The ecomodder calculator has a setting for rho with some preset values in the drop-down for different temperatures.
Which Ioniq do you have? Apparently the Blue model has better mpg, but I found this speed vs mpg chart from cleanmpg for the Limited trim level at 60F temperatures:
I can pretty closely duplicate those mpgs with:
Weight: 3200
Rolling Resistance: 0.007
Cd 0.25
Frontal Area: 25
Summer fuel
0.36 engine efficiency
0.91 drivetrain efficiency
1500 watts parasitic drag
1.225 rho (59F)
Speed_tested MPG_modeled MPG
50_____71.5______71.4
55_____66.0______65.5
60_____59.2______59.8
65_____54.4______54.5
70_____49.9______49.6
Using that model, here is how temperature affects mpg:
Speed_14F MPG__59F MPG__80F MPG
50_____68.1______71.4____73.3
55_____62.1______65.5____67.4
60_____56.5______59.8____61.7
65_____51.2______54.5____56.3
70_____46.5______49.6____51.4
In the "real" world with limited atmospheric wind, you should beat the modeled mpg because of the corridor effect (other cars on the freeway going to same direction create a bit of a tailwind)