Racing's chances of a comeback had all but been extinguished by the sending off of Domingo Cisma for two yellow cards six minutes before the second goal.
Angel Di Maria, back after 11 games out through injury, made an appearance from the bench and scored a stunning third before Benzema made it four late on.
Barcelona, who host Valencia on Sunday evening, have a mountain to climb in the title race as Jose Mourinho's men again strolled to three points.
They began at 100 miles an hour, and Ronaldo should have scored from inside the six-yard box on five minutes with a free header from a corner.
Within a minute, however, he had his goal. Kaka was played down the right side of the box by Benzema, and his hard shot from an angle was parried by Tono. When the ball came back to the Brazilian, he headed to the far post where Ronaldo was waiting to nod into the unguarded net.
Racing claimed offside, but with several players on, level or behind the byline/goal-line, it was almost impossible to tell even on replays.
The sides looked a mismatch for most of the evening, with Racing struggling to get out of their half while Madrid attacked at will down either flank.
On 26 minutes Cisma earned his first booking - and it was well-deserved. As a promising and rare Racing attack came to an end, Real broke and the left-back threw up an arm to handball deliberately and nip their counter in the bud.
Tono kept Benzema out with a scramble to the foot of his post while at the other end, Racing almost equalised when Iker Casillas did not deal convincingly with an Alvaro cross and Lautaro Acosta waiting beneath the dropping ball controlled before seeing a shot blocked.
Soon afterwards came Cisma's second yellow card. Ronaldo attacked down the right and crossed; before the ball was cleared it took a seemingly innocuous deflection off the hand of the defender, looking to block the delivery just outside the box.
Racing, already demoralised by their lack of competition despite being the last side to keep a clean sheet against Real - back in September - looked even more lost with 10 men.
Ronaldo won a free-kick off Alvaro and when Xabi Alonso passed it unexpectedly into the box to Benzema, the striker spun expertly with the ball and dinked a lovely finish from an angle over Tono. Sergio Ramos made sure with Bernardo looking to clear off the line, but Benzema seemed to be awarded it and was certainly deserving of it.
The second period was slower, with Madrid picking apart their opposition whenever they moved up a gear. Di Maria appeared for Mesut Ozil on 63 minutes and within 10 minutes he was on the scoresheet.
The Argentina star cut in from the right and unleashed a bending strike that Tono got fingers to but could not stop flying inside the post halfway up and rolling around the back of the netting.
Away centre-half Marc Torrejon was unfortunate to see a scissor volley head into the side netting but it was Madrid that looked the likelier to score again in the final quarter of an hour.
Ronaldo played a one-two with Benzema before Tono brilliantly blocked the Portuguese star's shot from close range then the keeper again somehow produced a point-blank stop when Madrid had four men in the box against him and a defender.
Seconds later though he was beaten by Benzema, whose strike took a deflection before heading into the roof of the net to put the seal on another win for Los Merengues in a season which will surely end in a Liga title, if not Clasico supremacy.