If it doesn't start on 0 then it won't be accurate when the car is running.
This usually happens for 1 of 3 reasons.
You have changed the dials so removed the needle and put it on in the wrong place.
The needle is touching the back face which is stopping it from moving correctly.
There is a connection issue so the rev counter lost power or RPM data.
Pop the front off of the rev counter. Make sure the face is sitting flat and the needle isn't touching the face at any point. Turn the car on so the engine in running and turn it off.
Manually push the needle around to 0 then try starting the engine again.
If the needle returns to 0 every time you turn the engine off, it's fixed.
You have 2 options for the 700cc Fortwo:
1# (2 people required) Reassemble the rev counter but leave the needle
and the cover off, drive at 30mph in 3rd gear and hold it steady, get your
passenger to gently put the needle on so it faces 3000rpm.
Stop the car, ensure the needle is seated far enough down but doesn't
touch the face, reassemble the rev counter.
2# (if you have no friends) Reassemble the rev counter but leave the cover off but the needle on, as above drive at 30mph in 3rd gear and hold it steady.
Carefully nudge the needle around to the 3000rpm mark, stop the car and check again,
if it still says 3000rpm at 30mph in 3rd gear you can reassemble the pod.
You have 3 options for the 600cc Fortwo
The 600cc smart has a different set of gear ratios so 3000rpm in 3rd gear isn't going to equal 30mph. Thanks to Aaron Ng for pointing this out.
1# Do the 30mph in 3rd gear trick, stick the rev counter to 3000rpm and know it's going to be a little bit wrong but it's better than nothing.
2# When you have been driving and the car is fully warmed up, allow the smart to idle until you hear the engine settle down to standard idle and put the needle on to face 850rpm. Again, not accurate but close.
3# Get an X-Gauge (if your year of car supports it), plug it in, select rpm.
Drive along at a steady 3000rpm on the X-Gauge and move the needle to show the same.
A few people have found that their rev counter suddenly becomes wildly inaccurate,
the needle gets stuck or just doesn't work at all.
Don't panic. Firstly you need to check the plug for the rev counter (circled in red),
it is connected to the back of the speedo pod so you'll have to take it off.
Once you have gained access to the connector make sure it is pushed in firmly and retest.
If it's still misbehaving disconnect the rev counter, start the car, turn it off and reconnect.
Still not working? Take the whole thing apart, make sure the connector inside is seated
firmly and when you reassemble make sure you don't push the needle on too far, if it
touches the face it's not going to work. If it's still not working your last hope is to check
the wires with a multimeter to make sure they aren't broken inside.
If your smart didn't come with a rev gauge and you have fitted a new one there is a good chance it won't work straight away, many of the newer smart cars have to be programmed to make use of the rev pod, this can be done by smart and other specialists.