So your goal in this thread is to wag your finger at everyone for having opinions, some of them based on experience, and not having run all the numbers in advance as a research effort.
I mean fundamentally you’re correct of course - the less facts and experience which are available, the less value there is in an argument or position. But this isn’t GQ, and there needs must be a large amount of opinion and WAG in these things. I assure you that if we were arguing this in GQ I would have at least some numbers myself to throw into the fray.
I contend, however, that some factors are prima facie benefits - for example, whether or not a tune-up which costs $200 produces less CO2 per mpg improvement than upgrading to a $20,000 new car. Sure you cannot easily exceed the baseline mpg of the car, but if that benefit is applied to a much broader population, the net effect is much greater. This goes back to the whole deal of “how many millions of barrels of oil could we save if people would just keep their damn tires aired up to the proper inflation pressure.”
Repairing a clogged catalyst is a more dubious benefit - some places charge as much as $1000 for a new catalyst, and you might only gain 1-2 mpg from the fix. But run the numbers with me - if we assume only 1 mpg per clogged catalyst, at a $4,500 maximum cash for clunkers benefit, that means the break-even is a 4.5 mpg improvement. BUT - that does not include the CO2 cost of producing the car, which as Cecil says here could be from 12 to 65 tons of GHG emissions per vehicle: What’s better for the environment, a scooter or a car? - The Straight Dope
Now if we take that old rule of thumb that 1 mpg over 10,000 miles is about a ton of CO2, and we assume a differential mpg between 4.5 fixed old cars/1 new car of 5.5 mpg (let’s round up to 6), and assume…shoot, let’s take a mid-range value of 35 tons CO2 per new car, and we assume 10,000 (aggregate) miles of driving per year, then even in the case of the dubious benefit catalyst replacement of just 1 mpg, we get a break-even of about 6 years until the new car mpg benefit.
I’m sure there are several ways we can run the numbers for a variety of improvements - tires can be a good cost-benefit, as higher-mileage and higher mpg-tires could improve economy by 2 mpg for, oh, $600 or so. YMMV (pun intended).