We present new gas flow models for the Milky Way inside the solar circle .
We use SPH simulations in gravitational potentials determined from the NIR luminosity distribution of the bulge and disk , assuming constant NIR mass-to-light ratio , with an outer halo added in some cases .
The luminosity models are based on the COBE/DIRBE maps and on clump giant star counts in several bulge fields , and include a spiral arm model for the disk .

Gas flows in models which include massive spiral arms clearly match the observed ^ { 12 } CO ( l,v ) diagram better than if the potential does not include spiral structure .
Furthermore , models in which the luminous mass distribution and the gravitational potential of the Milky Way have four spiral arms are better fits to the observed ( l,v ) diagram than two-armed models .

Besides single pattern speed models we investigate models with separate pattern speeds for the bar and spiral arms .
The most important difference is that in the latter case the gas spiral arms go through the bar corotation region , keeping the gas aligned with the arms there .
In the ( l,v ) plot this results in characteristic regions which appear to be nearly void of gas .
In single pattern speed models these regions are filled with gas because the spiral arms dissolve in the bar corotation region .

Comparing with the ^ { 12 } CO data we find evidence for separate pattern speeds in the Milky Way .
From a series of models the preferred range for the bar pattern speed is \Omega _ { p } = 60 \pm 5 Gyr ^ { -1 } , corresponding to corotation at 3.4 \pm 0.3 { kpc } .
The spiral pattern speed is less well constrained , but our preferred value is \Omega _ { sp } \approx 20 Gyr ^ { -1 } .
A further series of gas models is computed for different bar angles , using separately determined luminosity models and gravitational potentials in each case .
We find acceptable gas models for 20 ^ { \circ } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } % \raise 2.0 pt \hbox { $ \mathchar 316 $ } } \varphi _ { bar } \mathrel { \hbox to 0.0 pt { % \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt \hbox { $ \mathchar 316 $ } } 25 ^ { \circ } .
The model with ( \varphi _ { bar } = 20 ^ { \circ } , \Omega _ { p } = 60 Gyr ^ { -1 } , \Omega _ { sp } = 20 Gyr ^ { -1 } ) gives an excellent fit to the spiral arm ridges in the observed ( l,v ) plot .