Power Cord 65-watt vs. 90-watt

The 65W put out a lower power level than the 90.  There is no harm in using a 90W adapter in place of a 65, but you SHOULD NOT use a 65W adapter in place of a 90W unit.  The system may not run at full speed, the adapter may overheat, and the battery will charge more slowly (or may not charge at all when the system is powered on) if you do.

The Combo Adaptor works on AC and works in the Car.

Dell : Dell 65-Watt Combination Power Adapter

• Allows you to power your Dell notebook virtually anywhere - on the road, in-flight or at your desk
• Plugs into any standard wall outlet, auto outlet or air outlet
• Supports Latitude ExpressCharge for charging battery quickly

If you need to buy a new one I'd get that instead of the 90W

• Dell Laptop PA-12 AC Adapter with Power Cord.
• For Dell 5U092, F7970, PA-1650-05D, PA-12 Family
• COMPATIBLE MODELS: * DELL INSPIRON: 300M, 500M, 600M
• * Dell LATITUDE: X300, D400, D500, D505, D600, D410, D610
• Input: 100--240V, 50-60Hz. Output: 19.5V 3.34A
• 65-Watt Combination Power Adapter for Select
   Dell Latitude/ Inspiron/ Precision/ XPS Notebooks
  Manufacturer Part# : F2663
  Dell Part# : 310-4804

• PA-10 for Dell Inspiron 300m, 600m, 700m, 6400, 8500,
  LATITUDE D400, D500, D600, X300, 19.5 Volts 4.62 Amps 90 Watts

Manufacturer Part# : Y808G
Dell Part# : 330-1825

Manufacturer Part# : NF599
Dell Part# : 310-7699

Manufacturer Part# : UC473
Dell Part# : 320-1389

Manufacturer Part# : FF313
Dell Part# : 310-7698