As you might have read in the reviews, the XC40 is not very efficient. I think it's due to a combination of poor aerodynamics and some inefficiency of the drivetrain. The optimal speed will likely be slower than you'd like in the 30's MPH.
Maybe someone more patient than me can come up with a real "range vs speed" graph like others have done on ABRP.
I think that when you say optimal speed you don't mean the most efficient speed but the speed that will result in the shortest combined driving and charge time. As such there is not one optimal speed.
In general of you have a long distance between chargers ABRP will tell you to reduce speed to between 50 and 60. There is a connection here with the old federally mandated 55 MPH speed limit which was established to save fuel.
I used to have an ABRP trip that showed this, but EA plugged the gap. I'll have to look for another. Reducing your speed to gain time on your trip will only work for trips that are long enough to eliminate a charging stop and in areas where you have enough choice of chargers to drive further.
If you're just trying to be efficient try 55. It's a good compromise. Just do us all a favor and stay in the right lane. We don't want to get a reputation. 🙃
Many Tesla folks have found that since DC fast charging generally provides hundreds of miles per hour charging speed, you might as well drive fast to your next charger. It shortens overall trip time compared to driving slowly.
One of the things that makes this harder to answer is the finite number of charging stations. I tend to do it dynamically. Know where I am going. Look at the % on arrival then use extra speed to get their faster but also to drive that number down. Charging time at 10% is almost free per %.
Another option is planning things to do so your perceived time is good.
ABRP let’s you fiddle with max speed and also priority of over all time vs number of stops.
In general though if you were going to arrive at 15% let lose and seeing you can get it to 10% whilst being safe /)