Best rear gears for OD Trans

[leadheavy52]

I have all the bits and pieces to install my OD trans (R10-G1) and will be doing so in the next week or so. I did some research (fortunately) and it states that the Suburban, when ordered with the 18" wheels, would be geared at 4.78:1. Yikes! I have verified that mine was indeed ordered this way from the factory and would assume that the information from  the '52 model section of Allpar (included a screenshot from their site) would be correct in the ratio. If I install my OD trans, would most of the OD feature be negated by such a low rear gearing? I do have a '51 Cranbrook parts car that I can pull the rear axle if need be. It has the 6.70 x 15 tires. If I used this rear end, would they be the 3.93:1 or the 3.73:1 gears? I haven't done the "rotate the yoke" bit yet to count. Wanted to check here for the gurus' feedback first. 

 

Edited February 4, 2019 by leadheavy52
Misspelled words

[kencombs]

If you're keeping the big wheels, IMO, keep the 4.78.  Roughly the same RPM/MPH as the 4.1 and 6.70/15s.

[Plymouthy Adams]

.70 reduction is .70....whereas the 4.78 would be equal to approx. 3.35  in OD.....there are a few folks running 3.55 with the flattie but you will not be able to sustain OD in some conditions of load/incline.  You also will be running a taller tire so effectively it will help with top end though hurt you a bit off the line....your 52 would  have had stock without the 18's a 3.9 gear unless factory OD installed...then you would have had a 4.1   The factory made OD kits retroactive to the dealer for installing on slow to sell left over 51 models and are considered stock if you have a 51 with OD.  They DID NOT change the rear ratio given all I have read.  Given your car, how/where you plan to drive should have great weight in your final choice....

[leadheavy52]

Thanks for the input. I do plan on keeping the wheels and after the insight here, will most likely keep the 4.78s. I’d like to be able to swing 65-70 on somewhat hilly highways (New Mexico). If I need to, I can always drop the Cranbrook rear in if I need a little extra gear. Thanks! 

 

-Austin

[Plymouthy Adams]

that sounds to me to be the best plan of attack....why mess with prosperity?  As you stated, once you discover the driving characteristics of the car and they not to your liking...then you can craft a change...

[Silverdome]

With 30" tall tires running at 2800 rpm with .7 OD and 4.78:1 rear gears you would be going approximately 75 mph. The final gear ratio is lowered numerically with larger diameter tires which also lowers the rpms and affects your torque and horsepower.