Or buy for the right speedo gear for the 4L60. Same as you would do with a TH400, TH350 etc. You may need the right housing for the gear depending upon how many teeth you need. People will have them locally too like Rare Spares. Or get a housing and gear off an old TH350 or TH400. You'll have to pull yours out and count the teeth, work it logically. If yours is reading 120 at 100 then you are out by 20%, so you need 20% more teeth on the gear, or vice versa.

http://www.ebay.com.au/itm/700R4-4L6...item4845e47b5e