Briefly, nothing. A small 10 km/h change over 100 km/h will not make a big difference either to decrease oil consumption in case of Spain, nor on the Dutch highways — especially if the new limit will be coupled with tighter speed camera tolerances, which leaves you with pretty much the same de facto speed limit as today (I would say 135 km/h). But it does send a political message.
To look back a bit, note that highway speed limits range from 100 km/h (Norway) to 140 km/h (Poland) in Europe, as well as the infamous unlimited speeds on (parts of) the German Autobahnnen. This latter one-country exception provides the car industry with the excuse to sell cars all around the world which are carefully designed for breaking the law everywhere except certain German highway segments.
These differences, luckily, force us to stop searching for “best practices” (although an argument in the Netherlands was that “abroad” the limit is already 130 km/h — this fictional country named Abroad always intrigued me) and really think about what could be an optimal speed limit on the highway.
Capacity and consumption/emissions characteristics guide us to an optimal speed of about 80-90 km/h, above which the capacity is reduced and emissions per traveled km increase again. Traffic safety of course monotonously decreases by speed, and in fact the difference is also significant in this speed range: the Dutch Institute for Road Safety Research (SWOV) calculated that at the current speed of 120 km/h, a speed increase of 1 km/h (not 10 km/h) already means a 3,3 % increase in deaths.
On the other hand, the pleasure of speeding is hard to quantify, but travel time isn’t: in case of the Afsluitdijk, where the new speed limit is introduced, a time saving of only 1 minute and 10 seconds is possible over the 30 km length.
In short, although a general maximal road speed limit of 80 km/h does not seem to be politically feasible, any further increase above this range is also scientifically not justifiable — it is nothing else than a cheap way to please some voters. Surprise: there are elections today.
Update: just like most scientists, the “most senior traffic offence official” is also against the speed limit raise.