Welcome to the group,
Regarding the best MPH for your car, it will depend heavily on your car, when i started out i did a few test runs up and down a road with a ultragauge type device and worked out the most efficient speed to run at. For me it ended up being about 60kph which is where my auto transmission is in its top gear and has the torque converter locked.
Since i did that test i have overridden the control of the torque converter and the gear selection so its not so valid any more and it will be different for a manual transmission as well.
im not sure of the legality in your area about mirrors but i checked my road worthy rules for my country and state and saw nothing about mirrors so i took them off, other people in your area might be able to point you in the right direction here.
Some other easy tips for you
- pulse and glide, can get a fairly good gain but it can cause damage to your auto transmission but i have been doing it a while and i have had no issue yet (but i am happy to damage my car in the interest of doing fun things)
- A grill block is easy
- wheel covers are easy and give some good gain
- Car idle reduction and cold car idle reduction (personally gave me good results but that is mostly based on my low distance trips)
- Hydraulic Power steering to manual conversion (medium difficulty) but it can give some good gains
- If you do a lot of stop and go driving weight reduction can give you a little gain and is easy to do.
If your after a far more comprehensive list have a look at the wiki
Car MPG Efficiency Modifications Main - EcoModder
it has hours of effort and cost associated with mods that people have tried also some have directions how to perform the mod so you can work out how easy it is.
you can also gauge how popular some mods are by how many users have posted MPG benefit about the mod
There is also a hypermilling/driving techniques wiki but it doesn't yet have much more info than what is in the 100 + tips page
good luck