2.56 vs 2.73

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2.56 vs 2.73

Postby chuck79 » Sat Oct 15, 2016 11:37 pm

I'm putting more info together for my 'Bird, I've got the 2.73 or 2.79 and want to use a KC F 200 4R from a 1986 Regal. Standard rear ratio for that car (307 v8) was 2.56. Question is what would the difference be in highway speed reading between the two? As in what percentage? If anyone knows or can point me to the right website that would be awesome. Thanks
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Re: 2.56 vs 2.73

Postby Kenova » Sun Oct 16, 2016 10:36 am

Here is a calculator I like to use. It's easy to use and gives you more info than you need. :lol:

https://weddleindustries.com/gear-calculator

Ken
My other car is a Nova.
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Re: 2.56 vs 2.73

Postby Monza Harry » Sun Oct 16, 2016 10:36 am

Hi Chuck, you need way more info to give you a useful answer. You will need the revolutions per mile of the donor car and the same for your car. The math [arithmetic actually] is very simple your speedometer is looking for 1000 rev's/mile, so tire rev's/mi. X rear gear = driveshaft r's/mi. divide by 1000 and that will be your trans gear ratio. I believe the G-Body come with something like P205/75 R15 So side wall is 205mm X 75% X [both top and bottom] 2 so 307.5 mm [divide by 25.4 to go to inches] or 12.106" add the wheel dia. 15" = 27.106" tire height, now X pi [3.14159] 85.156" divide by 12 to go to feet = 7.096 feet per revolution of the tire. One mile is 5280 feet divided by the circumference of the tire [7.096] = rev's per mile of the tire which is 744.046 X 2.73 = 2031.246 rev's per mile of the driveshaft so your tranny/speedo gear will need to be 2:1 [2031.246/1000] I had a link saved with all available trans gears but that link is now dead so no need to share it :( this is just a sample work up you will need the actual numbers from the vehicles involved and my tire numbers are based on perfect tire geometry not likely accurate to any degree, some tire sites will list actual Rev's/mile that would be best course of action if available, if not my way will get you close enough to start. I hope this helps! Harry
Harry Finishes with: wrote:Sorry I had to "Fly" you will also have to figure this out for your 'bird as well as different height tires and a different gear(?) than the G-Body
Last edited by Monza Harry on Sun Oct 16, 2016 7:44 pm, edited 1 time in total.
I'm not a hoarder I'm a preservationist 78 Monza Spyder (Soon To Be 2+2 with Spoilers)
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Re: 2.56 vs 2.73

Postby chuck79 » Sun Oct 16, 2016 7:20 pm

Thanks guys, I'm going to save this info for later figuring out. It's more math than I'm used to. I'll look up (again) the info on the '86 Regal that this trans came out of. I have 14x6 Snowflake rims for my Bird and want to run around a 185/ 65 R14
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Re: 2.56 vs 2.73

Postby Monza Harry » Sun Oct 16, 2016 7:40 pm

Chuck I want to also point out that the rear gear you have is pretty steep for the OD trans you have: 2100RPM in "Drive" becomes 1400 in OD [@ 60MPH] that will be pretty "luggy" and will likely cause torque convertor issue being below even mild OEM TC stall speeds and may not have enough line pressures to activate the lock up. Your cam card may list your "Cruise" RPM's and make sure that isn't a Marine Cruise" as that will be much higher. And check my first post for my "EDIT" as I had to run the boss was calling [SWMBO] and I had to "Jet" Sorry for the incomplete info. Harry
I'm not a hoarder I'm a preservationist 78 Monza Spyder (Soon To Be 2+2 with Spoilers)
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Re: 2.56 vs 2.73

Postby chuck79 » Sun Oct 16, 2016 7:48 pm

Yeah I thought about that too. 4.1 v6 with an Isky cam 2 steps above stock. And a q-jet for now. Guy I bought it from way back when built it for a Regal. I was thinking about a 3.42 posi out of a s-10 too.
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