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Old 01-12-2015, 04:55 PM   #1
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gears vs. tire sizes

The best I can determine, increasing the tire diameter by about 2" would effectively be equal to a rear gear ratio change of from 3.31 to 3.55.
Is that right?
Does changing the tire size have the same effect as changing gears?
I know I must be missing something here.
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Old 01-12-2015, 04:56 PM   #2
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Larger overall tire diameter would equate to a lower numerical gearing.


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Old 01-12-2015, 04:58 PM   #3
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Originally Posted by Soccerluvr4 View Post
Larger overall tire diameter would equate to a lower numerical gearing.


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Yeah I can see where the rpm per mph is the same but is the effect the same?

Edit: like there's a torque change when you increase the diameter of the tire but I guess it is offset by the toque increase of the higher speed.???
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Old 01-12-2015, 05:01 PM   #4
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Try using a rpm calculator online. You can plug in different tire heights, and gear ratios to find your rpm at a certain speed. Easy to play around with to see how each affect each other.


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Old 01-12-2015, 05:01 PM   #5
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This is the one i usually use.

Engine RPM / Engine Speed Calculator
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Old 01-12-2015, 05:03 PM   #6
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Try using a rpm calculator online. You can plug in different tire heights, and gear ratios to find your rpm at a certain speed. Easy to play around with to see how each affect each other.


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Yeah thats what I did. That's why I had to check on the final results of power to the road.
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Old 01-12-2015, 05:11 PM   #7
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Having your tire size go up 2" would be like going from 3.73 to 3.31


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Old 01-12-2015, 05:31 PM   #8
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Having your tire size go up 2" would be like going from 3.73 to 3.31


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I'm looking at a few different sites and can't agree. (Yet) I'll look again. lol
Look at this site and match 27" with 3.31 to a 3.55 ratio and you get a 29" tire.
Gear Ratio and Tire Size Chart - HowStuffWorks
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Old 01-12-2015, 05:39 PM   #9
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I'm looking at a few different sites and can't agree. (Yet) I'll look again. lol
Look at this site and match 27" with 3.31 to a 3.55 ratio and you get a 29" tire.
Gear Ratio and Tire Size Chart - HowStuffWorks
I see where your confusion is.......

Say you have a 27" tall tire and 3.31 gears. Now, If you upgrade to a 29" tall tire to keep it the same rpms at the same speed you will then need the 3.55 gear. The taller tire counteracts with the numerically higher gear to give you the same rpm. Otherwise, if you upgrade to a 29" tall tire but leave your current 3.31 gearing in, it will now feel like a 3.15 or a 3.00 gear ratio.

Taller tire = lower revs on highway
Numerically higher gear = higher revs on the highway.
Numerically lower gear = lower revs on highway.

So if you were to change your 27" tire to a 29" tire AND say go from 3.55 to 3.31 it will drop the rpms in 2 different scenarios.

This is why if you wanted to say make your rpms higher on the highway, you have 2 options, either get a numerically higher gear OR a shorter tire. They both do the same thing. If you want lower revs on the highway you can either get a taller tire, or a numerically lower gear ratio. Both do the same thing.

Get what i'm trying to say?
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Old 01-12-2015, 05:59 PM   #10
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I see where your confusion is.......

Say you have a 27" tall tire and 3.31 gears. Now, If you upgrade to a 29" tall tire to keep it the same rpms at the same speed you will then need the 3.55 gear. The taller tire counteracts with the numerically higher gear to give you the same rpm. Otherwise, if you upgrade to a 29" tall tire but leave your current 3.31 gearing in, it will now feel like a 3.15 or a 3.00 gear ratio.

Taller tire = lower revs on highway
Numerically higher gear = higher revs on the highway.
Numerically lower gear = lower revs on highway.

So if you were to change your 27" tire to a 29" tire AND say go from 3.55 to 3.31 it will drop the rpms in 2 different scenarios.

This is why if you wanted to say make your rpms higher on the highway, you have 2 options, either get a numerically higher gear OR a shorter tire. They both do the same thing. If you want lower revs on the highway you can either get a taller tire, or a numerically lower gear ratio. Both do the same thing.

Get what i'm trying to say?
You're absolutely right when you said I was confused.lol
After your explanation and looking at it again it hit me.
I was just playing with the numbers to see what it would take in a tire size change to be equivalent to a gear change. I guess I was backwards, thats why it didn't seem to make sense.
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Old 01-12-2015, 06:51 PM   #11
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You're absolutely right when you said I was confused.lol
After your explanation and looking at it again it hit me.
I was just playing with the numbers to see what it would take in a tire size change to be equivalent to a gear change. I guess I was backwards, thats why it didn't seem to make sense.
It's tough to get old!

Haha. It's confusing at first. But once it hits you it all makes sense and becomes sooo much more clear.


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Old 01-13-2015, 01:07 PM   #12
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Let me add my 2 cents here for clarification.
A numerically higher gear is a lower gear and a numerically lower gear is actually higher numerically.

Confused?

4:10 is a lower gear than 3:55 but it's higher numerically because the numbers are bigger.
It's the ratio that matters. The pinion will turn 4.11 times while the axle will turn once.

Make sense?
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Old 01-13-2015, 01:14 PM   #13
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+1 to above.

Most people get confused by that so I was just saying "numerically" before all the gears.

It's weird


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Old 01-13-2015, 01:31 PM   #14
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So I have 3.15s and if I go up to a 305 tire. It'll be like having some 2.89s lol?


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Old 01-13-2015, 02:05 PM   #15
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So I have 3.15s and if I go up to a 305 tire. It'll be like having some 2.89s lol?


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The width doesn't matter too much. Its more the sidewall. Which remember is expressed as a %.

The difference between say a 275/35-18 and a 305/35-18 is only .8" so its not a huge difference. Might feel like going from a 3.15 to a 3.00. You most likely won't feel a difference.

Now say you went from a 275/35-18 to a 305/40-18 then its a difference of 2" and you will notice the difference.
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Old 01-13-2015, 03:53 PM   #16
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When I ordered my MPT tunes, they wanted to know the gear ratio as well as the tire size. All are factors in tuning for my V6 auto. Just 2 more cents
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Old 01-13-2015, 04:05 PM   #17
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When I ordered my MPT tunes, they wanted to know the gear ratio as well as the tire size. All are factors in tuning for my V6 auto. Just 2 more cents
That is mostly for speedo calibration I think.

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Old 01-13-2015, 04:11 PM   #18
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That is mostly for speedo calibration I think.

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Correct.


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Old 01-13-2015, 11:56 PM   #19
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Quote:
Originally Posted by Soccerluvr4 View Post
The width doesn't matter too much. Its more the sidewall. Which remember is expressed as a %.

The difference between say a 275/35-18 and a 305/35-18 is only .8" so its not a huge difference. Might feel like going from a 3.15 to a 3.00. You most likely won't feel a difference.

Now say you went from a 275/35-18 to a 305/40-18 then its a difference of 2" and you will notice the difference.

So right now I have a 295-35-18 and I'm gonna go to either a 305-45 or a 315-40. Will either make much of a difference?


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Old 01-14-2015, 12:49 AM   #20
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So right now I have a 295-35-18 and I'm gonna go to either a 305-45 or a 315-40. Will either make much of a difference?


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From the 295/35 to the 305/45 it will be a HUGE difference. The 315/40 will make a difference also, but not as much... Still noticeable though.
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