Car is slowly losing MPG...

[lsd]

Not seeing any issues here with the RX300 ratings at 17 city/21hwy.
You can try resetting your ecu by disconnecting the negative battery cable for sometime. Resetting the ecu should reset your long term fuel trims. It may improve your mileage but with your short commute it will drop with time.

 

[BUTCH1]

We're not talking about a 7L engines here that average 5 mpg, we're talking about a guy who has a vehicle that is suppose to average around 20mpg. 10% of 20mpg is 2mpg, 2mpg drop can easily be caused by a bad battery (not saying the battery is bad, but could be discharged).

Oh and btw, the calculated 10% fuel economy improvement on a 1.5L civic engine or 1L geo metro engine is when you go from a car that is operating normally with a reasonably charged battery to one where the alternator is completely disconnected. On a normally charged battery, the alternator isn't being fully loaded down all the time. When I disabled the smart key system on a friends Prius, due to the severely aged battery (he's using the original Prius Battery from 2004!) his fuel economy shot up from 40mpg to 47mpg which is a 17% penalty. And on a Lexus LS430, I disabled the Smartkey system which brought up the fuel economy from 17 to 18.4mpg which is around a 10% penalty. I really hate the Smart Key system and I wish it was still able to be disabled on newer vehicles.

More bunk, "Smart-key" systems have a "sleep mode" that is enabled after a car is inactive for a determined amount of time, unless he was keeping his FOB within transmitter range it should not have cause mileage issues.

 

[tortillasoup]

More bunk, "Smart-key" systems have a "sleep mode" that is enabled after a car is inactive for a determined amount of time, unless he was keeping his FOB within transmitter range it should not have cause mileage issues.

Smart-key systems don't go into "sleep mode" until about 5 days of non usage. If a car is driven about every 5 days, there is basically no energy savings.

 

[BUTCH1]

Smart-key systems don't go into "sleep mode" until about 5 days of non usage. If a car is driven about every 5 days, there is basically no energy savings.

There is still minimal draw, no more than any other BCM module is drawing. A receiver has to be alive to accept commands from a FOB, it doesn't matter if it's a "smart key" or just a regular FOB with lock/unlock/panic buttons. How long the battery will be able to function will vary wildly from maker to maker, the safest bet if your going on a long trip and wont be using the car is to use a battery tender or if you can live with the consequences take off one of the terminals.

 

[tortillasoup]

There is still minimal draw, no more than any other BCM module is drawing. A receiver has to be alive to accept commands from a FOB, it doesn't matter if it's a "smart key" or just a regular FOB with lock/unlock/panic buttons. How long the battery will be able to function will vary wildly from maker to maker, the safest bet if your going on a long trip and wont be using the car is to use a battery tender or if you can live with the consequences take off one of the terminals.

Smart Key systems use a lot more power than simply keyless entry. Smartkey is known for their high power draw which is WHY they used to have the ability to disable it. There is a big difference between smart key and keyless entry as Smartkey is constantly tracking where the key is located if there is a key in a given proximity. The Smart key system literally knows where the key is inside of the car as there are several proximity sensors in and around the car. This is why the key system knows that the key is in the trunk or outside of the vehicle. Unless the owner drives the vehicle every single day and does plenty of driving, I've found lots of dead batteries caused by the stupid smart key system.

The example of the Prius is an example of how high the parasitic draw is as it made the fuel economy drop to 40mpg from 47mpg on a weak battery. The keyless entry still has a draw but it's not nearly as high as the smart key system.