What is the relationship between Car engine size and RPM used?

RPM at a certain speed will be dependant on transmission and rear-end gearing. Lower gearing to the ground, higher RPM's

Not dependant on engine size.

The engine speed at a mph is dependent on the final drive ratio. This consists of transmission gear ratio, differential ratio, and it also depends on tire circumference Say that your transmission has a 1.00 to1.00 gear ratio in high gear. you have a 3.0 to 1 ratio in the differential and this assumes the manufacturers 27" tall tires for a 3.0 to 1 final ratio the engine will run at 2500 rpm at 60mph Put on a shorter tire and and your final ratio goes up in responce to the new tires size to say 3.25 to 1 and raises the rpm to 2800. Now the final drive ratios had to be different on the 1.8 powered vehicle. Was one a 5speed transmission and a 4 speed in the other? That would probably be your difference in the rpm's

Assuming the gear ratios, tire sizes, vehicle weight, etc. were close to being the same... Generally speaking, it would take a smaller displacement engine more RPM's to produce the same amount of power as a larger engine. In your situation, I would guess that the gear ratios & vehicle weights are quite a bit different.

It depends more on the gear ratios in the transmission and drive gears than the size of the engine.

RPM means REVOLUTIONS PER MINUTE, or otherwise means how many times the wheel can spin. on those car descriptions they put like 300hp@6800rpm right? well it means the wheels can spin up to 6800 times a minute, anymore and the wheels might pop off or tires will break. 300hp is just the engine power that can propel the car. that is why on the dashboard, one of them is usually a 1-8 and something with x1000. and the other is the mph.

It has nothing to do with engine size and everything to do with the gearing.