Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
STOP!! Never post your email address in open forums. Bots can "harvest" your email! If you must share your email use a Private Message or use the 
smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
I need to recalibrate my speedometer for my 1967 BJ8 after changing the rear end to a different gear ratio. What are the steps to remove it from the dash? How does one remove the long stem that resets the odometer? Do I need to disconnect the speedometer cable first from the back or after I pull the unit forward?
-
Hi Guest!
You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
There are some perks with a member upgrade!**Upgrade Now**
(PS: Subscribers don't see this gawd-aweful banner
Speedometer removal
- Thread starter Goldie
- Start date
-
- Tags
- speedometer
Share this page
Healey Nut
Luke Skywalker
Offline
Fairly sure a rear end ratio change will not require speedo recalibration , now if you change tire size etc then that's different . Changing the rear end ratio changes the motor rpm not the distance travelled by the number of wheel rotations . I'm no science major so some of the other whizzes on here can chime in and correct me .
A simple call to Nisonger Instruments and Peter can confirm it .
If if you insist on removal it's a 5 min job disconnect the cable pull the panel light bulb and remove the thumb screws and bracket . You don't need to remove the reset stem for the odometer .
D
Duane_Rhynard
Guest
Guest
Offline
Goldie,
Anthony Rhodes has put together a nice article on speedometers (Link Below), I hope it helps!
https://obswww.unige.ch/~wildif/cars/docs/Smith-jaeger_speedo_repair.pdf
Bob_Spidell
Yoda
Offline
It does. With a taller rearend--e.g. going from a 3.91 to a 3.54--you get approx. 11% more rotations of the rear wheels--and more distance traveled--for the same gearbox/OD output speed (and vice versa). The speedometer can be changed by moving the indicator needle and/or tweaking the watch/return spring (I've never dared mess with that). The odometer and trip meter require a changing of their driven gears. Or, you can get a ratio adapter that goes inline with the cable.
Keoke
Great Pumpkin
Offline
Goldie:
Disconnect the speedometer cable first from the back.
Then remove the speedo's retainer bracket and pull the speedo out.
Then I call Nisongner and tell them what i need them to do.
Course now you down there in Virginia and White Post is just around the corner from you.
Disconnect the speedometer cable first from the back.
Then remove the speedo's retainer bracket and pull the speedo out.
Then I call Nisongner and tell them what i need them to do.
Course now you down there in Virginia and White Post is just around the corner from you.
Offline
Nisonger will confirm that either a tire size change or axle ratio will require re-calibrating the speedometer.
Peter will ask you to count the revs of the speedo cable when you push the car 52'-9.5".
This will handle your current axle/tire setup.
Friends and I have been through this several times.
Offline
i just done this and it's easy. The hard part was paying...I had to have a new expensive thingy installed in my speedo.
Offline
Let me correct you. The speedometer connects to the tail shaft of the transmission and effectively reads the driveshaft rotational speed, not the wheel rotation.
Healey Nut
Luke Skywalker
Offline
Correct that's why I don't think you need to recalibrate as the rotational speed of the transmission doesn't change with the different rear end gear .
Offline
How do you figure that? For each revolution of the rear wheel with a 3.9 rear end, the transmission output shaft rotates 3.9 times. With a 4.11 rear end, it rotates 4.11 times. That sounds different to me.
RAC68
Darth Vader
Offline
I totally agree with John but was slower to respond
The speedometer does read the rotations of the transmission tail shaft but, keep in mind that the actual speed of the car is determined by the distance covered per unit of time (miles/hour). In this case, we are changing the rear ration and this change will alter the number of tire revolutions per trans tail shaft revolution. As a result, the 3.54 rear will turn the driving wheels a greater number of revolutions then the original 3.9 rear and result in a lower then actual speedometer reading.
Ray(64BJ8P1)
The speedometer does read the rotations of the transmission tail shaft but, keep in mind that the actual speed of the car is determined by the distance covered per unit of time (miles/hour). In this case, we are changing the rear ration and this change will alter the number of tire revolutions per trans tail shaft revolution. As a result, the 3.54 rear will turn the driving wheels a greater number of revolutions then the original 3.9 rear and result in a lower then actual speedometer reading.
Ray(64BJ8P1)
D
Duane_Rhynard
Guest
Guest
Offline
The rotational speed of the gearbox does not change but your speed over the ground does! That is why you need to recalibrate the speedometer when you change rear end ratios or tire size. If you have an original parts manual, you will see their are different speedometers for the different rear end ratios.
Offline
I've recalibrated on the following occasions: 3.54 rear end; Toyota .85OD trans & 185-70 tires; Toyota .78 OD Supra trans & 185-70 tires; conversion to 180-85 tires - each time the speed of the car relative to the revs of the transmission tailshaft changed. The speedo is basically a tach that measures engine revs over distances which can vary according to axle ratios and tire size.
That's why it's important to measure over the 52+ ft per above as Nisonger and the other outfits recommend.
If you have a Garmin, you can verify these changes.
That's why it's important to measure over the 52+ ft per above as Nisonger and the other outfits recommend.
If you have a Garmin, you can verify these changes.
Bob_Spidell
Yoda
Offline
... but the distance you move forward increases approx. 11% for the same rotational speed of the gearbox--going from a 3.91 to a 3.54--hence the speedometer will indicate 11% low. When I put a 3.54 in my BJ8, I had the speedo recalibrated--by Klaus at Palo Alto speedo--and he put the new gears in for the odometer and tripmeter, but neglected to recal the speedo itself. A friendly CHP informed me of the ommission.
Put another way, it takes 3.91 rotations with the stock rearend to move the rear wheels one circumference--approx. 82" with a 26" dia. tire--but only 3.54 rotations with the taller rearend, so your speedometer 'thinks' you're going slower down the road than you are.
bob hughes
Luke Skywalker
Offline
Nice one Keoke,
I just use the local speed recorder mounted on a lamp post, not connected up to any recording device, just advises you of your speed :angel:- check it against your speedo and that's it Q.E.D.
Bob
I just use the local speed recorder mounted on a lamp post, not connected up to any recording device, just advises you of your speed :angel:- check it against your speedo and that's it Q.E.D.
Bob
Changing the rear end ratio will effect the speedometer reading and require recalibration. The speedometer is calibrated for a specific rear axle ratio such that a specific number of turns on the driveshaft will result in a specific speed based on the speed of the rear wheel. By changing the differential ratio the tire speed (rpm) will change in relations to driveshaft speed (rpm). Said in more understandable way, the distance covered by the vehicle in in the same amount of time will be less or more based on the ratio change as Bob mentioned above. Since speed is a measure of distance per unit of time, whenever you change the time required to cover a distance or the distance itself the actual speed is altered. In addition, it should be noted that calibrating the needle to tweak it to the proper speed is able to be done when the speedometer gears are correct for the ratio of the differential. You will notice that the speedometer has numbers on the face, which indicate the differential ratio it will work correctly for. Without having that correct the odometer reading will also be inaccurate. The miles registered under the different ratio will be less or more based on which way the differential ratio was changed. As indicated by others in the above posts, there are some good websites on this. Nisonger in Mamaroneck can answer what needs to be changed to correct for the change in differential ratio. I hope I didn't confuse the issue.