Speedo problem
#1
03-03-2012, 04:50 PM
Speedo problem
Well got the gears set in to calibrate my speedo but found out that there was only 1 size larger (21 tooth) than I had already. Thought last time I looked I had a 17 but turns out I had a 20. Problem is, speedo still shows about 12%, or so, faster than the GPS. So, where do I go from here to slow the speedometer down? New drive gear? I have a GPS I can use but really would like the speedometer to be at least close to correct.
#2
03-03-2012, 08:04 PM
Posting Legend
Well got the gears set in to calibrate my speedo but found out that there was only 1 size larger (21 tooth) than I had already. Thought last time I looked I had a 17 but turns out I had a 20. Problem is, speedo still shows about 12%, or so, faster than the GPS. So, where do I go from here to slow the speedometer down? New drive gear? I have a GPS I can use but really would like the speedometer to be at least close to correct.
Note that I haven't read these, but saw them recently and filed it away in my rememberer. And, I'm astounded that I remembered them!
#3
03-04-2012, 01:00 AM
Thanks for the link Gary. Those are some handy calculators, shows I'm about 15% off on the speedo. Problem is I already have the biggest gear on it that's available (21 tooth). And at approximately 3mph per tooth I need a 24-25 tooth. I did find a 23 tooth at the Bronco Graveyard but it says it's for the AOD so don't know if it would work in my 4 O/D trans. So that leaves me with only 2 options if I want to correct it, change the drive gear or re-gear the rear axle. Unless someone have another option I might try.
#4
03-04-2012, 07:59 AM
Posting Legend
You don't think there is any chance that the "calibration" range would bring it in? Here's a response to a question of mine:
Yes there is a way.
For the Odometer, you need to replace the cable drive gear with one of the correct color. You can get it close, but you cant get it 100% unless you are real lucky. Give or take 0.5 per mile is ideal, but 1 mile variance is acceptable.
For the speedometer, there is a spring on the front of the speedometer. You have to remove the face and the needle (being careful not to break it) and turn the spring either left or right to slow it down or speed it up.
What I have done is get a variable drill and set it up and lock it to turning the speedo at 60 MPH. Then by using a clock, make sure at 60 MPH the odometer is turning 1 mile every minute. If not adjust the spring by turning it slightly either left and right and try again. It's very tedious, you will have to speed or slow the drill up or down to match the reading of 60 MPH for every adjustment, and I don't recommend it without having patience.
After you get the speedometer calibrated with the odometer on your bench, IE turning 1 mile a minute at 60MPH...
Then you can adjust the odometer by swapping cable gears.
For the Odometer, you need to replace the cable drive gear with one of the correct color. You can get it close, but you cant get it 100% unless you are real lucky. Give or take 0.5 per mile is ideal, but 1 mile variance is acceptable.
For the speedometer, there is a spring on the front of the speedometer. You have to remove the face and the needle (being careful not to break it) and turn the spring either left or right to slow it down or speed it up.
What I have done is get a variable drill and set it up and lock it to turning the speedo at 60 MPH. Then by using a clock, make sure at 60 MPH the odometer is turning 1 mile every minute. If not adjust the spring by turning it slightly either left and right and try again. It's very tedious, you will have to speed or slow the drill up or down to match the reading of 60 MPH for every adjustment, and I don't recommend it without having patience.
After you get the speedometer calibrated with the odometer on your bench, IE turning 1 mile a minute at 60MPH...
Then you can adjust the odometer by swapping cable gears.
#5
03-04-2012, 03:11 PM
I'm not sure if the calibration is off but that's not what caused the problem. I might be able to adjust the speedo but not sure how I would have to go about doing that. That's because the speedometer and odometer are both off. It would require that I uncalibrate it enough to compensate for the difference but, the odometer would still be off. The problem is that my truck came with 2.47's and 215 tires. It now has 3.55's and 235 tires. There just isn't a driven speedometer gear large enough to correct for that large a rear gear change.
#6
03-04-2012, 03:21 PM
Posting Legend
#7
03-04-2012, 03:30 PM
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#8
03-05-2012, 02:53 PM
Lead Driver
kedwinh,
Been there, done that!
1984 F150 2.47 gears changed to 3.55 several years ago and that is a 43.7% change. I, too, went from 215/75 to 235/75 tires. At 55 mph, the speedometer indicated 79 mph!!!
I took the truck to a speedometer calibration shop where they cut the speedometer cable and installed a "unit" which corrected the speedometer. This "unit" is relatively small and is like a small transmission and reduced the cable speed appropriately.
This shop had been in business for years and calibrated police and State Trooper vehicles. I was given a certificate stating that the speedometer was completely accurate @ 55 mph.
You might go online and look under speedometer calibration.
Been there, done that!
1984 F150 2.47 gears changed to 3.55 several years ago and that is a 43.7% change. I, too, went from 215/75 to 235/75 tires. At 55 mph, the speedometer indicated 79 mph!!!
I took the truck to a speedometer calibration shop where they cut the speedometer cable and installed a "unit" which corrected the speedometer. This "unit" is relatively small and is like a small transmission and reduced the cable speed appropriately.
This shop had been in business for years and calibrated police and State Trooper vehicles. I was given a certificate stating that the speedometer was completely accurate @ 55 mph.
You might go online and look under speedometer calibration.
#9
03-05-2012, 03:59 PM
#10
03-05-2012, 06:27 PM
Look at some of the places that cater to road rally folks, they used to carry the little gear boxes. Now you know why GM and Chrysler went to really large diameter speedometer driven gears. If you want to change the drive gear, you need to know whether it is built into the output shaft or not. If it is built in, you have a problem.
#11
03-05-2012, 06:43 PM
Posting Legend
#12
03-05-2012, 08:07 PM
I have the SROD 4o/d trans. Don't know if the drive gear is removeable or not. But think I'm going to track down a speedo shop and see about an adapter like 1986F150six was talking about. I'm off a little over 15% right now and running the biggest driven gear I can find, 21 tooth. So unless I can find a shop that can do that I'm looking at new rear gears, new drive gear or just live with it.
#13
03-06-2012, 11:52 AM
D8TZ-17285-A .. 2.75 thru 3.55 8T Nylon DRIVE Gear-Gray / Obsolete / 613 (!) available NOS ~ Closest sources to Arid-Zona are in TX
ANTIQUE AUTO SUPPLY in Arlington TX has 1 = 817-275-2381.
PARTS INTERNATIONAL in Farmers Branch TX has 2 = 888-727-0418.
KINSEL L/M in Beaumont TX has 2 = 800-816-2894.
But it may be cheaper if bought in Concord NC: CARPENTER NOS OBSOLETE PARTS has 569 of the little charmers: 800-476-9653.
> > This is what your truck have now: E1TZ-17285-B .. 2.47 9T Nylon DRIVE Gear-White. < <
#14
03-06-2012, 05:35 PM
Alright! Thanks NumbersDummy. With the hunting I have done on this I figured that was the problem. Just couldn't get up high enough with the trans in the truck to see what was in there now. I did find a speedo shop up in Northern Phoenix that does the calibrations for $100. But where I am to where he's at is a 150 mile round trip, or about $50 in fuel. I will give them a call.
Your little book doesn't tell how the gear is held on does it? Unless it's pressed on I might be able to do it without dropping the trans. But even if it is that gives me a reason to change the tranny fluid.
As for Arid-Zona, it's still winter here. Only got up to 82 today..
Your little book doesn't tell how the gear is held on does it? Unless it's pressed on I might be able to do it without dropping the trans. But even if it is that gives me a reason to change the tranny fluid.
As for Arid-Zona, it's still winter here. Only got up to 82 today..
#15
03-06-2012, 06:23 PM
Kedwinh, it will either have a snap ring behind it or a little metal spring clip that holds the gear on the output shaft. Once you pull the extension housing off, the gear is right there. If you need specifics on removal I can probably find something for you either in AllData or one of my Ford manuals.