Diff Ratio

AussieEtype · Post by **AussieEtype** » Mon Sep 16, 2013 2:37 am

My series 3 is a standard UK spec auto. Somewhere in my handbook which I cannot find at the moment I seem to remember tha UK spec cars with auto had a diff ratio of 3.0??:1.

So firstly - can someone tell me what the ? number is in the diff ratio? I thought it was either 3.02 or 3.08 but I cannot remember.

Secondly, to confirm the number where would I find this marked on the diff or what is the procedure of working it out excatly - testing input rotation vs out put rotation will not be accurate enough for my purposes.

I have put in a local getrag 5 speed that does not have a speedo drive so need to program an electronic speedo that I have grafted into a series 3 speedo housing with an original style face.

Thanks

Garry

abowie · Post by **abowie** » Mon Sep 16, 2013 4:45 am

3.07.

There should be an aluminium tag attached to one of the rear cover bolts with a fractional number on it. Top number divided by bottom gives your ratio.

abowie · Post by **abowie** » Mon Sep 16, 2013 4:48 am

MarekH · Post by **MarekH** » Mon Sep 16, 2013 7:40 am

Testing the input and output turns ratio will be accurate enough because the differential crown and pinion wheels only came in certain sized teeth, so if you think the answer is 3.5, it'll be a 3.56 or if 2.8, then it's a 2.88 etc.

kind regards
Marek

PeterCrespin · Post by **PeterCrespin** » Mon Sep 16, 2013 12:11 pm

Yup, what Marek said, because the odds are your diff tags may be missing. Available 4HA ratios were 3.54; 3.31; 3.07; 2.88 and turning the wheels will be accurate enough to decide which one is fitted. Having said that, you really don't need to worry about two decimal point accuracy! Tyre diameter changes from new to worn would change it by more.

If running an electronic speedo it ought to be possible to 'tune' the head against a GPS reading? The later XJ-S and XJ40 cars which used Getrags sans speedo drive, had a sensor in the diff cover plate reading a matched toothed ring inside. Anyone fitting a cable-less Getrag could do worse than transfer the relevant diff internals to their E diff.

Be interesting to read a write-up here of how you get on with this.

Pete

AussieEtype · Post by **AussieEtype** » Mon Sep 16, 2013 1:37 pm

Thanks for all the input - looks like 3.07 it is.

The electronic speedo I have needs to be set up via dip switches rather than calibration against a GPS - basically need to tell it how far it travels for each pulse from the sender. The sender/pickup for the is not in the gearbox but has a ring gear on the front UJ yoke of the driveshaft.

Thanks for the information.

Cheers

Garry

lowact · Post by **lowact** » Thu Jun 17, 2021 3:27 am

My diff had three tags:

46/13 was the ratio (= 3.54).
BP-L i guess is what it is, the "P-L" probably stands for "Power-Lok"
I've no idea what the 7.5 tag signifies.

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