Ssangyong Rexton issue on speed

I bought this car from someone with some issue with oil cooler,then i fix but after fix i drove this car its Automatic then have problem with speed and revs,when im driving at 100km/h then rpm will be 2900,but i have brother with same car when its on 100km/h his rpm will be 2500,then we thought was diff issue we change 3 rear diff ratio 4.89 but same speed and we realise this guys changed the engine its 2.9 from musso sport but they have same bhp and torque but seemz small turbo,i dont know does it contribute and transfer case was played with and we dont know does it have ratio mybe they played with it but not yet with better idea gearbox its 4 speed with overdrive....any help or idea