Will changing the differential ratio alter the speedo?

[BenBS]

Hi guys!

Long time no post..

As you probably won't recall, I asked quite a while ago for some opinions on an E28 525i. Well, I bought it!

So although this question relates to an E28, it's the same thing underneath really.

So, will changing the diff for a different ratio alter the speedo reading? I'm looking at an E12 M535i LSD.

(EDIT) Nevermind to the question below, I've realised it won't fit since it uses mechanical speedo drive. But the original question still stands, will the diff change the speedo reading?

Which brings me on to my second question for the BMW gurus out there, will an E12 diff fit an E28? I don't see why not, since it seems you can bolt pretty much any BMW diff to any BMW if you change the output flanges.

Some photos for your enjoyment. :) Unfortunately the car is a bit less presentable than when these photos were taken as rust is taking hold, but that will be fixed, and it will be glorious once more!

http://homepage.ntlworld.com/astonman/picmanoulton

[Travelfeet]

According to past posts, though not my personal experience, the speed senser in the diff. should keep the speedo accurate - if the wheels/tire combination used match the size of the original application.

[Jake]

So, will changing the diff for a different ratio alter the speedo reading? I'm looking at an E12 M535i LSD.

Hi Ben,

You can swap out diffs with no affect on speedo readings. The pick up measures half shaft speed so only modifications from that point out (i.e. wheels) affect speedo readings. I have no experience with the E12 unfortunately so I can't help yah with the compatibility question.

HTH,
Jake Larsen

[Captain Case]

Hi guys!

Long time no post..

As you probably won't recall, I asked quite a while ago for some opinions on an E28 525i. Well, I bought it!

So although this question relates to an E28, it's the same thing underneath really.

So, will changing the diff for a different ratio alter the speedo reading? I'm looking at an E12 M535i LSD.

(EDIT) Nevermind to the question below, I've realised it won't fit since it uses mechanical speedo drive. But the original question still stands, will the diff change the speedo reading?

Which brings me on to my second question for the BMW gurus out there, will an E12 diff fit an E28? I don't see why not, since it seems you can bolt pretty much any BMW diff to any BMW if you change the output flanges.

Some photos for your enjoyment. :) Unfortunately the car is a bit less presentable than when these photos were taken as rust is taking hold, but that will be fixed, and it will be glorious once more!

http://homepage.ntlworld.com/astonman/picmanoulton


On the e34 the rpm/crankshaft position sensor is used for the speedo. this means that speed is calculated from rpm, which in turn means that altering any ratio after the crankshaft will change the speed reading, including diffs, tyre-circumference, etcetera.

[Captain Case]

On the e34 the rpm/crankshaft position sensor is used for the speedo. this means that speed is calculated from rpm, which in turn means that altering any ratio after the crankshaft will change the speed reading, including diffs, tyre-circumference, etcetera.

correction for the above: the rpm-sensor is NOT used for the speedo. The speedo sensor is indeed located on the diff itself. Sorry bout that...