Converting to a GPS based speedometer

Bmachine

Bmachine

Well-Known Member
Site Donor $
Site Donor $$
Messages
3,729
Reaction score
2,041
Location
Northern California coast
So the main reason we cannot use a 5 speed out of an 80's car is because of the lack of mechanical speedo drive on the bell housing.

Then how about this option: There are plenty of GPS speedo apps for the iPhone. Is there a way to connect that to a digital-to-mechanical converter box like this one:

http://www.dakotadigital.com/index....ategory_id=-1/home_id=-1/mode=prod/prd886.htm

This particular one only lists Ford and GM speedo cable connectors but I imagine someone offers a VDO one.

Has anyone done that? If so, this would open up a larger set of 5 speed bell housing options while retaining the original speedo in the car, wouldn't it?
 
Another option might be to find an early BMW electronic speedo with similar gearing and use the e9 face on it...

Then find an interface between the iPhone GPS app and that electronic speedo.
 
Bo, the speedo drive can be added to the later boxes, it is the same whether 4 or 5 speed. Just need to make sure the ring gear is on the trans shaft.
 
I can confirm that the 265/5 dogleg or 265/6 OD units will accept the mech speedo drive, the latter being in the '80-'82 and 85 M30, as well as the 1982 528e (different bellhousing). The 260/5 from the 83/84 M30s may have retrofit capability but the 260/6 from the 86 and up cars definitely does not.
 
You also need to match the speedo to the differential. So - if the 5 speed OD comes from a car that had a 3.07 differential, and your car now has a 3.45, the reading of the speedo will be ~12% higher than actual speed - unless the speedo is re-calibrated by someone like Hollywood Speedo. My car has the 5 speed overdrive, and the differential in the car when I purchased it was 3.64. The speedo read 20% faster than the road speed. I changed to a 3.07 diff and now it is exactly correct via roadside radar and GPS. The engine makes enough torque and horsepower to still feel strong with the diff change. A 3.25 would have left me with a 6% higher speedo reading than road speed unless I'd removed the speedo and had it re-calibrated..
 
Last edited:
Gary Knox said:
You also need to match the speedo to the differential. So - if the 5 speed OD comes from a car that had a 3.07 differential, and your car now has a 3.45, the reading of the speedo will be ~12% higher than actual speed - unless the speedo is re-calibrated by someone like Hollywood Speedo. My car has the 5 speed overdrive, and the differential in the car when I purchased it was 3.64. The speedo read 20% faster than the road speed. I changed to a 3.07 diff and no it is exactly correct via roadside radar and GPS. The engine makes enough torque and horsepower to still feel strong with the diff change. A 3.25 would have left me with a 6% higher speedo reading than road speed.
Click to expand...

From BMW2002Faq.com. When the time comes i'll compare the backside of both of my speedos. I presume things work the same on an E9 as they do on a 2002, and it is quite easy to recalibrate given the right parts...

The speedo/odo is made up of two major parts. There are the display wheels. These are a set of numbered wheels with dogs between the wheels to keep them from turning backwards and to allow the lower digits to sequentially advance the higher digits. The speedo needle is driven directly. All the display wheels are the same independent of the final drive ratio of the car.

The 'head' is what contains the gearing that translates a revolution of the speedo shaft into needle movement and rotation on the worm gear that drives the numbers. Fortunately this head is held onto the display portion of the speedo/odo by two screws. You can change the 'gearing' of the speedo to better correspond to your final drive by changing out the drive 'head'

Say you have a '72 with a 3.90 E21 LSD, in theory, all you need to do is get a '76 speedo, remove the head from the display, put it onto your speedo/odo and you're set.

The craft portion comes from the fact that building speedo/odo 'heads' seems to be an imprecise art. As best I can figure, VDO builds and then tests the heads and then stamps the appropriate ratio on the metal backing plate. There is a range of ratio's that seem to be available, and the calibration of any particular 'head' seems to be a little suspect.

I've done this with a W=1.393 speedo (from a '76) and it registers about 10% fast with my setup (185/70-14). I also have a w=1.3 speedo that I'm going to try this weekend.

I have some pictures if you're interested, contact me off list and I'll send them to you.
Click to expand...
 
Gary,

All US 3.0CS came with the 3.64 differential, your speedo must have been swapped before you got her. It is common for them to be off around 5% though.
 
Chris,

The takeoff for the speedo is on the back of the transmission. Thus, I think it is configured to the differential (and to a small degree, the tire circumference) of the vehicle in which that transmission originated. Thus, a 265/6 trans, such as mine from an early e28 would be geared to produce the rotation of the cable in relation to the mph of that diff and tire circumference, and I believe those cars had a 3.07.

So - I believe the speedometer gear, the transmission take off gear and the differential ratio/circumference all need to be coordinated to have an accurate mechanically driven speedometer.
 
I looked into this a while back and found some info I posted here:
http://www.e9coupe.com/forum/threads/getrag-260-6-overdrive-5-speed-1988.20991/#post-153177

I did manage to get a Getrag 265/6 without a speedo drive and then sourced the parts from Carl at La Jolla. It was not cheap but since the parts are much in demand and in short supply, I felt it was better to have pros who know exactly what I needed source them for me. I would have searched for a considerable time before locating them myself.

Ian
 
Whether it is a 1 speed, 3 speed, 5 speed or whatever speed there is no impact on the speedo. The gear inside is the same for the speedo in the early and late trannys. It is simply counting revolutions of the driveshaft.
Speedos are keyed to diffs ring and pinion ratios.
What IS impacted by trannys ratios and gears is the engine speed relative to mph.
 
Maybe I'm 'lame' on this, (and I'm really stupid to differ from sfdon!!!), but we agree that the driveshaft rotates at a different (higher) speed with a 3.64 at 60 mph than it does with a 3.07 at 60 mph.

Thus, since it is the direct connection to the drive shaft that is sending a signal (mechanically in our case) of driveshaft rpm to the speedometer, it seems to me that both the rpm of the engine, and the rpm of the shaft turning the cable drive to the speedometer are affected by the ratio of the differential that is driving the car. Slower rpm of the driveshaft on a 3.07 means slower turning of the speedo cable drive, means slower rotation of that shaft at the speedometer, resulting in less circular rotation of the needle (lower indicated speed).

Must be a hole in that logic somewhere!!!

The only REAL FACT I know is that my speedometer indicated a speed that was ~20% higher than the GPS speed with the 3.64 diff. With the 3.07 diff, it indicates the same speed as the GPS. NO CHANGES anywhere other than the diff. By the 4th grade math I took 69 years ago, 3.64 divided by 3.07 equals 1.19. Thus, at 2500 engine rpm, the car is traveling 19% faster.

Cheers guys,
 
Last edited:
As Don stated, engine and trans rpm does not matter, it is output shaft/driveshaft RPM that drives the speedo. Again, US coupes had the 3.64, it is hard to believe the speedo could be off 20% if the speedo was also original. I believe you, not questioning that.
 
Last post, as my old stubborn Scottish head believes the output speed of the tranny (which drives the speedo) changes the speed of the car when the differential ratio or the tire circumference changes.
 
All this sort of becomes irrelevant with a GPS driven speedo, right?

As long as the mechanical speedo is calibrated to display exactly what the GPS tells it to, you can change your diff ratio or tire size all you want I would think.
 
OCCoupe said:
North Hollywood Speedometer can modify your speedo to work off a GPS unit. Call them and they will give you the details.
Click to expand...

Sounds great Mike. I wonder if they have an interface with the iPhone. That would be one less box to buy. Of course, then you have to bring and hook up your phone everytime you drive the car. But If you have your previous generation phone, you could leave it in the car permanently. IIRC the GPS on the phone works even if that phone is no longer connected to a mobile provider. (<-- Am I wrong on this?)
 
Last edited:
You must log in or register to reply here.
Back
Top