Since it's Monday and I'm bored...let's give this a whirl.
An average cow produces about 75 or so gallons of methane per day. At a density of 0.7kg per cubic meter, that converts to 0.29 pounds of gaseous methane per day. This comes to around 150 lbs of methane per year.
My 4x4 F150 with the 5.4L V8 produces about 600grams of greenhouse gases per mile driven. I drive my truck about 10,000 miles per year. That's about 13,000 lbs of CO2 per year.
According to sources...methane is 23 times more harmful than carbon dioxide (the main greenhouse gas produced by cars). So a cow produces an equivalent damage of 3,450 lbs of CO2.
Now...I'm a meat eater in every sense of the word. I consume large amounts of red meat. I'm probably in the upper 25% of the population when it comes to red meat consumption. Let's say I consume 0.25 pounds of beef per day. I probably don't...but ocassionally when I crave a juicy steak...I'll consume a whole pound in one meal...I do eat quarter-pound and third pound burgers fairly often...some days I each chicken or fish or pork...some days I don't eat meat (rarely)...so we'll use a conservative estimate. That's 91 lbs of cattle per year. Your average steer will produce around 500 lbs of consumable meat...so one cow would feed me for 5 years.
Conclusion. I don't think eating as much red meat as I do is quite as damaging as driving my F150.