Force = Mass x Acceleration

According to google, a steam roller is about 20 tonnes or 20,000 kg.

the acceleration of gravity is approximately 9.81 m/s^2

therefore the force that Jotaro exerted had to be equal or greater than the force of gravity forcing the road roller down.

F = 20,000 x 9.81

F = 196200 Newton’s

however, this is the amount of force needed to lift the road roller, however, DIO is on top of the roadroller. it’s safe to assume that Dio is around 85kg seeing as he’s around equal size and strength to jotaro, who is 82kg.

therefore an additional 82kg(mass) x 9.81(acceleration) = 804.42 N(force) would be needed just to lift the road roller and DIO’s body, not including the force of DIO hitting the road roller.

Interestingly enough, the 19704.42 is far lower than amount of force needed to shatter diamonds, which we see Jotaro do in the High Priestess Arc (the giant teeth are supposedly as strong as diamond) diamonds can withstand around 2 megapascals of force which equates to around 2,000,000 Newton’s, meaning that Jotaro can exert even more force than the calculated 197,000 Newtons.

to calculate the force DIO is exerting is a bit tougher. we know that with THE WORLD he can exert around the same force as STAR PLATINUM, meaning that he can exert at least 2,000,000 Newton’s, same as Jotaro. HOWEVER, he is also sitting on top of the road roller, which has a downwards force of 196,200 AND DIO himself punching the roadroller.

this means that the minimum force on either end is 2,000,000 Newton’s upwards for Jotaro and 2,196,000 Newton’s for DIO.