Oh wow!
 

So, I completely forgot, since I got my insight I haven't been calculating the 4.6 offset on my speedo, so it turns out I have been doing a lot better mpg wise then I thought! Woo-hoo!

Woohoo!!!

Now update your fuel log so your signature can show 2.3% ABOVE EPA!!!

I was under EPA ratings before updating, now I'm over :p

How does one do those calculations? GPS says my Civic shows 5 MPH slower than actual.

Also, could this be corrected with a larger/smaller tire? I'd love for it to be correct

Well i just did mine because my car has tires that are 4.6% bigger, so for every 100 miles I travel, it's 104.6.

Can you elaborate? For every 100 miles on the odo, you actually go 104.6?

So, are bigger tires better for MPG?

Does that make the speedo read faster or slower than actual?

Just because the speedo does not read correctly does not mean the ODO is off as well. My Focus reads 2mph high but the ODO is spot on. Verify miles traveled with your GPS before doing math that might not be needed.

Good advice, I'll check this as well