Auxiliary transmission calculating gear ratio

[alfio]

Hello all,

I'm considering using an auxiliary transmission but have questions about how to calculate actual gear ratio once the Brownie gets into the mix. For example, my truck has an overdrive gear of 1: 0.80. The auxiliary I'm looking at has an overdrive of 1: 0.86.

Am I correct in thinking that the new gear ratio (5 speed and Brownie in overdrive) would be (roughly) 1 : 0.68?

Thanks
A.

 

[KansasIDI]

Hello all,

I'm considering using an auxiliary transmission but have questions about how to calculate actual gear ratio once the Brownie gets into the mix. For example, my truck has an overdrive gear of 1: 0.80. The auxiliary I'm looking at has an overdrive of 1: 0.86.

Am I correct in thinking that the new gear ratio (5 speed and Brownie in overdrive) would be (roughly) 1 : 0.68?

Thanks
A.

Yes. 0.688 to be precise.

I have a Spicer 5831, has a 0.73 overdrive. My ZF5 main transmission in my 91 that I’ve thought about putting that brownie box in, has a 0.77 overdrive. Combined, I’d have 0.584, which is very steep. However, I have 5.13 gears in the rear axle, and since it’s a 2WD truck, a brownie such as that would be a viable solution.

5.13 differential gears, 0.77 overdrive with 32.8” tires, 2630 RPM at 65 mph

5.13 differential gears, 0.584 overdrive with 32.8” tires, 1995 RPM at 65 mph

Much better for the highway. And many more gear selections for towing.

I still don’t know whether or not to put the brownie in… maybe I should just sell it, now that I’ve bought a different truck that will be much more highway friendly and even more versatile once I get it ready to roll…

 

[Clb]

Marlin (rip) of marlin crawlers old website had a great gearing calculator...

 

[KansasIDI]

I use Spicer’s…

Spicer Transmission Ratio RPM Calculator

Use the Spicer Transmission Ratio RPM Calculator to determine your engine’s RPM based on the transmission gear ratio, tire height, MPH, and ring and pinion gear ratio.

spicerparts.com

 

[Clb]

Marlin adds in a doubler.
So now we need one for overdrive...