I ran one last year outdoors on tarmac (UK) and it was very competitive against the M05 and Top Sabre I was racing against. I did have a little setup help from ABC USA, here's the info:

1)
Front spring >> Yellow or Red. Grid need soft front suspension on the bumpy surface.

2)
Rear spring >>Yellow. Cause present battery (Li-Po) is light weight, Rear suspension needs soft spring too.

3)
Stock Hard grease is Fine, Also, all the suspension need smooth moving.

4)
*** Most important is Smooth moving suspension, Especially, King-pin and king-pin ball. Please check your car,

5)
Tires. You can choose your favorite tires for matching on your track surface. But, under setting, Please try same tires for front and rear, you can find different from same tire traction.

6)
Additional weight, Grid is light weight chassis. If your Grid doesn't have stability. Please add 100g weight to the center of the car,

7)
Gear Diff, You can use #5000 - #10000 oil
By the way, Ball diff is better than gear diff at all the surface. Easy to get traction.