at or near MSL, the C172 does 105 knots at 2350 RPM. The C172R's I've noticed cruises at just under 120 knots for the same power setting
The C172R has a coarser pitch propeller than the C172N. Both have 160hp engines, but
at a given RPM the R will develop more power than the N. To compare like with like, refer to section 5 of the flight manual to determine the RPM giving 75% power for each aircraft. You'll find that the speeds don't change much, but the R model is quieter at that speed due to lower RPM.
I tend to target 100kts on the downwind leg in these aircraft, and set whatever RPM is required to achieve it.