Deceptively easy one this 
Assuming speed is constant (i.e. not accelerating or decelerating in each gear) If you are in 4th gear at 4000rpm at 50% throttle you will be using exactly the same fuel as in 3rd gear at 4000rpm at 50% throttle. The reason for this is that the engine would have exactly the same load on it, and therefore producing exactly the same amount of power.
HOWEVER if you look into this more you will see that it is not possible to create both of these on the exact same bit of road, and your MPG readings would be totally different. This is where the visualisation might become tricky.
First of all in any case imagine the engine is at 4000rpm – and we will ignore wind resistance. You choose 3rd gear, and you find a road where the car maintains 4000 rpm at 50% throttle. If you were to work out torque and rpm at wheels after the drive has left the gearbox this would equal the load required to pull the car along this bit of road. Lets say it’s a hill. Now you choose 4th gear. The ratio of engine speed to wheel speed is now smaller. The result of this is 1, the engine has obviously dropped RPM and two, due to the lower ratio in gearing the is not enough torque to maintain your current speed at the 50% throttle position. Two choices, 1 increase throttle to get back to 4000 rpm at a higher speed, or two find a different bit of road (smaller hill) where you can achieve a 4000rpm at 50% throttle again. If you now find a smaller hill, where you can again reach 50% at 4000 rpm, and you work out power at the wheels this will be the same as it was in third gear. (Power = torque x rpm) However since you are now travelling faster, that is greater MPH but less load (not as steep hill) your MPG will be much better.
Make sense?
Ed
