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I don't get the right answer for this question. I don't see where I'm going wrong.

I use this equation to get the engine rpm for each gear ratio:A vehicle has the following specifications

Drag coefficient = 0.33

Chassis dimensions 1.52m x 1.40m x 4.20m (w x l x h)

Rolling resistance = 200N

Vehicle's mass = 1200kg

Transmission efficiency at converting engine power to vehicle motion = 96%

Gearbox ratios: 3.25:1 (first gear), 1.9:1 (second gear), 1.25:1 (third gear), 0.94:1 (fourth gear)

Final drive ratio = 4:1

Rolling radius of wheels = 0.25m

The engine power is seen to be approximately constant at its maximum value Pmax = 59.4hp over the engine speed range 3000-6000rpm. With the engine at maximum power Pmax, the vehicle can maintain a speed of 40mph when climbing a hill at 20 degrees to the horizontal.

Q) Determine which of the four gearbox ratios can be used to maintain the speed of 40mph up the incline.

Take air density to be 1.2kg/m3, 1mph = 0.447m/s, 1hp = 0.7475kW

engine speed (rpm) = gear ratio X final drive ratio X vehicle speed X 60 / (2*pi*rolling radius)

So for first gear I get an engine rpm of 8880rpm

Second gear = 5190rpm

Third gear = 3414rpm

Fourth gear = 2500rpm

So my answer to is that gear 2 and gear 3 fulfill the conditions. However the answer says its gear 4. Any thoughts as to what I'm missing?

Thanks.