This research compares CO emissions of old and new technology vehicles using field test. Benefits of replacement old passenger cars with new ones are estimated. Two representative cars are considered in field tests. Three factors are considered in the comparison; speed, Revolution per minutes (RPM), and relative humidity. Using mobile vehicle emission detector, more than 1000 readings are taken for each vehicle. Analysis proves that CO emissions from old technology vehicles are much more than that from new ones. Differences range between 1.5 to 20 times with an average value of 3 between both vehicles. Analysis also proves that old passenger cars are responsible for 81.9% of the total CO emitted from passenger cars. Research concludes that replacement of old passenger cars will save 60% of the total passenger CO emission in Egypt. It is recommended to replace cars with model 1990 or before.