In the old days, automatic transmissions were scorned by any self-respecting hot rodder. He’d rather push his car than use a slush box. But times have changed—especially with the new generation of four-speed overdrive transmissions. The automatic is now very much an accepted performance component.
If you’ve ever driven a car with an automatic with overdrive, you know how nice it is to cruise down the freeway at 70 mph with the tach just nudging 2000 rpm as opposed to a screaming 3500 rpm or more. Add a lockup torque converter that saves another 200 to 300 rpm, and you have a slick package that, with a little installation ingenuity, can be adapted to virtually any street machine or street rod.
In this short piece, we’ve included not only gear ratios for all the popular GM automatics but also some basic dimensions that will help when it comes time to swap a TH700-R4 in place of that pedestrian Powerglide. A quick glance at the chart showing overall length and rear crossmember position will reveal that a TH700-R4 is a close cousin to the TH400 (mounting configuration, anyway), while the TH200-4R is virtually a bolt-in replacement for the TH350 by just moving the transmission crossmember rearward.
The new generation of overdrive automatics may be the key to updating your current ride. With some aftermarket hop-ups, it could be strong enough to take the abuse of even a mild big-block!
Thanks to Gene Christensen at Auto-Rite Transmissions for his help with this story.
