Gear ratio change? Tech Library chart?

[Jmazda94]

Has anyone used the gear ratio chart in the tech support library? I recently upgraded to 31'' tires from 225/75/15 (which i think are stock). I went to the conversion calculator in the library and 225/27/15s have a dia. of 28.3. After entering new/old dia. and gear ratio 4.10 on a 4.0L 4x4 it gave me a new ratio of 4.49 as optimal for tire size. The cross referencing that number with the gear ratio chart
http://www.therangerstation.com/tech_library/gearandtirechart.html
With 31s on the closest ratio is 4.11 to what I am using now and its right in the middle for best overall performance. With 28s and 4.11 ratio it is in worst fuel efficiency...

so I am wondering if I am 1. reading it correctly 2. needing to charge ratio's maybe to a 4.27 or 4.56?

 

[alwaysFlOoReD]

I tried to duplicate your findings but couldn't find the calculator that uses gear ratio input. However, I'm running 4.10 w/4.0l/5spd-od in my b-II and it is a great all around set-up for me. I'm running 31x8.5xr15" Baha Claws.
So....I don't understand what info you're looking for. Are you worried that the gearing isn't good for your set-up? What is your primary purpose for your truck?

Richard

 

[Jmazda94]

Sorry i messed that up a bit...originally I was running a 235/75/15 which is 28.9 dia. now running a 31. Current gear ratio is 3.27 open dif. Checked chart again and it checked out right now that I have correct info. But to run 31's at peak performance (according to chart) i would have to switch to 3.91's or 4.11's. I guess i was just wondering if that chart was correct when i posted the first thread. To change gears in the diff. is it as simple as just changing the gear wheels or is new axels..etc. needed?

 

[4.0B2]

4.10s and be happy.
if you do 33's go to 4.56's.
etc.

 

[alwaysFlOoReD]

The simplest way I found was to buy used rear axle/front pumpkin with the correct ratio. 4.10 is a stock oem ratio, so is available used at junkyards, but is getting harder to find. Find the door sticker code and with a copy of the axle code chart;
http://www.therangerstation.com/tech_library/Axles.html
you can hunt down what you need at a junkyard.
It's still a lot of work swapping out axles but it's pretty basic - no setting up of gears required. I had problems with corroded brake fittings and lines breaking.
Hopefully you have something close to you in alaska, and you could try to get in touch with other members living up there and see if they can give you leads.
Good luck,

Richard

 

[Jmazda94]

Cool, thanks for the replies. Is there an axel code on the door? I found the ratio number 91..would it change much even?

 

[alwaysFlOoReD]

Yes there is a axle code on the door sticker.
Door code 91 equates to a 3.27 ratio, which you said you have, so that works.
"would it change much even?". Would what change much even?

Richard

 

[JoshT]

I think these guys have given you pretty good guidance on which ratio to get, but they don't seem to have answered your underlying question.

so I am wondering if I am 1. reading it correctly

I think you were misunderstanding the purposes of the calculator and chart.

The Chart gives suggested gear ratios for a given tire size and RPM at 65 MPH. It doesn't care what the stock tire size or gearing is, and won't help you keep the speedometer right. The coloration is basically suggested RPM ranges for different uses. You want a higher ratio for better economy and highway driving (blue range), or a low ratio for better towing, power and acceleration (red range). For a general purpose vehicle you want to get somewhere in the middle (green range).

The calculator is meant to suggest a new gear ratio to keep stock performance with the new tire size, and as a side effect keep your speedometer closer to accurate. It compares your old tire size and gear ratio to the new tire size to suggest a new gear ratio that will keep the engine turning at the same RPM for a given speed. I won't help you determine the best gearing or tire size for a particular purpose.

As for the ratios, with the 28" tire the 4.10 gears were a towing ratio. By moving up to the 31" tire you've changed that 4.10 to a DD ratio. Theoretically you have increased the economy and top speed of the truck, and decreased it's towing capabilities and acceleration. What does this mean in the real world? Not much, you'll probably never notice the difference. What you are more likely to notice is that the speedometer is off, keep this in mind as it affects the odometer and any MPG calculations you may attempt. There are two ways to fix it, change the driven gear in the transmission (I can't tell you which one to use), or change the gearing in both axles to what's recommended by the calculator.

Hope that helps a little.

 

[alwaysFlOoReD]

^^^ good summation.

Richard

 

[4.0B2]

Good luck finding a TTB 4.10 gear set in a junkyard...

 

[Jmazda94]

Well I guess the hunt begeins..thanks!