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Old 04-08-2013, 05:15 AM
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Originally Posted by starin View Post
Hi Guys and Gals,

So i have a 1968 firebird with a BOP TH350 it, i'm planning on throwing a 200r4 in it in a few months because the RPM on the freeway is so high. The th350 transmission is used, no idea how many miles. The rear diff is a gm 12bolt it was rebuilt before i bought it but no idea what the gears it has in it.

so at 56mph i am taching 3500 rpm and i have 28 inch tall tires, i used several gear calculators and everything says that equates to 5.21 gears. is it possible that my gearing in the diff can really be that high or maybe my transmission is not shifting into second gear??

Your math is right, but is the tach?

If the trans is in second, the gear ratio would be a more realistic ~3.5:1. But like poncho said, checking the gears yourself is a good idea.

I'd also be checking the shift linkage to be sure the trans is actually shifting into high gear.
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