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Okay, so I just went from a 36x12.50x16.5 to a 42x15x16.5 TSL. I was running 4:10s, but were a little sluggish. I have a D60 and 14BFF. Which gears would you run? 4:56, 4:88, 5:13, other
(I don't know how to setup as a poll.)
Thanks.
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I'm sooooooooo glad i saved this:
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Zakk’s Dissertation on why 5.13’s are the best choice for a 700R
Recently, I made the switch to 5.13’s in my D60/14B combo. I run a mildly build TH350 and 39.5” TSL’s. The 4.10’s I did have were killing everything: the motor, the trans, freeway “ get up” …all of it was terrible.
So, after typing this up a few times, I will commit this document to the annals of time, and have a friend of mine host it.
The problem with larger tires and a 700R and larger tires, is that you can’t seem to get enough RPM’s to get the OD to properly engage.
Dr. Watson of ORD fame has this problem with the Art Carr 700R and his 42’s. There is just not enough RPM’s to shift in OD. So what little freeway driving he does do, is all done in Drive.
When we mix this with tires in the 35-38” range, 5.13’s are a great choice.
Assuming a true tire dimension of 35”, a vehicle with a small block motor mated to a 700R would spin 2241 RPM’s. Running with 4.88’s would have the engine humming along at 2132 RPM’s, only saving 109 revolutions of the motor per minute. All are calculated at 65 MPH.
Assuming a true tire size of 37”, 5.13’s would net 2120 RPM’s at 65 MPH.
Most small block motors are comfortable running between 1800 and 2800 RPM’s. 5.13’s with the addition of OD allow this number to be on the lower end of the spectrum, while allowing for maximum acceleration and off road performance. Gearing can make a motor that is slightly lighter on power really come alive, as I have found with my mild small block 350 with approximately 285HP at the flywheel.
In closing, unless you bought your axles with 4.88’s already installed, there is no reason that this author sees to not go with 5.13’s from your present gear ratio.
Let the flaming begin….
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not sure what tranny you are running, but the idea is the same, especailly since you got the 42's. 5.13's are the ONLY choice, or swap in a D70 rear and run 5.86's.
