3:73 geared rear end?

[Pete]

I was wondering if anyone could tell me how a 3:73 gear ratio rear end with an original Studebaker 289 cu. in. R2, auto trans., will preform? What can you expect for reasonable speed/rpm in each gear ? What kind of Fuel economy say at 70 mph? Is it geared to low to be a touring car? thanks for you help again.

Pete

[okc63avanti]

Pete,

My 63 Avanti before starting its restoration was an R1 with PowerShift Automatic and AC. It was equipped with the standard axle for an automatic with AC which is a 3:31 ratio rear-in. The R2 vs R1 will have no effect on the RPM you are turning, just how past you get there! The 3:73 axle would be pretty standard for a R2 as it will allow faster acceleration and performance. I'm not sure what your RPM will be with the 3:73 but it should be higher than what my Avanti was turning at the same speed, if we are both running the same size wheels and tires.

My R1 with auto and 3:31 rear end would run about 2900~3000 RPM at 70 MPH. If you are wanting to lower RPM for economy look at going to the 3:31 or 3:07 rear end and changing the trans to a 4 speed GM automatic. I'm changing my Avanti with restoration to a Tremec TKO with final 0.68 overdrive ratio, will increase my rear end to the 3:73 and will get both the performance and lower RPM at highway speeds.

If you send me your tire size, I can calculate your RPM at various speeds if I know what transmission you would like to run and what combination rear end gears. I've written an Excel spreadsheet to help me do the math.

Just let me know ... John

[okc63avanti]

Pete,

OK I did some checking and ran the math through my spreadsheet here's a table of results for a Stude PowerShift and a GM Turbo700 4 speed automatic. These calculations are for P215/70R15 Tires with a diameter of 26.85" I hope this helps

PowerShift final ratio = 1.0

RPM @60 MPH @ 70 MPH

3:31

2486 / 2901

3:54

2659 / 3102

3:73

2802 / 3269

GM T700 final ratio = 0.70

RPM @60 MPH @70 MPH

3:31

1740 / 2030

3:54

1861 / 2172

3:73 1961 / 2288

Edited April 30, 2010 by okc63avanti

[Pete]

Pete,

OK I did some checking and ran the math through my spreadsheet here's a table of results for a Stude PowerShift and a GM Turbo700 4 speed automatic. These calculations are for P215/70R15 Tires with a diameter of 26.85" I hope this helps

PowerShift – final ratio = 1.0

RPM @60 MPH @ 70 MPH

3:31

2486 / 2901

3:54 2659 / 3102

3:73 2802 / 3269

GM T700 – final ratio = 0.70

RPM @60 MPH @70 MPH

3:31 1740 / 2030

3:54 1861 / 2172

3:73 1961 / 2288

[Pete]

Pete,

OK I did some checking and ran the math through my spreadsheet here's a table of results for a Stude PowerShift and a GM Turbo700 4 speed automatic. These calculations are for P215/70R15 Tires with a diameter of 26.85" I hope this helps

PowerShift – final ratio = 1.0

RPM @60 MPH @ 70 MPH

3:31

2486 / 2901

3:54 2659 / 3102

3:73 2802 / 3269

GM T700 – final ratio = 0.70

RPM @60 MPH @70 MPH

3:31 1740 / 2030

3:54 1861 / 2172

3:73 1961 / 2288

Hi John,

I knew there was a smart guy out there! Thanks for your detailed calculations and effort that answers my question rather completely!

Pete

[jtjim]

I have owned a 63 GT Hawk, R2 4-speed-3.73 axle, and a 64 Avanti R2 Automatic 3.73 axle. In both cars the engine turned just about 3000 rpm at 65 mph. It would be nice to have an over drive gear. However it will run all day long at that rpm without harm. I am in process of building a 53 champion with R1 engine and 4-speed--with 3.31 axle. I think it will be a nice cruiser.

[PackardV8]

FWIW, I swapped in a GM 700R4 and the overdrive completely changed the feel and the comfort of the car on the highway. Reducing the RPMs in overdrive changed the highway noise from a cacophonous racket to a bearable roar. bad news is even though the RPMs were reduced 30%, the fuel economy did not increase 30%.

The only thing I miss about the PowerShift is the way it would bark the right rear going into second gear at anything more than half throttle. That was fun.

jack vines