Benefit = get some more power, since in stock form alot of it in the 22re is closer to the higher RPMs.
Downside = poorer gas mileage.

O/D is basically just 4th gear. Lets you decide if you want the idiot tranny in 3rd or 4th. Nice on the highway b/c you can cruise @70 mph and low RPMs. Personally, w/ stock tires I almost always leave mine on, and leave ECT off. Improves mileage. However, you'll never feel like you're getting somewhere fast, if that's an issue for you.

With 200+ Billion electrical parts, the world most complicated machine is inside your own skull.

Question Reality.
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'89 Rnr DLX "SR4.5", 32s w/ 5.29 locked f/r blah blah