Speedo Gear Drive For 3.55 Diff.

[redskatejbf]

I have a 3.55 Diff. that I will fit in the near future, but I do not know the correct nylon gear drive for the speedo. Do I need a 23 tooth black gear or a 25 teeth blue gear?. The G/box and wheels / tyres are standard original fitment. Could someone advise me please.

Regards, John.

[ppnelan]

You need the black 23 tooth one.

Matthew

[bcmc33]

I have been thinking about this subject since taking my diff apart recently.

The crown wheel has 39 teeth and the pinion 11 teeth which makes it a 3.545:1 diff. Brian Buckland notes this as 3.54:1, and says as a late Ford change for RS2000/Capri it can also be fitted to the Elan.

When my car is back on the road I plan to conduct GPS speed tests to see if the speedo is correct.

If it isn't correct, and as I plan to fit Minilites and new tyres, the periphery of which may be the secret in getting the speedo more accurate.

Comments most welcome.

[msd1107]

A previous post:
http://www.lotuselan.net/forums/viewtopic.php?t=16106
had a spreadsheet for determining speedometer drive and driven gear teeth numbers.

The ford transmission has a 7 tooth drive gear.

For 155 series tires, either 3.54 (11/39) or 3.55 (9/32) use a 23 tooth driven gear, as has already been said.

If you go to the Spyder Minilites at 185/60-14, you are in luck as that combination will use the same 23 tooth gear.

If your speedometer is accurate now, it does not need to be recalibrated, as the tire rev/mile figures are almost identical between 155-13 and 185/60-14.

For those who are curious, a 3.77 (9/34) takes a 24 tooth gear, a 3.9 (10/39) takes a 25 tooth gear, a 4.11 (9/37) takes a 26 tooth gear, and a 4.56 (9/41) takes a 29 tooth gear.

David
1968 36/7988

[ppnelan]

Does that answer your question, John

Matthew

[SADLOTUS]

Tyre sizes will also effect your speedo calibration. About quarter of the way down is a tyres size calculator to check the rolling radius and tyre size alternatives of those 18inch chromies anyone may be contemplating.

http://www.carbibles.com/tyre_bible_pg2.html

[redskatejbf]

Wow... What can I say, but thanks very much to you all. QED as they say in Latin. (I think).

Very much appreciated, John.