The trick to using motion blur is to apply it selectively. The effect we're trying to replicate is that of panning the camera to follow the action, using a shutter speed of about 1/10th of a second to blur the background while the subject remains relatively sharp.
To do this we need to make a selection to separate the car from the background. In GIMP, as in all editing programs, the best way to do this is to use the Polygonal Lasso tool. Select the Lasso tool from the tool palette. The icon looks, not too surprisingly, like a rope lasso. With the Lasso tool selected a couple of tool options become available. Check the Feather Edges box, and set the Radius slider to 2.0 pixels. This will slightly soften the edges of the selection and help to blur out any roughness. Also check the Antialiasing box, which will also help to straighten out any kinks in your selection and make the curved sections smoother.
Zoom in on the starter image (use the keyboard shortcuts – and +), and click the lasso tool around the outline of the car, trying to keep as close to the edge as possible. This is a bit fiddly, but it's really the best way of making an accurate selection. You can use the space bar to move around the image as you go.
When you've gone right around the outline of the car you'll need to de-select the areas within the outline of the roll-cage. To do this, hold down the CTRL key on your keyboard (Mac users use the Apple key), and you'll see a small minus sign appear next to the cursor. Select around the holes in the roll cage and these areas will be removed from the selection.
We've now selected the car, but in fact we want to apply the motion blur effect to the background, so we need to invert the selection so that everything except the car is selected. To do this we go to the Select menu and click on Invert, or simply use the keyboard shortcut CTRL+I.