The current how to thread seems out-dated, so I figured I could make a new/updated one and try to make it a little simpler. Please note, if you have under 5 posts, you won't be able to have the link on your sig, but the image should work fine.
At the top of the site click the Garage link (red box in photo)
Click the view button next to your vehical
Note down your vehicleid number
Go to the User CP and click Edit Signature
Copy and Paste the below text and edit the two spots 9999 shows up to what you saved earlier
HTML Code:
[url=http://ecomodder.com/forum/em-fuel-log.php?vehicleid=9999]
[img]http://ecomodder.com/forum/fe-graphs/sig9999a.png[/img][/url]
*** New Members with under 5 posts use this ***
HTML Code:
[img]http://ecomodder.com/forum/fe-graphs/sig9999a.png[/img]
Preview it and if it shows up right then save it!
This should work for other forum sites that use BBcode as well.