what is the gear ratio of a 1978 cutlass

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re: "What is the gear-ratio of a '78 Cutlass?" I'll assume you're talking about the rear differential, not the transmission. Not sure if they(Oldsmobile), marked the "rear-ends" the same way as they did in the mid seventies(73-77). However if they did, you'll need to safely lift the car so the rear axel. assy is about 16" off the ground. You probably dont have to absolutely do this, however since there may be some cleaning/scraping involved it makes this alot simpler. On the passenger side axel tube about 12" away(towards the tire), from the diff. housing there is a axel code stamped on the tube, facing down. It was used by Olds.(GM), in identifying manufacture, build date, ratio. You may have to scrape/brush/clean off a large area to read this, a bright light will help. An "O"(a letter,not a zero, means built by Olds.), is usually the first thing you'll see followed by a build sequence. It goes #1 - Jan., #2 - Feb. and so on. Then a letter, "A" - first week, "B" - second week and so on, so "04C", would be April, 3rd week. Not sure if they put the year of manufacture, if you see an "8" by itself most likely the year.Then a ratio code, not sure about "G" body codes, but "A" body's went something like this: A(A, B, C, D),2.56, 2.73, 2.95, 3.08, so an "A A", would be an 2.56 non-Anti-Spin "open" type diff. and so on. If it has a "C" as the first letter then it is a Anti-Spin(this is what Oldsmobile called its locking differential, and is what most people refer to as a "posi"), followed by a lets say "D", so "C D" would be a 3.08 posi. So the whole sequence may look something like: O04CCD, also look for a small aluminum tag attached to the diff. housing cover(9 o:clock bolt location), it should have the ratio, if its at the 5 o:clock position it may be a "posi" tag.

I wanted to found out what’s my ratio on my 78 cutlass it had a 260,now it’s has a 350. Unbelievable unless in reading it wrong but I Spond one wheel rpm equals 1:25 rpm on the pinion. That’s Puny