Predicting Monthly Car Sales for Brands in US: First Step

I've set out to produce monthly forecasts of monthly car sales by brand in the US. So far I've made a SUTSE dynamic linear model (code on Github) and created a Shiny app (http://sweiss.shinyapps.io/carvis/) as a prototype (no predictions yet).

Basically, I want to combine my interest in cars with my interest in stats.

My current roadblock is the how slow the code runs, but I intend to have a speedy implementation using RStan by next month.

If anyone has tips or suggestions, please comment!

Predictions for September:
Acura & 13764.75 \\
  Audi & 15929.24 \\
  BMW & 26445.80 \\
  Buick & 23725.69 \\
  Cadillac & 16722.01 \\
  Chevrolet & 204636.69 \\
  Chrysler & 26336.27 \\
  Dodge & 51082.20 \\
  Fiat & 4408.64 \\
  Ford & 228880.56 \\
  GMC & 50361.17 \\
  Honda & 143487.90 \\
  Hyundai & 67112.10 \\
  Infiniti & 10366.93 \\
  Jaguar & 1162.72 \\
  Jeep & 60577.05 \\
  Kia & 53863.38 \\
  Land.Rover & 4736.08 \\
  Lexus & 30614.06 \\
  Lincoln & 7766.23 \\
  Mazda & 29544.19 \\
  Mercedes.Benz & 28693.43 \\
  Mini & 5455.48 \\
  Mitsubishi & 6706.69 \\
  Nissan & 125118.79 \\
  Porsche & 3914.65 \\
  Ram & 39194.54 \\
  Smart & 1258.65 \\
  Subaru & 46186.21 \\
  Toyota & 213761.40 \\
  Volkswagen & 31581.44 \\
  Volvo & 4809.14 \\