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Originally Posted by USMC20yrs
(Post 4114946)
First you are going to need to do some research; you are going to need to know the gear ratio of your transmission when it is in drive. You can get this within this forum. Then you need this formula: MPH x Drive Gear Ratio x Diff Gear Ratio (divided by) Tire Diameter x 336 = RPM . This will give you the RPM of your current set-up (for the MPH number I use the speed limit). Then by working the formula in reverse, inserting your new tire diameter it will tell you what gear you need to maintain the same level of performance. For example: My JK is '13 6sp, stock tire dia is 29", 6th gear is 0.797, diff gear is 3.21; at 55MPH my RPM is 1613.6. So, taking that RPM as the starting point divide 1613.6 by 336 x by 33.4" (new 295/75R16 tire) divided by 0.797 divided by 55 MPH = 3.65 diff gear ratio needed. Since there is no 3.65 gear ratio available I would need a 3.73 gear in order to have the same performance as the stock set-up. However, something to also take into consideration is the increase in the weight of the tire and wheel (twice the weight of stock in my case). This will effect performance and MPG as well as braking. All this said, in my case, 295/75R16 with 3.73, I'm satisfied with it.
I hope this has helped and I wish you luck with YOUR decision. Keep the shiny side up and don't forget to wave.:standing wave: |
Originally Posted by Tooadvanced
(Post 4115300)
Much much much easy way is to take new tire size , yours is 33.4÷29(old tire size) then multiply by current gear ratio which is 3.21which is actually 3.697
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Originally Posted by Stephane87
(Post 4115341)
Why do you divide by 29? My stock 2010 JKU sport came with 32"...was is already an upgrade that I didn't realize? Because if I do the calculations with 32" as the stock size then a 40" tire would only need something close to 4.66 (gears)....doesn't make sense?!?!?! |
Originally Posted by ChairRepairHeatnAir
(Post 4115354)
Size isn't the only part of that equation. You also have to think about rotating mass. A 40 is a lot heavier than a 33. Any extra weight in the tires is like having 5 times that weight difference in the back of your Jeep, therefore making the need for a higher gear ratio to turn the extra mass.
My example would be: 33" / 32" X 3.73 = 3.84 (stay at 3.73) or 33" / 29" X 3.73 = 4.24 (go up to 4.56) |
A good way to find out you originally equipped tire size, is to go on to jeep.com and look up your build sheet using your vin#
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Originally Posted by durffjk
(Post 4115381)
A good way to find out you originally equipped tire size, is to go on to jeep.com and look up your build sheet using your vin#
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Originally Posted by Stephane87
(Post 4115377)
Ok, but do I still do that calculation base on a stock 29" or 32" (in my case)? My example would be: 33" / 32" X 3.73 = 3.84 (stay at 3.73) or 33" / 29" X 3.73 = 4.24 (go up to 4.56) |
You can also find out what gears you have when looking at your build sheet.
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With the 3.8 auto and 35s you need a minimum of 4.88s. 5.13s would be a better option but people don't like running them in a D30.
It's not complicated. Also you need to look at actual tire size when mounted and not what the manufacturer specs say. There are a million gear threads. And those people who are chiming in about running manuals or the 3.6 you aren't really helping the guy. I would leave the gears as is with 33s |
Like Joe said ..... Don't need to make this so complicated !!!!
I have an 09 JKU auto I ran 5:13 gears in my Dana 30 on 35's I did eventually blow the gears wheeling. So if you wheel it I'd probably stop at 4:88's tho the 5:13's were nice. I now have a PR44 up front and 37's and the 5:13's still works really well with my set up. If you're staying with 33's I'd save the money and not re gear. |
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