Whether you just like to keep a close eye on how your engine is running, or fancy profiling your own needles, fitting an air/fuel gauge is avery worthwhile investment. For the small cost of buying and fitting one it could save you a lot of money in the long run. It will let you know when your engine is running lean, which is vital on any engine especially a Turbocharged one.
It will also allow you to set your fuelling up correctly and make your own needles to suit your engine, allowing you to constantly develop your engine .

You will need both the air/fuel gauge and a Lambda sensor. The Lambda is a device that is fitted into your exhaust pipe and connects to the gauge. This reads the ratio of air and fuel in your exhaust gases, and allows you to analyise them via the gauge.

There are two types of gauge/sensors available, the narrow band type & the wideband.
The wideband type is far more accurate & is the better option, though they can cost about 6x more than there narrow band counterpart, though the prices have dropped quite considerably over the last few years.
If you can afford it the wideband is the one to have, if you can't then the narrowband is still much better than nothing. They can be bought for a little as £25 for the gauge & the lambdas can be removed from cars at the scrap yard. Otherwise it's possible to make your own, as shown below.

Wideband Air Fuel Gauge.
I'm currently using the AEM wideband gauge & Bosch sensor. This is an excellent piece of kit & allows very accurate metering of the fuelling.

More info soon !

Narrow Band Air Fuel Gauge.
Air fuel gauges can be bought quite cheaply from places like ebay for around £30 for a basic one. Though the one i bought features nineteen LED's giving a greater range for accuracy and was £100.
Whichever you choose, any gauge is certainly better than none !

Pictured here is a homemade air/fuel gauge made by a friend of mine Gregor.
The LED's going from lean to rich.

LAMBDA SENSOR.
Most modern cars are fitted with these, this allows the engines ECU to control the fuelling. In which case you can collect them very cheaply from scrap cars, though they do deteriorate with use so it is usually best to invest in a new item. They cost around £30 for a four wire unit.
Lambdas can come with a varying amount of wires, from 1 to 4 are the most common, though some do have 5.
Lambdas with heater wires fitted allow the lambda to warm up quicker and to keep a more steady reading. Ones without heater wires require the exhaust system to heat the unit so will take a while to get up to proper working temperature.

Lambda Wiring.

• 1 Wire Sensor - Black - Signal to gauge
• 2 Wire Sensor - Black  - Signal to gauge
•                      - White - Earth
• 3 Wire Sensor - Black  - Signal to gauge
•                      - White  - Heater x2 
• 4 Wire Sensor - Black  - Signal to gauge
•                      - White - Heater x2
•                      - Grey   - Earth

Bear in mind this may not be correct for all Lambdas !

Want to profile your own needles ? click here for more info.