Hit an open highway, set the cruise on 60-65mph, then reset the average mpg read out. after about 15 miles on the highway, you will then know what the highway miliage could be. It should be right at what the sticker says.
Of course certain factors will make it vary like; head or tail wind, weather conditions, extra weight the vehicle is carrying, turning off the A/C, and traffic conditions.
If my VHO Silverado can hold 19-20 at a smooth 65mph, then your TB SS should too.

Jim

 

i can barely get 12 outta my vho

 

LOL. Your just going to have to change up your driving habits a bit if you want better. It will do it. I average 16.5+ on a daily routine. That's from the Houston medical center back to the north side of Houston.
Keep your top speeds a 70mph or lower. If your cruising over 2000 rpm's, your just basically burning excessive fuel. These engines get best fuel economy at 1900 rpms at cruise. When you accelerate from stops, keep the rpms under 2000 between shifts.
If you want better, try it.
Just take a look at some of my pics of my gauge cluster. There is one of them where I'm at an average of 18 mpg (about 2/3 of hwy driving). Then there's another pic showing my daily commute average mpg numbers at 16.7 mpg. The sticker MPG numbers that came on these vehicles are not really all that far fetched. It can be done.

Jim

 

Mine is stripped down- I dont have calculated MPG or a tripmeter

 

I took my TB SS on a round trip to Ky in December. I averaged 16.3 on the road, cruising at 79 most of the way. Around town I am getting 14-15.

 

That's a nice looking ride, congrats and good luck with it

 

65-70 cruise flat road about 10mi, showed avg 20.9, not bad at all!! but since that ride home from the dealer my left foot will not allow a repeat of that event

 

Them things are F n sweet

 

absolutely beautiful. I would go with an STS kit before heads and cam though. Do it. You would definately have the title for a while

 

nice choice!