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How do bigger tires affect your gear ratio?


 
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djgbsbj
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PostPosted: Tue Mar 24, 2009 11:46 am    Post subject: How do bigger tires affect your gear ratio? Reply with quote

Do bigger tires lower the gear ratio or raise it?
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ehirner
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PostPosted: Tue Mar 24, 2009 12:59 pm    Post subject: Reply with quote

They don't change the gear ratio at all. The gear ratio in the diffs stays the same. Bigger tires require more effort to get moving and keep moving, not to mention travel more distance in a single rotation. All of these combined, bigger tires decrease performance.
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djgbsbj
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PostPosted: Tue Mar 24, 2009 5:40 pm    Post subject: Reply with quote

So to get the same performance you would have to go to a lower gear?
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TJ Apex
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PostPosted: Tue Mar 24, 2009 9:13 pm    Post subject: Reply with quote

ehirner is correct, tire size will not change the gear ratio, But it will change your overall ratio (engine rpm, trans, front & rear diff, and tires).
Example: If you had 30" tires and you were travling at 60 mph at 2500 rpm, then do nothing else but have 35's put on, then go 60mph and you'll note that your tach reads 2000 rpm.
You'll also notice that going up a hill will be more difficult and you may even need to down shift, simply because your engine is not in it's power band.

Larger tires will put more load on the engine, this is generaly the reason why people that run 33's or larger tires need re-gear to keep the same overall ratio.

Note also that when you do go to larger tires it will also change your speedo and mileage readings. If you have the 35's on and your speedo indicates 60 mph your true speed may be 68 mph, same with your odom, if it indicates 10 miles you actually went 13 miles.

Hope this helps
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jppilot61
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PostPosted: Wed Mar 25, 2009 3:04 am    Post subject: Reply with quote

djgbsbj wrote:
So to get the same performance you would have to go to a lower gear?


Lower gear ratio is actually higher number gear

4.11 is lower gear ratio than 3.55

See http://www.jeep4x4center.com/jeep-tires/tire-gear-ratio-chart.htm

I hope I am correct in my statements here, I am not a gear expert and have not yet regeared (i run 4.5 " lift and only 32" tires).
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DavidXJ
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PostPosted: Wed Mar 25, 2009 7:07 am    Post subject: Reply with quote

To get the same performance, you would have to go with lower gears, which means a higher number. (3.88:1 to 4.11:1, for example).

There is a way to figure out what new gears you would need in order to have approximately the same final gear ratio you had before the tire change. Keep in mind, that even with the same final gear ration, larger tires have more rolling resistance, so it might even be advisable to go a set lower than you were before.

The formula to figure out your new gears is:

(New Tire Diameter/Old Tire Diameter) x Old Gear Ratio = New Gear Ratio

Here is a link that has a new gear ratio calculator to make it easy: http://www.4lo.com/calc/geartire.htm Keep in mind that they are looking for tire diameter in inches, so if your stock tires have a number like 265/70R15, or similar, your going to have to get out a tape measure and actually measure the tire. Although, there are calculators for that too... from the same website. http://www.4lo.com/calc/gearmetric.htm
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Orangeamc13
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PostPosted: Wed Mar 25, 2009 8:08 am    Post subject: Reply with quote

TJ Apex wrote:

Example: If you had 30" tires and you were travling at 60 mph at 2500 rpm, then do nothing else but have 35's put on, then go 60mph and you'll note that your tach reads 2000 rpm.



It is the reverse of this! You have it backwards! Wink
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kaos
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PostPosted: Wed Mar 25, 2009 4:05 pm    Post subject: Reply with quote

Orangeamc13 wrote:

Quote:
TJ Apex wrote:

Quote:
Example: If you had 30" tires and you were travling at 60 mph at 2500 rpm, then do nothing else but have 35's put on, then go 60mph and you'll note that your tach reads 2000 rpm.


It is the reverse of this! You have it backwards! Wink


Actually, I think he is right, but the way he describes it is a bit confusing. To do the experiment suggested you would have to have a GPS unit or some other way to measure the Jeeps true speed, as the speedo will be skewed once you put on the bigger tires. If you did that and drove at 60MPH true speed, you would find that both the speedo and tach would read lower with the bigger tires.

If, on the other hand, you drove at 60MPH according to the speedo, you would find that the tach reading would be either exactly the same, or, if you were driving an auto and the converter hadn't locked up yet, somewhat higher. More to the point, the true speed would also considerably be higher.

Hope I made it clearer, rather than muddy the waters even further Smile .

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ehirner
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PostPosted: Wed Mar 25, 2009 4:33 pm    Post subject: Reply with quote

Orangeamc13 wrote:
TJ Apex wrote:

Example: If you had 30" tires and you were travling at 60 mph at 2500 rpm, then do nothing else but have 35's put on, then go 60mph and you'll note that your tach reads 2000 rpm.



It is the reverse of this! You have it backwards! Wink

No, he's right.

Same gearing and same speed, a vehicle with larger tires will have less engine RPMs than one with smaller tires.
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TJ Apex
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PostPosted: Wed Mar 25, 2009 4:50 pm    Post subject: Reply with quote

If you have a Jeep with a 30" tall tires, the engine has to rev higher than the same jeep shod with a 35" tires.
I did experiment with my GPS to find this out, it's suprising how far off speedo can get after going to larger tires.
I currently have 31's and when the speedo reads 60mph my GPS reads 66mph. When I had 32's it was 8mph off at 60mph.

There is one small but good thing that no one's thought of, the odometer slows down too, so if you go to sell your ride, the registered mile will be lower than the true mile, unless you re-gear the speedo. Giving it to the man...Wink
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Orangeamc13
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PostPosted: Thu Mar 26, 2009 8:20 am    Post subject: Reply with quote

ehirner wrote:
Orangeamc13 wrote:
TJ Apex wrote:

Example: If you had 30" tires and you were travling at 60 mph at 2500 rpm, then do nothing else but have 35's put on, then go 60mph and you'll note that your tach reads 2000 rpm.



It is the reverse of this! You have it backwards! Wink

No, he's right.

Same gearing and same speed, a vehicle with larger tires will have less engine RPMs than one with smaller tires.


Sorry, brain fart! Wink Had a baby yesterday. Lots on the mind!
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ehirner
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PostPosted: Tue Mar 31, 2009 5:51 am    Post subject: Reply with quote

Orangeamc13 wrote:
Sorry, brain fart! Wink Had a baby yesterday. Lots on the mind!

Congrats. I hope he or she is healthy.
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TJ Apex
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PostPosted: Tue Mar 31, 2009 9:43 am    Post subject: Reply with quote

I second that...
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Orangeamc13
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PostPosted: Tue Mar 31, 2009 10:47 am    Post subject: Reply with quote

It's a boy! He's very healthy. thumbs
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