fuel mileage computer

Hey guys, I have a mpguino arduino based fuel mileage computer in my car and I would like to have some form of data logging capabilities with it. What kinds of options do I have to say, store information from each tank reset? I may also work on another arduino to record data in the form of ambient temp, humidity, baro, (maybe some form of rain sensor?) To observe what weather conditions effect fuel efficiency and how much so. Any ideas would be greatly appreciated.

There's all kinds of data loggers available.
Adapt one to add a keypad and LCD display to enter whatever data you want.
I just keep a little note book in the console of my Jettta Turbodiesel Sports Wagon and note the date, how many miles at a refuel, how may gallons, and any notes (big snow storm). Plug into excel occasionally to see if there's much variation.
Cold seems to impact mileage - I'm seeing more 50mpg trips now that it's warmer (20 mile commute), vs 42-44 in the winter months.

I have a log book but I'm looking for some more accuracy. Is it possible to connect the datalogger to my mpguino? How would that be done?

Don't know. Got a link to data on mpguino? I'd never heard of that before this topic.

There are three unused holed in the board labeled Rx, tx, and ground. I didn't know if I could use these to communicate to another arduino or a data logger or something

MPGuino appears to be a '328 & LCD screen. I don't have access to the manual so I don't know what might be output on the Tx pin - you could connect Tx & Gnd to Rx & Gnd on USB/Serial adapter (FTDI Basic for example) and try some different baud rates on the IDE Serial Monitor to see if any usable messages are being transmitted. If you have access to the MPGuino code you could do some reprogramming to output what you wanted and use another Arduino/SD card to log whatever you wanted to with extra sensors, etc.
This board I offer would let you connect up all kinds of things, and could use the SD card for datalogging. I didn't have vehicle install in mind when I designed it, so you'd probably want to add some more protection if powering it from the vehicle 12V supply.
Available with & without screw terminals, and with & without USB/Serial module

(5V regulators are installed, were missing when pic's were taken)