Google this for the pdf but I'll paste the conclusion
Effect of intake air filter on fuel efficiency
4.1 CONCLUSIONS
The goal of this study was to explore the effects of a clogged air filter on the fuel economy of vehicles
operating over prescribed test cycles. Three newer vehicles (a 2007 Buick Lucerne, a 2006 Dodge
Charger, and a 2003 Toyota Camry) and an older carbureted vehicle were tested.
Results show that clogging the air filter has no significant effect on the fuel economy of the newer
vehicles (all fuel injected with closed-loop control and one equipped with MDS). The engine control
systems were able to maintain the desired AFR regardless of intake restrictions, and therefore fuel
consumption was not increased. The carbureted engine did show a decrease in fuel economy with
increasing restriction. However, the level of restriction required to cause a substantial (10–15%) decrease
in fuel economy (such as that cited in the literature3,4
) was so severe that the vehicle was almost
undrivable. Acceleration performance on all vehicles was improved with a clean air filter.
Once it was determined how severe the restriction had to be to affect the carbureted vehicle fuel economy,
the 2007 Buick Lucerne was retested in a similar manner. We were not able to achieve the level of
restriction that was achieved with the 1972 Pontiac with the Lucerne. The Lucerne’s air filter box would
not hold the filter in place under such severe conditions. (It is believed that this testing exceeded the
design limits of the air box.) Tests were conducted at a lower restriction level (although still considerably
more severe than the initial clogged filter testing), allowing the air filter to stay seated in the air box, and
no significant change was observed in the Lucerne’s fuel economy or the AFR over the HFET cycle.
Closed-loop control in modern fuel injected vehicle applications is sophisticated enough to keep a
clogged air filter from affecting the vehicle fuel economy. However for older, open-loop, carbureted
vehicles, a clogged air filter can affect the fuel economy. For the vehicle tested, the fuel economy with a
new air filter improved as much as 14% over that with a severely clogged filter (in which the filter was so
clogged that drivability was impacted). Under a more typical state of clog, the improvement with a new
filter ranged from 2 to 6%.