Axle gearing, ratios, 35s, and model differences
I was a bit bored and mulling over how everyone and their brother expresses that new gears are basically required if you go to 35s, even in Rubis. I have a Rubi with a 6-speed and have been entertaining the idea of 35s – and of course, wondering if I really need new gearing as part of the package. So I put together a hypothetical comparison!
From an academic standpoint, if you want to maintain stock performance when going from 32s w/4.10s to 35s…people will say you need 4.88s to regain stock performance, probably because the added rotating mass and unsprung weight, but from an academic standpoint the equivalent new ratio is 4.48.
Here’s the hypothetical situation: Your buddy has a Sahara with 3.73s and you have a Rubicon. Both are manuals, and you both are going to 35s. You are keeping the 4.10s, he will re-gear. Both have the factory transfer cases at 2.72:1 and 4.0:1 ratios, respectively.
Adding 35s to a 4.10 axle brings the equivalent gear ratio to 3.75, so you’re pretty much even with your stock Sahara buddy with his 32s. The 35s go on.
Sahara wants to maintain stock overall ratios, and the calculator tells him that the equivalent axle ratio with 35s will be 4.08. So he buys a set of Rubi take-off 4.10s and installs them along with the 35s.
Although not technically related to tire size…your Rubi’s overall crawl ratio has been reduced from 73:1 to 67:1. Still a very low ratio suitable for heavy wheeling work. Sahara’s crawl ration is 45:1, roughly the same as stock.
In 4-lo and in first gear, at 3000 RPM your Rubi’s speed has increased from 3.9 MPH with stock tires to 4.27 MPH with 35s, less than ½ MPH difference. Sahara’s speed under the same conditions is 6.28 MPH.
At 65 mph in 6th gear, Rubi’s RPMs have dropped from 2350 to 2150. Stock with 32s and 3.73s, Sahara’s RPMs under the same conditions is 2138 (and the same now due to the gear change). This puts Rubi’s highway revs well within the factory spec range.
Conclusion? Putting 35s on a stock geared Rubi only very slightly changes the the 4-lo ratios and speeds, where it still has a distinct advantage over the Sahara at least on paper. Outside of 4-lo, it behaves just like a stock Sahara with 32s and 3.73s which is obviously well within the vehicle design limits and specifications. Your ‘butt-dyno’ probably will tell a slightly different story with the added unsprung weight and rotating mass, but I’d hardly consider a gear upgrade a necessity for a Rubi to move to 35s.
Now, if you have an AUTO….let’s just say I feel your collective pain. Plug in those ratios and the game changes big time. In that scenario I can totally see why you could consider gears a necessity. A stock Rubi crawl goes from 73:1 to 46.5:1 when you have an auto…for a Sahara you’re now at 28.8:1 from 45:1. Yikes.
If you’ve read this far, you’re as bored as I.