How to choose your Gear Ratio

Very well done video with great information. Now I just need a company to build some 5.29 or 5.13 gearsets for my axles
 
How did we select our gear ratio?.. Jeep.com > 2017 jeep Wrangler Unlimited Sport > Powertrain > 3.73 Gear ratio

Two Reasons for selecting the 3.73 gears:

1) More aggressive tires in the future
2) Possibly adding a tow package later.
 
It's usually the pinion gear that becomes the weak point as the gears get numerically higher, it gets smaller in diameter. Having said that, I ran 4.88's with 36" TSL's on my old YJ with Dana 30 front/high pinion D44 rear...the rear is an old Currie axle, the high pinion was a good idea, bad execution as it works the coast side of the gears. For my application it met my needs as I had done a spring over axle (SOA) conversion on that rig. My driveline angles with a conventional low pinion rear axle brought the suck. My current '05 Rubicon is running 5.13's in the stock Dana 44 housings with metric 35's.

Another consideration with drastic gear changes is your driveshafts themselves. We learned this the hard way with our Jeepspeed car. We decided to run 7:1 gears in our pigs for a couple of races, the first race in that configuration was the three day Vegas to Reno "The Hard Way" stage race, followed by our next MDR race. In both races we tossed our front driveshaft and pole vaulted the car over it destroying the transfer case. We couldn't understand what the issue was. After talking to the guys at JE Reel (our driveshaft sponsor), we realized that the front driveshaft was not of sufficient strength (tube diameter) to allow for the faster driveline speeds with the numerically higher gear set. Essentially the driveshaft was spinning fast enough that it bowed in the center enough to pull the splines apart. Who knew...expensive lesson.
 
A friend posted this a while ago. Yellow for increased gas mileage. Blue for improved towing. Black, a decent mix.
gear-chart.gif


http://southwestpowersports.com/GEARCHANGE.HTML
 
...Can't see any link in the first post (might be my protection software).
What has worked very well for me over the last 30 years;
Determine the level that you are going to abuse the vehicle (includes towing).
Verify your axel assembly strength or select a compatible axle for strength and track. (10 bolts, Dana 35s and pre 1995 Dana 30s are at best marginal for 33s, IMO)... do your research!
Select your desired maximum tire size, consistent with the above & make it/them fit.
Charts like the one posted above may leave you undergeared for some overdrive transmissions (42rle and other modern/excessive overdrive transmission owners beware). I calculate the tire rolling diameter (always less than the measured tire size which is normally less than that on the side wall and engine rpm at interstate cruise in overdrive and select the axle gearing for a good compromise between power and economy on the highway (helps to know your engine's power and torque curve; underpowered engines often need lower axle gearing)... You are close if you can shift into overdrive with acceptable street performance at 45 to 50 MPH, after the mods are complete.
Set your crawl ratio by changing the transfer case gearing...(much more necessary with high (<4:1) first gear manual transmissions) Most automatics slip more than enough in first gear to make this unnecessary.
I recently re-geared my Explorer to 4.11s for 31-32 inch tires and generally consider 3.73s fit only for tires in the 29inch and smaller range (with moderate horsepower ~200 hp engines)... High torque engines (big diesels) can often turn larger tires with smaller gear ratios.
Proper ring & pinion tooth engagement (I.E good setup) will keep internal breakage to a minimum... It is somewhat advanced assembly..

Enjoy!
 
I went with 456 on a 35in tire and now I have to pull the teardrop in overdrive on the interstate... I'm a firm believer in pulling in drive for 1 to 1 when towing to keep down heat in the auto transmission, so the JK axles going under now are 410's..... Going to loose power but it's all compromise at this point.
 
I went with 456 on a 35in tire and now I have to pull the teardrop in overdrive on the interstate... I'm a firm believer in pulling in drive for 1 to 1 when towing to keep down heat in the auto transmission, so the JK axles going under now are 410's..... Going to loose power but it's all compromise at this point.

Gearing always seems to be a compromise :coffee
 
So if I’m reading this right, I went from 31” tires to 35” tires. Currently running a 3.42 gear. I should be running a 4.10 if I’ve read and calculated it all correctly!?

On my Tacoma the above chart suggests that the OEM 3.91:1 with my 33" tires would get me the better fuel economy but that's not true. I've a huge improvement going to 4.88:1

This gear ratio calculator is based on a from/to tire size and suggested 4.27:1 when I went from 31" (265/70R16) and 3:91:1 to 33" (255/85R16) which was what I had originally asked for when I went to East Coast Gear Supply. They suggested going to 4:56:1 because I would not be able to feel any difference with 4.27:1. However they realized the 4:56:1 ratio wasn't available with e-lockers for the Taco and suggested 4.88:1.

The calculator is designed to return the truck back to what it was. If you like how your truck handled the 31" tires then following the conversion suggested should get you what you want. One of the things I've said that I missed with the 4.0L in the FJ is that it had the torque and HP in the lower end and the the 3.5L in the Tacoma seemed to be lacking. Going to 4.88:1 fixed that short coming of the 3.5L.
 
I’ve no idea how it drove on 31s! It actually says I should go to 3.91 but that is available for the GMC 4x4. Next closest is 4.10
You should be okay. Send East Coast Gear Supply an email. Won't hurt to ask.
 
Back
Top Bottom