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post #1 of (permalink) Old 09-11-2003, 05:10 PM Thread Starter
 
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table to determine best gear ratio for a tire size

I've seen a table befor that is a guide to what gears you should have for a tire size. anyone know of one?

I've got 35's and stock 3.75 gears. just wondering if I should go to 4.11's or 4.56's.
I know that the lower (numbricly higher) gears will have higher rpms at 60mph, but if I recall, they gain back the toruqe at lower speads. Right???

thanks
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post #2 of (permalink) Old 09-11-2003, 05:16 PM
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Re: table to determine best gear ratio for a tire size

check out the jeeptech.com. They have a gear ratio calculator. It's a bit conservative compared to my 258. But it will give you a good rule of thumb.
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post #3 of (permalink) Old 09-12-2003, 08:36 AM
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Re: table to determine best gear ratio for a tire size

Pretty decent calculator (includes options to click your drivetrain if you don't know the ratios) [img]images/graemlins/40BEER.gif[/img]
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post #4 of (permalink) Old 09-12-2003, 08:52 AM
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Re: table to determine best gear ratio for a tire size

http://www.4wheelparts.com/Tirel_Gear_Ratio.asp


go here web page
post #5 of (permalink) Old 09-12-2003, 10:43 AM Thread Starter
 
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Re: table to determine best gear ratio for a tire size

cool thanks.

now, what would be the best gears? it does see street time so 4.88's are gonna be a little low.
is there really much of a difference between 4.11 and 4.56?
currentyly i'm running 3.75's dana 300, and the t999 auto tranny running on 35's.

thanks for the help!!
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post #6 of (permalink) Old 09-12-2003, 11:42 AM
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Re: table to determine best gear ratio for a tire size

I would compare where you are rpm wise vs the charts first. The charts are a rule of thumb. Your actual rpms can vary. Mine was off by 200 rpms. Second, get a feel for what your rpms are now at different sppeds so you will have a starting point. Decide if it is currenlty too low, too high or just right. This will determine where your target rpms should be.
My Jeep sees alot of hwy miles. I like to drive it to where I hunt and so on. So I'm not gearing it as low as others would for a trail only rig. I am in the process of going to 4.10's and 35's. With my engine and where I drive this is good.
If you don't drive on the hwy alot and spend more time off road, then go to the 4.56's or lower. It'll pay the benfits offroad.
Just an example off of JeepTech.com. 4.10's, no overdrive and 35's gets 2575 RPMs at 65mph. 4.56's, no overdrive and 35's gets 2850rpms. This is 275 rpms difference for just between 4.10 and 4.56's. This gives you an idea of the delta between the gears. Don't forget, you need to consider your starting point now and relate it to the chart. Different engines have slightly different rpm power bands. Some guys 258's can drive decently up to 3400 rpm's. Mine hates it above 3200. Just do your homework and choose wisely. [img]images/graemlins/grin.gif[/img]
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post #7 of (permalink) Old 09-12-2003, 01:53 PM Thread Starter
 
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Re: table to determine best gear ratio for a tire size

well right now, with the 3.75 gears, it doesn't feel bad up to 60 or so. I don't drive the highway alot, mostly just surface streets but, Sometime I use the highway so, pushing 2800+ rpms is kinda high. I do need to get a tach on there to check what i'm running though.

offroad, is have the lower gears just help with the crawl ratio? what about low end torque?
I know on steep decents, I have my foot on the brake alot, but would changing these gears really help THAT much??

thanks
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post #8 of (permalink) Old 09-12-2003, 02:18 PM
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Re: table to determine best gear ratio for a tire size

If you crawl, wheel torque at low speeds will be increased due to the fact that you will be operating your vehicle within the engine's powerband (higher rpm) at the same low wheel speeds. You won't be lugging the engine as much.

Most importantly for crawling, lower gearing will also help out when your current gears don't allow you to go slow enough without clutching. With a good crawl ratio and bottom end torque at the wheels, you can lug over obstacles with much more control than with higher gearing.

Low gearing is very important on technical stuff. I run 4.88's with my 35's, and run about 50 rpm/mph ([email protected],[email protected]). If you dont do much technical driving, or play in the mud a lot, the extra wheelspeed of 4.10's is an advantage. (4.88 is about 20% lower than 4.10)

I don't take interstates, and drive mosly at about 55 mph in it. It's not too bad on the highway, and makes a good amount of difference on the trail. I went from 4.10's to 4.27's, and now the the 4.88's. My crawl ratio is about 85:1, and I'd eventually like to get 3.15 xfer gears to get down to 100:1.

On the surface roads in town, 4.88 is no less convenient than any other gearing. You just shift a little sooner and spend time in 4th gear when someone else would be in 3rd. I only notice a different when approaching highway speeds.

post #9 of (permalink) Old 09-14-2003, 12:36 AM
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Re: table to determine best gear ratio for a tire size

I've got 4.10's and 33's. I went to 33's before switching gears from 3.55's. The only difference I really noticed was the reduction in highway speed. I should have either gone to 3.73's or stayed with 3.55's. If you do any highway driving, I wouldn't worry with going to lower gears. With the auto trans, it's not enough to make a difference to me. Going from 3.73 to 4.1 will take you from about 24.43:1 to 28.86:1. If you keep the 3.73 and put 4:1 in the transfer case, that will take you from 24.43:1 to 37.3:1. If you have enough power in high for the highway, I would regear your low range instead.
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post #10 of (permalink) Old 09-15-2003, 04:32 AM
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Re: table to determine best gear ratio for a tire size

[img]images/graemlins/tongue.gif[/img] Here is a little-known method of enlightening yourself as to what is going on with your Jeep NOW, and at the same time experiment on paper with other setups. The key to this is TURNS PER MILE of the tires. Go to the tire dealer and get into their books and find the TURNS PER MILE of the tire you have now, and the TURNS PER MILE of the tires you MIGHT WANT TO RUN. The formula is: TURNS PER MILE X REAR END RATIO = ENGINE RPM @ 60 MPH You can see that if you want the engine to run at the same RPM at sixty MPH, you use that as the constant and swap tire sizes and rear end ratios until the numbers work out. OR.....if you think you want deeper gearing, just multiply the tire TURNS PER MILE by the proposed new gearing and you have the new RPM. [img]images/graemlins/smile.gif[/img] Hardly anyone looks at TURNS PER MILE, but that is a useful statistic if you are juggling tires and gearing. [img]images/graemlins/crazy.gif[/img]
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