why offroad mpg is listed 17/20 vs 18/23 for limited

I have the tonneau on my TRDOR DCSB 4x4 and manage to get 19-21.5 mpg hwy going 70-75. A lot better between 55-65. The wild part is that with this auto, I'm averaging 19.7+ on my 20 mile commute of 50/50 Atlanta traffic on city roads and a couple of Highways. My 14 was a 6spd manual without a tonneau (same options otherwise) and that thing would get 14.7-17.5 on the same commute and 18-19 at best on 100% highway. City mileage out here can fluctuate from parking lot to low traffic through rolling hills with a million street lights. The 16 auto runs around 1800 rpm at 70 while that 14 was at 3000. The gearing is a huge factor. Whether it's the tonneau, 2 gears of overdrive or whatever, I'll take it. Will also be installing that front air dam that all the sport owners appear to be removing for looks.

 

I seem to get 20MPG combined in my 15 DCOR and am working on ways to get better. On the highway in Washington because of all the hills sport mode in 5th seems to be the best.

 

As of late, my Limited 4x4 seems to struggle to get 17 mpg around town. Granted it is a lot more stop and go driving as of late.

I have the OEM cover from the factory, not the cheap add on from the distributor. Yeah it closes the bed, but the thing is heavy.

 

lol, this was never a "post your mileage" thread. I think there's already a few of those, aren't there?

 

Here is an article from July 2015:
"Another interesting line was that bit about a $650 OEM tonneau cover (you know, one of those flat things that goes over the bed) had to be sold on 30% of Tacomas to meet CAFE requirements.

I’m told this means the EPA lets Toyota advertise certain MPG figures achieved with the cover, as long as less than a third of Toyota’s truck are sold with said accessory. We’ll be looking into this a little more closely and circling back."

http://truckyeah.jalopnik.com/report-2016-toyota-tacoma-priced-from-22-200-to-39-3-1716230202