Get ready for a lot of figures because they tell the story about automatic transmissions. First and most important, everybody including the Automatic Transmission Rebuilders Association agrees that 90% of failures are due to overheating. The oxidation rate (which determines the useful lubrication life) of ATF doubles for every 20 deg. F. temperature increase over the normal operating level of 175 deg., which means fluid that's formulated to last 100,000 miles (a distance frequently recommended by car makers as the change interval) at 175 deg. will only go 50 grand at 195 deg., or 3,000 at 275.

A survey of trans shops some years ago revealed that the average unit has less than a 50/50 chance of going 40-50,000 miles, one in three won't make it past 40 grand, and one in five won't see 30,000. Also, while pre-'78 cars needed transmission work at an average of 68,000 miles, later models gave out at 39,500 (just as you suspected).

The average cost of a transmission repair is interesting too. Between '79 and '82, it went up 50% -- from $300 to $450. But that's nothing. It's way up over $1,000 now, and projections for the future say it'll continue to rise dramatically. Blame the increased number of FWD transaxles and complicated O.D. units.

These numbers mean you might be saving yourself the biggest repair bill you've ever seen by changing the ATF and filter more frequently, and/or by installing an aftermarket cooler (O.E. units obviously don't make it).

So, people who tow trailers aren't the only candidates for coolers anymore. The only trouble is deciding which type to use, the traditional tube-and-fin design, or the stacked or embossed plate units. The former is well-proven -- it's saved many a transmission. The manufacturers of the latter claim that their configuration is 30% more efficient than tube-and-fin, and cite their units' small size as an important feature in today's fully-packed cars. Also, they say it's strong enough to drive over.

My opinion? Well, the test results I've seen are nothing if not inconclusive. Apparently, both designs do the job they're intended for, and are far superior to anything you'll find in the bottom of a radiator, so you won't go wrong using either one.

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