You didn't mention whether the car was equipped with an automatic transmission or a manual transmission. The autos were equipped with a 2.59:1 ratio as standard equipment I believe, with 3.07:1 as an option. The manual trans equipped cars had 3.07:1 as the standard ratio. However, I could be wrong.
Ken, the Black Book lists a 3.07 rear as performance option G92. For 88, it also says that coupes with this option had 245 hp, 5 more than all other cars, due to less restrictive mufflers that were deemed too loud for the base model. Was this option only for automatics?
Yes, G92is the option code that's listed in the Black Book, for both your '88 and my '87. But my build sheet lists code G44 for the 3.07:1 ratio axle, and G87 as the code for the ring gear 8.5". My window sticker doesn't include that type of information.
As for the ratios for automatic equipped cars, if I'm not mistaken, the 2.59:1 or 2.73:1 ratio axle was standard, with the 3.07:1 axle as an option. The 3.07:1 ratio was standard for the Z51 equipped coupes, which were exclusively manual transmission cars.
It's curious that you bring up the muffler thing again. The horsepower increase was attributed to the mufflers having two additional, functional outlets on Z51 equipped cars (The Z52 and convertibles were equipped with mufflers that had the middle two outlets blocked to reduce resonation.) Initially the horsepower increase from 235 to 240 in '87 was attributed to the aluminum heads, then in '88 they say it's due to the mufflers. :eyerole