its right?
Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
Unfortunately, that's exactly what you got to do, if you calibrate it yourself.
Removing the speedo from the cluster each time.
If you find out how fast you are going, you can use your tach to read your speed and make the speedometer head calibration easier.
For a loose example, if you are going 30MPH when your tach reads 2000RPM in 3rd gear, you can use the tach to calibrate the speedo, by getting the speedo to read 30MPH when your tach reads 2000RPM in 3rd. That takes a lot of guesswork out of the equation.
Your RPM, and MPH may vary, but you get the Idea hopefully.
Removing the speedo from the cluster each time.
If you find out how fast you are going, you can use your tach to read your speed and make the speedometer head calibration easier.
For a loose example, if you are going 30MPH when your tach reads 2000RPM in 3rd gear, you can use the tach to calibrate the speedo, by getting the speedo to read 30MPH when your tach reads 2000RPM in 3rd. That takes a lot of guesswork out of the equation.
Your RPM, and MPH may vary, but you get the Idea hopefully.
