Just wondering why my car charger charges my iphone 4 like molasses compared to the supplied wall charger. I can be on zero battery and charge it in about 45 minutes with the wall charger. It seems to take twice that long in the car!
A automobile can provide WAY more amps than an iPhone could possibly use, 40A to over 120Amps. The iPhone uses about 1/2 amp (.5 amps or 500 milliamps, however you care to think of it as. My car charger charges the same or perhaps a little faster than the home charger. He probably has a defective charger, try another one, from a friend perhaps.
There are low and high output charges. eBay is notorious for having charges for $1.99. Rest assured, it will take forever to fully charge with one of these. Compare the miliamp output of chargers. Best bet, get one from your carrier or apple.
I use an official charger in my car. The cable with one of those usb to 12v things (an apple one). It's always charged my 3g, 3gs, and now 4g way slower than the house. I always thought it was because a car had so much less power. Oh well...
Make sure your car charger outputs at 1A to get the full charging speed. My advice is to go for one of the HTC car charger adapters (CC C200 or CC C300) as they're fairly cheap and are definitely 1A because that's what the Desire needs to charge and run Sat Nav at the same time.