The ECU in most fuel injection systems are programmed to go into an open loop mode when cruising at 60 or 70 mph in 5th and save gas. If they lower the speed limits, a LOT of car will burn more gas simply due to the fueld delivery of their engines. True, the RPM will be lower, but the lack of the programmed constant load at 60-70mph will alter the method of fuel delivery and make mileage worse. It's partially related to the reason cars get better highway mils than city. True, the city requires stop/go and vast momentum adjustments, but most city driving (shifting at 2500) keeps the RPM way lower than average top gear cruising at 75 but requires more fuel to combat the change in load and throttle position and the ECU preps the injectors to be ready for these changes in demand making them a little less steady and a bit thirstier.
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