[allen smith]

I have an opportunity to purchase a 12 bolt posi unit in very good condition, my car has A/C, and the rear has a 3.31 ratio i think. What was the steepest gear available in 1970 for a car with A/C?

[TR]

3.23's I believe is the most "gear" you can get with A/C.

[TJH]

With a 12 bolt, I believe what you already have were the tallest offered.

[allen smith]

Thanks, I may go ahead and spring for the posi 12 bolt. I just did not want to over burden the compressor with too tall of a gear. I suppose that is why there was a gear limit with A/C cars???

[getmygoat]

I think they just assumed A/C cars were not used for ultra-high performance applications.. I don't think you would overburden the compressor, as the POA? would just vent the excess pressure.

[4SLASH6]

I always just believed it was better to keep the a/c car's engine running a bit cooler. could there be possible overheating issues with max a/c and higher rear ratios

[70RAIII]

I'm running a/c and 3.31 gears (Chevelle differential) in my GTO. It does run hotter when the A/C is on, especially on the highway...over 200 degrees. I switched to a 160 degree t-stat and it's been working fine but on real hot days with the A/C on the temp will still creep up to 190-200 degrees at highway speeds. I'm actually going to switch out this rear-end before this season and go with a BOP differential with 3.08 gears. Hope this helps and good luck!

[70461goat]

1970 A/C equipped GTO with a 400 engine usually had 3.23 or "economy gear" gears installed. This is what my PHS papers say. This is with Turbo 400 transmission. Also check timing for your application and fan clutch for proper operation. the heavy duty clutch is what you need for A/C cars. Also make sure your fan shroud is on. In west Texas where it does get pretty hot during the summer, my temps run about 200-205 on a 100 degree day on the highway with A/C on. I also use water wetter and not much coolant with distilled water and a rodney red radiator. When its hot outside, sometimes it's hard to keep em cool. thanks.

[poncho-mike]

I don't have my info available on the GTO, but I would guess the 3.31 is the highest gear you can get with a/c.

I know that the 1970 Grand Prix with 455 came with 12-bolts. Most were 3.07s, but I have pulled 3.31 posi's from a/c cars.

I suspect the 70 GTO 455 cars came with 3.31s for a/c cars.

Mike

[T.D.Vick]

See attached chart. It gives the whole breakdown.

[allen smith]

Thanks to everyone for all the help. I really want a 3.08 gear to save a little on the fuel cost this summer; therefore I`m luke warm to the 3.31

[getmygoat]

One other point.. if you are concerned with overheating on the highway with the A/C on, you could always get an aluminum radiator...

I had a '69 GTO, Turbo 400, A/C... It ran very hot on the highway with 3:23 gears (over 230 sometimes - ouch!).. Tried the cast impeller thing, etc.

Finally bought a rodney red ($500).. after that it never went over 190.

Of course, a lower gear ratio offers other benefits, longer engine life, better MPG, etc.