4 to 5 speed swap speedo correction

[WantingAfunE21]

Legion,
I'm saving for parts to complete this swap, and am wondering what needs to be done to make a getrag 245 show the correct speed with the 3.64 rear end. I did not find any information on this. Thanks in advance.
-Roy

[Thecatmilton]

I think the speedometer should have no issues and remain the same. If you changed differential ratios, then I think the reading will change.

[WantingAfunE21]

I think the speedometer should have no issues and remain the same. If you changed differential ratios, then I think the reading will change.

My thought was the speedo gear in the transmission that drives the cable is for the 3.91 that came in later cars, while my car has the 3.64. Causing the speedo to be off, is this incorrect?

[Thecatmilton]

My thought was the speedo gear in the transmission that drives the cable is for the 3.91 that came in later cars, while my car has the 3.64. Causing the speedo to be off, is this incorrect?

I'm not sure, but I think the speedometers are calibrated to the differential ratios instead of the gears. This is the conclusion I came to when I tried to put a 120mph speedometer in my car. It did not read properly.

[okieflats]

My thought was the speedo gear in the transmission that drives the cable is for the 3.91 that came in later cars, while my car has the 3.64. Causing the speedo to be off, is this incorrect?

Right-O. You're going to be off a bit. You're going to be traveling faster than your gauge indicates.

[Layne]

My thought was the speedo gear in the transmission that drives the cable is for the 3.91 that came in later cars, while my car has the 3.64. Causing the speedo to be off, is this incorrect?

That is incorrect. The speedo itself is different to accommodate the different diff ratios. There's a "W factor" usually found on the back, but sometimes printed very small on the face. A 3.64 diff with E21 sized tires should be W=1.3. Basically it's the internal gear ratio of the speedo itself.

The driver gears in the transmission are all the same.

[WantingAfunE21]

That is incorrect. The speedo itself is different to accommodate the different diff ratios. There's a "W factor" usually found on the back, but sometimes printed very small on the face. A 3.64 diff with E21 sized tires should be W=1.3. Basically it's the internal gear ratio of the speedo itself.

The driver gears in the transmission are all the same.

Perfect, Thanks! I was coming to this conclusion myself as I realized the part numbers for the speedo drive gears in the transmissions are interchangeable, and the pinion gears themselves had to be similar to mesh properly.

[Somjuan]

That's what I figured, but then what makes them innacurate over time? If mine's to be believed, I go around 90-100mph at all times. Wildly innacurate

[320iAman]

You have a Getrag 240 5 speed installed from 242 4 speed, did this issue start then or was it present all along ? I know that there is adjustments on the speedometer/odometer unit,, when mine was rebuild due to a plastic gear wearing out , the re-builder asked me if the speedometer was accurate, I said yes so no adjustments done.

Randy

[WantingAfunE21]

You have a Getrag 240 5 speed installed from 242 4 speed, did this issue start then or was it present all along ? I know that there is adjustments on the speedometer/odometer unit,, when mine was rebuild due to a plastic gear wearing out , the re-builder asked me if the speedometer was accurate, I said yes so no adjustments done.

Randy

It could be this.... or he is going 90-100 at all times.

[okieflats]

If it's reading high take it apart and spray with WD-40 to lube it. Friction will make it read high. Magnetic field is supposed to move the needle. If there is friction that will make it move more and read high. Or maybe you just need to slow down.

[Somjuan]

It could be this.... or he is going 90-100 at all times.

Now we all know that's just not possible

It's been off no matter the tranny by about five to fifteen miles an hour. The faster I go, the more it's off.

[Tom D]

the speedo gear records the rotation of the output shaft and so it doesn't matter if you're running a one speed or a seven. the only thing that affects the speedo is a change in ratios after the transmission. i.e. tire profile and diff.

most auto makers set the speedo to read greater then the actual speed, including the E21.

if you change from a 3.64 to a 3.91 your speed will read 7.4% greater, but if going from a 3.91 to a 3.64 your speed will read 6.9% less. either way a 7% error is only 3.5 mph at 50 and hardly worth fretting about.

[WantingAfunE21]

Thanks Tom, I'm keeping the 3.64. I'd like to make that a limited slip at some point though

[320iAman]

When did the problem start of the speedometer being off kilter before the 240 change or after ?

I'm not saying the transmission is the cause, simply finding out whether its recent or been there for some time.

If it was after it could be running a 240 with a 3.64 diff, unless he changed to 3.91 Diff. Running a 240 with a 3.64 or 3.49 ect could jump the speedometer around.

79's ran 3.64 with 242's as I recall, 1980 marked the change to 3.91's and 245's and 1983 to 240's.. If he changed to 3.91 with 240 then its in the lines, the speedo or most likely Ithe odometer/speedometer unit.

From hemmings:

At first blush, you'd think that the 320i remained the same all its life, but this is not the case: Model year 1980 saw some invisible evolution take place under the skin. First, the engine dropped to 1.8 liters (though the 320i moniker remained), and now used a three-way catalyst and Lambda sensor to monitor the air-fuel mixture to make 100 relatively green horsepower. A five-speed Getrag gearbox with .81 overdrive replaced the four-speed, the 3.64 rear was chucked out in favor of a 3.91 rear on the five-speeds (3.64 remained on automatics). Even so, the performance difference wasn't that noticeable: Thanks to the new rear gearing, a higher redline (6,800 RPM vs. 6,400 RPM) and a weight drop of 150-plus pounds, Car and Driver was able to get their 1980 model to 60 MPH and through the quarter-mile tenths faster than their 1977 model. Other differences included changing the side-view mirror from a pedestal-mount to an A-pillar-mount and a new instrument panel that better integrated the radio and air conditioning. Base price rose to $11,465.



Randy

[Tom D]

^ do you mean when did it start displaying false speed? always.

most auto makers have optimistic speed reading, at least they did 20-30 years ago. i think it was probably pressure by law enforcement/goverment to make people thing they were driving fast in order to slow them down.

the newest vehicle i own is a 2014 bmw motorcycle and its speedo reads just around 10% fast.

[320iAman]

Yes, when did the the speedometer/odometer unit start showing 90 mph driving around town- at town speeds ? I recall he did the 240 change as I posted there two 320i's in the Roseville Pick n Pull that I seen while I was up there white one and a black or blue one and he asked me if a 5 speed transmission was in one of these, I do not recall the 3.64 to 3.91 differential or 3rd member change. He hustled up there and bingo a 240 was waiting.

His surprise nearly equaled mine when for over 5 years of looking at 320i's and finding all automatics, I found a 1980 BMW 320i Getrag 245 and while I was taking it out a 1981 or 82 320is guy showed up and told me his LSD had to be rebuilt for $900 and his 245 had to be rebuilt for $1400 or so and then he tried to tell me it was a 4 speed to change my mind about getting it, I told him I have a 1980 and its a 245 and then he just said well nice find.. The 245 I have inside on the kitchen floor and all new seals put it, including the speedo seals, its not going to be in the elements outside,its filled with T-oil and I shift and turn all the gears every now and then, I paid $153 for it out the yard and was offered $300 for it the very next day at a local BMW dealership when I was there ordering new speedo seals and there holder.
I have the shifter, shifter platform,linkage,foam, and transmission bar/holder hooked up, I got those the next day at same pic n pull.


Randy

[okieflats]

... I found a 1980 BMW 320i Getrag 245 and while I was taking it out a guy showed up and he tried to tell me it was a 4 speed to change my mind about getting it, .....

Well! He obviously didn't know who he was dealing with! He thought you were some amateur.

..... The 245 I have inside on the kitchen floor and all new seals put it, .....

So you're one of those guys? Major car parts inside the house, like a Transmission in the Kitchen? I met a 2002 owner in west Los Angeles who was like that. How does your wife feel about having a transmission in the kitchen? Does she think the transmission gear oil puts off a smell that does not complement her cooking?

[320iAman]

Well! He obviously didn't know who he was dealing with! He thought you were some amateur.

He hoped I didn't know it was 5 speed and could convice me it was a 4 speed





So you're one of those guys? Major car parts inside the house, like a Transmission in the Kitchen? I met a 2002 owner in west Los Angeles who was like that. How does your wife feel about having a transmission in the kitchen? Does she think the transmission gear oil puts off a smell that does not complement her cooking?

I have a complete cylinder head in the front room from the same car-all packaged/boxed up,someone was nice enough to take it off and put it the trunk , I snagged it and was gone,,$100( gave them a cracked core back so less than $100, ), I got another one I'm running and had resurface and so forth,,cost was $50 half off day for tool kit members and still has the plastic / wax piece on the back for warranty--over heating purposes. It took me a while to catch up on why these cylinder heads warp--ever since then no more warping or any to speak of. Hint: bimetal motor is not the reason, cast iron long blocks will do the same thing and some cases even more so.

Randy