Speedo TPM for disco diffs and 750 16 tyres

Hi does anyone happen to know what tpm speedo i would require to fit disco diffs to a series 3 88 with larger 750 16 tyres? I was reading this earlier
Series Land Rover Speedometers | My Series III 109 Land Rover
which would suggest that a 109 speedo would allow for the 750 16 tyres but i wasn't sure what additional drop i would require if i was to use disco diffs.

The standard speed is 1536 tpm
the 109 speedo is 1408 tpm meaning that a 128 tpm adjustment is required for the tyres.

If anyone could help me or explain how to do the calculation that would be fantastic.

thanks
aaron

I make it : 1408*3.54/4.7 = 1060.5

Now where to find such a speedo...I want one but for standard little tyres....

D

ah cheers mate, there are a few 1000tpm speedos about, i think there was an mgb one like this
http://www.ebay.co.uk/itm/Mg-mgb-12...arts_Vehicles_CarParts_SM&hash=item3f21166618

It will just depend if the speedo cable will fit.

oh and for you there is a 1200ish i think mgb one which wouldnt be too far out then.

MGB SPEEDO 1280 | eBay
thats 1280