Greetings, I have an old Mopar with a 360, 727, 8.75 out of a 64 Plymouth with 3.23 gears. The tach says about 2900 at about 65 mph. I don't have a working speedometer yet. Anyway, would simple math tell me have many r's if I change to a 2.91 or 2.76? (Experts, sorry if I don't have the numbers correct). Oh, the rear tires are 225x70x15. Thanks. I am thankful.

Go to http://vexer.com/automotive-tools/speed-rpm-calculator you'll like the lay out, easy & Mopar oriented..

not sure what you're asking?? if i read it right, the rpms would be lower at 65mph with 2.xx gears.

The math said going from a 3.23 to 2.91 is 90%. If swaping rear end, would 2900 rpm be 10% less with the higher gears, 2900 x .9 equals 2610 rpm? Thanks for the info!

Well first off 3.23 gears, 27.4" tall tire & 1-1 high gear shows you should only be spinning 2600 rpm's at 65 so if your tach is correct your convertor is slipping quite a bit... 2900 should give you about 74 mph..

65 with 2.91 = 2370 rpm

65 with 2.76 = 2200 rpm

what kind of Tach do you have? I would bet its off..

It's an old SunPro. I wired the lights to it and the dial pointer goes to zero when the lights are turned on. It shows okay when the lights are not on. Not sure how accurate it is.

most factory and old aftermarket tachs are wildly inaccurate. Trust the math, not some ancient $20 tach.


Dave

I might be easier to get some taller tires, 29 - 30" to lower crusing rpm. Thanks for all the info,

I have been looking all over for this thread...LOL