Remember... When calculating your trip odometer you have to adjust for the tire size if you've changed it...

For instance.. I just filled up today... 20.8 gallons... I had 315 on the trip

Because of my tires, 315 is really 346.5
I get that from the fact that when I'm going 60 on my speedo, I'm actually at 66... so 66*315/60 = 346.5

346.5/20.8 = 16.65 mpg for this tank

not 15.14 which some may have thought

SO maybe 13 mpg is really someone getting 15 or 16 with 33 inch tires... ?

You can use the infamous miata tire size calc to figure out your speedo difference from stock tires to your new tires...

btw.. who's getting 23? are you running 13 inch wheels and tiny tires?