MyE28.com

Ok, so after moving from a 3.25 to a 3.46 diff, my speedo is reading way too high. Has anyone calibrated their speedo to adjust for a high/low reading? If so, is it possible to do it ourselves, or are special tools or equipment needed?

Changing diffs will have no effect on your speedometer reading. They use the exact same pickup wheel and sensor.

Kyle in NO wrote:Changing diffs will have no effect on your speedometer reading. They use the exact same pickup wheel and sensor.

Sorry to tell ya, but yeah it did. Added around 15 mph at cruising speed on the freeway. Unless of course the universe decided to just change my speedo calibration on the day I changed my diff....

Then you have other problems.

The diffs are designed specifically to be interchangeable without changing the speedo programming. Something else is going on.

The "pulse generator" in the diff is the same part # on every e23, e24, e28, and e30 ever sold. It reads the output shaft speed IIRC, so the gearing of the differential doesn't matter.

Did you change anything else at the same time you changed your differential?

Dunno. Exorcism needed maybe.The new diff is out of a 635, but that shouldn't(?) make any difference. I'd swap my 3.25 back in to see if it changed back, but I'm too damn lazy.
Oh well, it is what it is. I don't look at my speedo often enough to worry about it.

Jeremy wrote:
Did you change anything else at the same time you changed your differential?

Shocks and struts, that's all. I think starting last night, the world just started driving a bit faster, and I missed the memo.

So you have no quantifiable evidence that it is off? I think you should try a measured mile with a good stopwatch. At the very least, write down the road speed and engine speed across a range. Then just do the math to see if they match up within a few percent.

mooseheadm5 wrote:So you have no quantifiable evidence that it is off? I think you should try a measured mile with a good stopwatch. At the very least, write down the road speed and engine speed across a range. Then just do the math to see if they match up within a few percent.

No, my proof is my own veloc-o-meter. I drive the same route all the time, and I know how fast I've been traveling, up to yesterday. Today, on that same route, which always moves at about 65mph unless there's an accident, I looked at my speedo and it says 80. I know what 80 feels like, and this wasn't it. In fact I was part of the slower traffic. If I was going eighty, I would have been passing cars easily.

Maybe another way would be to calculate the rpm. With a 3.46 and a 260/6 trans, what should the mph be at 3000rpm? I can take that and compare it to the speedo reading and get back to youse all.

I would look here, back at the source. That's where you were just dickin around .

This is from Rod Paines site when he changed out his diff.


The metal blades opposite the ring gear (BMW call this the Pulse Spider) is the speedometer sensor trigger for the sensor housed in the rear cover. Don't forget to check the sensor terminals, including cleaning the terminals to insure good electrical contact. The signal is a simple on/off magnetic switch but it travels a long way through some pretty small gauge wire, so clean connections are important, as is the condition of the rubber boot cover, to prevent water from entering the connector socket.

Just a thought

I think you're crazy. If the speedo still works, its still reading the same as it ever was.

Only thing that it could have been is if your old diff was missing a tab or two on the pulse generating ring. That would be the first time I've heard of that.

BuzzBomb wrote:Maybe another way would be to calculate the rpm. With a 3.46 and a 260/6 trans, what should the mph be at 3000rpm? I can take that and compare it to the speedo reading and get back to youse all.

About 64mph in 4th gear with 225/50-16s and about 79 in 5th.

mooseheadm5 wrote:

BuzzBomb wrote:Maybe another way would be to calculate the rpm. With a 3.46 and a 260/6 trans, what should the mph be at 3000rpm? I can take that and compare it to the speedo reading and get back to youse all.

About 64mph in 4th gear with 225/50-16s and about 79 in 5th.

LOL...you really were going 80 pal.

Ok everyone. False alarm. Everything is fine. I think my last bag of coke had too much baby laxative. I'm better now.

BuzzBomb wrote:Ok everyone. False alarm. Everything is fine. I think my last bag of coke had too much baby laxative. I'm better now.

At least you realized it before you called us all incompetent a-holes who didn't know what we were talking about!

So I guess you need to recalibrate your veloc-o-meter, eh?

BuzzBomb wrote:Ok everyone. False alarm. Everything is fine. I think my last bag of coke had too much baby laxative. I'm better now.

You should've kept playin' 'em. This thread had epic potential.

Thanks guys, but I know when to admit I made a mistake. I just got on the fwy and compared the gps speed and the cluster speed, and it was about 3 mph off. Between the 16" tires and the gps being a mile or so per hour off, I gotta assume it's pretty close to accurate. So, it turns out after all, it was a screw loose behind the steering wheel.

BuzzBomb wrote:Thanks guys, but I know when to admit I made a mistake. I just got on the fwy and compared the gps speed and the cluster speed, and it was about 3 mph off. Between the 16" tires and the gps being a mile or so per hour off, I gotta assume it's pretty close to accurate. So, it turns out after all, it was a screw loose behind the steering wheel.

It happens to all of us at some point.

BuzzBomb wrote:Thanks guys, but I know when to admit I made a mistake. I just got on the fwy and compared the gps speed and the cluster speed, and it was about 3 mph off. Between the 16" tires and the gps being a mile or so per hour off, I gotta assume it's pretty close to accurate. So, it turns out after all, it was a screw loose behind the steering wheel.

I did this exact same thing on the way to Blacksburg last weekend, and my speedo was off 3 mph at an indicated 80 mph (GPS Speedo said 77).

desmofan wrote:

BuzzBomb wrote:Thanks guys, but I know when to admit I made a mistake. I just got on the fwy and compared the gps speed and the cluster speed, and it was about 3 mph off. Between the 16" tires and the gps being a mile or so per hour off, I gotta assume it's pretty close to accurate. So, it turns out after all, it was a screw loose behind the steering wheel.

I did this exact same thing on the way to Blacksburg last weekend, and my speedo was off 3 mph at an indicated 80 mph (GPS Speedo said 77).

That's the "standard deviation" for the two 535i cars I've had as well. It reads 2-3 mph higher than actual at 80 mph.

Jeremy