Europe has been under exceptional sunlight for the past ten days, cf. Eumetsat's picture. According to French media, this prevents smog from escaping major hubs. In an attempt to reduce this air pollution, some cities have lowered their speed limits, as RFI, Radio France Internationale, reports:
Paris police lowered the speed limit for cars to 20 km/h in some areas and banned trucks weighing over 3.5 tonnes from entering the city after a peak in air pollution in December 2013.
The question is meant to focus specifically on the 10 km/h arbitrary choice: it makes sense that lowering speed limits by 50 km/h will result in much less pollution. But is 10 km/h enough to have any substantial change? How likely is it that some drivers may be confused at to which gear to be in at 60 km/h instead of their usual 70: if they're driving with a higher or lower RPM than the optimal operating RPM then wouldn't these drivers actually pollute more? (Cf. this answer which specifies that at least the highest gear coincides with getting the maximum mileage -- and therefore lower consumption -- at the maximum allowed speeds https://skeptics.stackexchange.com/a/3355/2086 .)