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How do I calculate final gear ratio?

Posted By: moparcanuk

How do I calculate final gear ratio? - 02/07/12 03:35 AM

F70/14 tires have a diameter of 24.6 inches. That tire diameter gives me the factory 3:23 ratio.

I am currently running a 3:23 gear with a 28 inch diameter tire. How do I calculate my final gear ratio?
Posted By: moparcanuk

Re: How do I calculate final gear ratio? - 02/07/12 04:00 AM

I found this equation which answers my question:

http://www.4lo.com/calc/geartire.php

Which gives me a final gear ratio of 2.84.

(28/24.6)x3.23 = 2.84

Attached picture 7057799-geartire-formula.gif
Posted By: Cab_Burge

Re: How do I calculate final gear ratio? - 02/07/12 05:04 AM

Quote:

F70/14 tires have a diameter of 24.6 inches. That tire diameter gives me the factory 3:23 ratio.

I am currently running a 3:23 gear with a 28 inch diameter tire. How do I calculate my final gear ratio?

Dude, the final gear ratio remains the same until you change the rear gear ratio What are you trying to find out, engine RPM lost at a steady tire speed, or distant traveled with the taller tires for the same engine RPM or what?
Posted By: moparcanuk

Re: How do I calculate final gear ratio? - 02/07/12 05:11 AM

I was trying to find out what the actual gearing is with a 3.23 gear and 28 inch tires. Since the F70/14s (24.6 inch diameter) that came stock from the factory are smaller in diameter than my 28 inch tires, the final gear ratio on the road has to be higher than 3.23. I now know that it's equivalent to a 2.84 gear.
Posted By: DaytonaTurbo

Re: How do I calculate final gear ratio? - 02/07/12 05:14 AM

That doesn't make it equal to a 2.84 gear. It may make it equivalent to a 2.84 gear with 28" tires but that info isn't terrible useful because most of us pick a tire size and gear around it. I find this to be more useful and entertaining:

http://vexer.com/automotive-tools/speed-rpm-calculator
Posted By: moparcanuk

Re: How do I calculate final gear ratio? - 02/07/12 05:20 AM

I'm not trying to match tires and gears!!

It's a matter of curiosity. I just wanted to know what my final gear ratio is with a 3.23 gear and 28 inch diameter tires. A 3.23 gear with 28 inch diameter tires is equivalent to a 2.84 gear where the tires meet the road. A larger diameter tire will produce a higher gear on the road surface. It has to. It's simple mathematics. That's why cruising at 2500rpm with a 28 inch tire will give you a higher speed than a 24.6 inch tire.
Posted By: DaytonaTurbo

Re: How do I calculate final gear ratio? - 02/07/12 05:26 AM

[sigh] I wasn't disagreeing with you on what you think I am but it's not worth the effort to articulate my point.

Posted By: moparcanuk

Re: How do I calculate final gear ratio? - 02/07/12 05:29 AM

Sorry you feel that way. It's a simple discussion.
Posted By: Twostick

Re: How do I calculate final gear ratio? - 02/07/12 05:52 AM

It's still 3.23:1. The only thing you have calculated is what gear ratio would give you that speed with a 26.whatever tire.

Kevin
Posted By: MuuMuu101

Re: How do I calculate final gear ratio? - 02/07/12 05:58 AM

I like this calculator. Plug in numbers and have fun. Idk what you are trying to do. I think you are reverse thinking this just for curiosities sake. Instead of finding the change in rpm with the new tires you are looking for the change in gear ratio, if I understand you correctly.

Well, here it is:
http://www.csgnetwork.com/rpmcalc.html
Posted By: moparcanuk

Re: How do I calculate final gear ratio? - 02/07/12 06:10 AM

I have to agree with Daytonaturbo.

Attached picture 7058065-double-facepalm.jpg
Posted By: MuuMuu101

Re: How do I calculate final gear ratio? - 02/07/12 06:30 AM

This thread is an utter FAIL on so many levels.
Posted By: Dodgem

Re: How do I calculate final gear ratio? - 02/07/12 10:11 PM

Well if it took 2650 rpm to go 60 with 24.6 tires it will now take 2277
if you were going 60 with the 24.6 tires you will now be going 68 ??
Posted By: Clanton

Re: How do I calculate final gear ratio? - 02/07/12 10:34 PM

Quote:

I found this equation which answers my question:

http://www.4lo.com/calc/geartire.php

Which gives me a final gear ratio of 2.84.

(28/24.6)x3.23 = 2.84


and that is what i have read that every 1" tire change will change the gear .10 like you did.dont let them get in your head and mess thing up.
Posted By: moparcanuk

Re: How do I calculate final gear ratio? - 02/08/12 06:35 AM

Quote:

Well if it took 2650 rpm to go 60 with 24.6 tires it will now take 2277
if you were going 60 with the 28 tires you will now be going 68 ??




At 2650 rpm with 28" tires - Yes!
Posted By: phantomx

Re: How do I calculate final gear ratio? - 02/08/12 07:10 AM

The rpm with your 28" tire is the same as it would be with your stock tires and a 2.84 gear. A 28" tire doesn't magically make your 3.23s into 2.84s, it just makes your 3.23s feel like you kept your old tires and swapped to 2.84s. All you are doing is calculating the effective gear ratio difference due to your change in tire diameter.
Travis..
Posted By: mikemyke71

Re: How do I calculate final gear ratio? - 02/08/12 12:05 PM


Put the Car in Gear, push peddle on right down to the floor, if front tires come off the ground,, you did a good job...
Posted By: 70Cuda383

Re: How do I calculate final gear ratio? - 02/08/12 02:02 PM

Quote:

The rpm with your 28" tire is the same as it would be with your stock tires and a 2.84 gear. A 28" tire doesn't magically make your 3.23s into 2.84s, it just makes your 3.23s feel like you kept your old tires and swapped to 2.84s. All you are doing is calculating the effective gear ratio difference due to your change in tire diameter.
Travis..



Guys, I am pretty sure the original poster is well aware of this fact, and he simply wanted to know what his new tires would FEEL like in terms of rear gear ratio.


we all talk about "going from 3.55s to 3.91s really wakes up your car and gives it a stronger off the line punch" well...what DOES that mean, really? how do you put a number on a feeling? all the OP is trying to do, is put a number on a feeling.

Most car guys will talk about what their rear gear ratio is when in casual conversation. For example, I say my Dakota has 3.92 gears in the rear axle. I don't bore the person to death in a casual conversation by saying "my dakota has a 3.92 gear ratio in the axle, I'm running a 28.3" tall tire, and my first gear ratio in my transmission is a 3.48, my second gear ratio is a...."

because what use is the rear gear ratio, when you can have a 3.55 gear set in the axle that drives like a 3.92 simply by having a transmission with lower gear ratios.

by trying to put everything in terms of "effective rear gear ratio" it's just a way to standardize what everyone is talking about.
Posted By: phantomx

Re: How do I calculate final gear ratio? - 02/08/12 11:35 PM

Quote:

I'm not trying to match tires and gears!!

It's a matter of curiosity. I just wanted to know what my final gear ratio is with a 3.23 gear and 28 inch diameter tires.




Yes, but this statement right here leads me/us to believe that the OP feels that his car has 2.84s due to the tire change. It does not, it is only equal to how the car would feel with 2.84s and the original size tire. If his car originally had shorter tires the 28"ers would make it feel more like 2.60 or 2.70, or if he had say 27" tires originally the 28s would feel more like a 3.0 gear. It's a simple comparison derived from his original tire diameter.
Travis..
Posted By: moparcanuk

Re: How do I calculate final gear ratio? - 02/09/12 02:22 AM

Quote:

The rpm with your 28" tire is the same as it would be with your stock tires and a 2.84 gear. A 28" tire doesn't magically make your 3.23s into 2.84s, it just makes your 3.23s feel like you kept your old tires and swapped to 2.84s.



D'uuuuuuuuuuuuuuhhhhhhhhhhhhhhhhhhhhhhhh. I'm not so stupid to think that my gear 3.23 gear would magically turn into a 2.84. C'mon guys, a little thought before posting.

Quote:

All you are doing is calculating the effective gear ratio difference due to your change in tire diameter.
Travis..



That was my question in my first post!!!!!

Attached picture 7061087-double-facepalm.jpg
Posted By: MuuMuu101

Re: How do I calculate final gear ratio? - 02/09/12 03:00 AM

You're overkilling the double facepalm picture.
Posted By: moparcanuk

Re: How do I calculate final gear ratio? - 02/09/12 04:16 AM

Can you blame me??????
Posted By: cogen80

Re: How do I calculate final gear ratio? - 02/09/12 02:09 PM

Quote:

F70/14 tires have a diameter of 24.6 inches. That tire diameter gives me the factory 3:23 ratio.

I am currently running a 3:23 gear with a 28 inch diameter tire. How do I calculate my final gear ratio?



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