IMO, 20 mpg is very optimistic. Based on my experience with the Gen 1 Tundra and the 4.7, you should average about 16.5-17 on the highway and 13.5 -14 around town.

And, btw, noticed this is your first post, so welcome to TTZ!

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2007 Tundra Limited Double Cab 4x4 Super White
Volant CAI
Pioneer AVIC Z-2 (hard drive upgrade to AVIC Z-3)
Key FOB Tailgate Lock (courtesy of Toxarch)
OEM back-up camera w/flip down monitor microswitch mod
Sirius Satellite Radio
WeatherTech Floor Liners
20" OEM alloy wheels
Line-X Bed liner
UWS Black Powdercoat Low Profile Toolbox
DU-HA underseat storage unit
JC Whitney Black Nerf Bars
Lund Interceptor II Bug Shield
Tekonsha Voyager Brake Controller