Like Neil said, if you don't already, definitely need to get an SG/UG. That will help you determine best driving practices, even for a 5 mile drive. I would also do all maintenance work you can. Check the oil (level and "freshness"), transmission oil (level and "freshness"), battery voltage level, you said you checked the brakes, but what about the tire threads?, also 38psi might not be the best pressure for you (a SG/UG would help you determine this).
After maintenance, you can do some MODs. If you are only doing 5 mile trips, I would try to seal up the engine bay as much as possible, so the engine gets to operating temp further. So grill block, belly pan, etc.... Some other aerodynamic mods would be helpful (full belly pan, rear diffuser, kammback, etc.).
As Neil mention, if you run the defroster all the time, you might be running a/c without even knowing it. You'll have to take out the a/c belt, a/c fuse or cut the a/c wire from the defroster.
What about weight? Are you carrying a lot of stuff or is the car basically empty?
Hope this helps! I'll probably come up with more ideas as soon as I press the submit button!
