Speedometer recalibrate for different diff ratio - DIY?

[Stevehose]

Has anyone done this yourself? I know these can be sent out along with a big check to one of the speedo rebuilders, but was wondering if this is a possible DIY. Since going to a 3:45 diff, my USA 3:64 speedo is at least 6mph too slow and is eventually going to get me in trouble with the law.

 

[eriknetherlands]

Unfortunately no. But I'l likely have to do it as well. Taking a guess, the speedo has to be pried open, mechanicals reviewed, and a gear needs to be swapped out?

 

[deQuincey]

Has anyone done this yourself? I know these can be sent out along with a big check to one of the speedo rebuilders, but was wondering if this is a possible DIY. Since going to a 3:45 diff, my USA 3:64 speedo is at least 6mph too slow and is eventually going to get me in trouble with the law.

oh, that is fine, if you have identified the difference then you can simply slightly turn the scale counter-clock wise the equivalent of 6mph,
and to make it absolutely elegant, then you turn the whole instrument clock wise the same angle so that noone would notice

(p.s. hereby i declare that this is a joke, just to clarify it to the censor without-senseofhumour that is out there)

 

[adawil2002]

Yes, I have done this myself according to the guide I received from North Hollywood Speedometer.
Need to include differential ratio, tire & wheel sizes & overall average distance of 4 wheel-rotation- index results.

For wheel indexing:
Needs to be on a flat/level surface.
Place a piece of tape or a chalk mark on the floor or pavement & index that mark on the front tire.
Push the car so the indexed wheel does 2 full revolutions then mark that.
Measure the distance from the 1st mark to the 2nd & write it down.
Will need to push the car back to the 1st mark after 2 full revolutions, the mark should line up exactly with the 1st. If not, will need to add the difference.
Need to push back to the 2nd mark, write distance.
Push back to the 1st again & write the distance.
Average the 4 results & send all the measurements & overall average to your preferred Speedometer Shop.

Fair warning, despite doing this, Veronika our 1968 1600 Cabriolet, I was told the gear needed doesn't exist. So despite working. It was not properly calibrated.
Athena's speedometer is correct 100kph = 62.5 mph. Veronica's is way off 100kph = 55mph so I use a Garmin GPS as the speedometer now, even though the odometer is spinning faster than it should.

 

[TodB]

Silly question, if the speedo is off is the odometer also incorrect?

 

[adawil2002]

Silly question, if the speedo is off is the odometer also incorrect?

Yes. With older BMWs the speedometer is driven by the transmission based on the differential & 14 inch wheel with a 195/70 14 tire. If one changes wheels & tires to +1 or +2 the overall wheel tire should be as close as possible to revolutions per mile as the original set up.
265 5-speed gear ratios are only different from the 4-speed as 5th gear is .08:1 as the "overdrive" increasing fuel economy from 17.5 mpg to 23 mpg.

If one only changes from 14" 195/70 is 815 revs/mile to 16" 205/55 to 811 revs/mile the 4 fewer revs/mile will .06 to 1km or 1 to 1.3 mph faster on the speedometer.

 

[rsporsche]

(p.s. hereby i declare that this is a joke, just to clarify it to the censor without-senseofhumour that is out there)

and just who might you be referring to? i for one could see your tongue firmly implanted in your cheek from across the ocean ...

 

[rsporsche]

Silly question, if the speedo is off is the odometer also incorrect?

yes

 

[rsporsche]

Steve, if i remember correctly its only 1 tooth different in the speedo between 3:45 and 3:64. you might be able to call odometer gears and get one of each ... and put the one with 1 more tooth in than the one you take out.

 

[Dick Steinkamp]

Steve, if i remember correctly its only 1 tooth different in the speedo between 3:45 and 3:64. you might be able to call odometer gears and get one of each ... and put the one with 1 more tooth in than the one you take out.

Good point since you can't do adjustments at the "fractions of a gear tooth" level.

 

[rsporsche]

the gears are 25 bucks each ... so you don't want to be buying a bunch of different sizes. Steve, try to take the speedo apart and count the teeth on yours. Odometer Gears might be able to help.

 

[deQuincey]

and just who might you be referring to? i for one could see your tongue firmly implanted in your cheek from across the ocean ...

oooh, not you mon ami….but yes I have something in mind

 

[Stevehose]

am contemplating the pros and cons of this operation, mostly cons at this point so may just live with it for a while.