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Recognising New And Old Style Paddle Wheels
Take a look at the pastic cowel at the back of the wheel, where the paddles exit:
Squared off - meaning it is an old style paddle wheel with bolted in paddles.

Rounded off - meaning it is a new style paddle wheel with clipped in paddles.

Paddle Resistances
This is not public knowledge, the people who know like to keep it to themselves. It's part of their evil scheme to take over the world, probably. In the years I have owned a smart, I have read many supposed truths about the values of these resistors. The problem is that all of the 3rd party gear change boards use different values, somewhere along the line the truth was lost in the confusion.
Smart Standard Paddle Wheel (squared - old style)
Horn - 0 Ohm
Up paddle - 470 Ohm
Down Paddle - 150 Ohm
Nothing pressed - no connection
Smart Standard Paddle Wheel (rounded - new style)
Horn - 2.68K Ohm
Up paddle - 1.6K Ohm
Down Paddle - 890 Ohm
Nothing pressed - 8K Ohm
Many thanks to Jan J for these figures.
Smartshift, Smartronik, Smartkits
Although the Smartshift and the Smartronik are now discontinued, they are widely believed to
share the resistances of the new 'copy' replacements sold by Smartkits and Smartarse Design.
Horn - 1000 Ohm
Up paddle - 0 Ohm
Down Paddle - 4300 Ohm
Wiring Differences
There is one major difference between standard and third party wiring.
The standard is wired in parallel, third party is wired in series.
This means is the down paddle on the third party wheel actually uses two resistors to
achieve the 4300 Ohms. It passes through a 3300 Ohm resistor before going through the
1000 Ohm resistor used to trigger the horn, therefore giving the 4300 Ohms required. 700cc Paddle Wheel The setup for the 700cc wheel is different as it has a constant resistance. 
Resistor Codes
470 Ohm is also known as 470R (Resistance), making it easier to type.
470 Ohm is stated on SMD resistors by the numbers 471, thats 47 and 1 lot of zero (multiplier).
150 Ohm is also known as 150R and 151 on SMD resistors. 15 and 1x zero (multiplier).
1000 Ohm = 1K = 1000R = 102 (10 and 2x zero as the multiplier).
3300 Ohm = 3K3 =3300R = 332 (33 and 2x zero as the multiplier).
'K' stands for thousand and is used instead of the decimal point to stop it being missed out.
It might sound daunting but I can assure you it's quite easy, i'll hold your hand, don't worry.
Resistor Colours
Because of the size of resistors, you distinguish the ratings using coloured bands.
This is ok if you aren't colour blind. Why they don't just write it on the resistor i'll never know.
The system works on four, five or colour bands. Each colour represents a number.
I will cover four and five bands here as six is rare and used for specialist resistors.
Can I Use The Horn Buttons Instead?
Yes, you certainly can. You can adapt the steering wheel so the right horn shifts up a gear,
the left horn shifts down a gear and the centre horn sounds the horn.
Switches 1, 2 and 3 are the original horn buttons, after the mod, 1 and 3 become up and down gear.
Here is the link that shows you a few ways to add your own steering wheel shift.