Compact Arduino GPS Speedo + and More

So I decided to build an device I could use in my car, a GPS speedo looked like a good idea.The Parts I used in this build: -(fake) Arduino Nano v3 ($10,99). -MT3339 GPS module. -Graphic LCD + Connector ($5,00). -Plastic enclosure ($4.58). -1 x 6 pin switch. -1 x Push button. -1 x 470 Ohm resistor. -2 x 10K Ohm resistors. -A small Mini USB cable (came with Arduino) And a USB power pack for portable power (4000mAh), But in the car I could be powered of a 5v outlet (phone charger). I need to confirm this, but this device uses less than 100mA Max. in combination with an 4000mAh power pack I would last a minimum of 29 Hours.Power consumption -GPS : 25mA on Acquisition, and 20mA on Tracking. -GLCD : Display 3-5mA, Backlight 40-45mA. -Arduino : 20mA running As you can see, the Backlight is good for 40% of the total consumption. Im looking a way to switch on/off the backlight with the Arduino. if this work I the consumption drops to 50mA and it would last for 50 hours. But at the moment all my output pins are used. 10 by the GLCD, 2 by the GPS and 1 for the button. I could switch the button to an analog pin. But I used pin 2 for interrupt, for later switching between modes. So I will keep it this way and leave the backlight on. 29 hours is acceptable :)”