How to figure gear differences?

LBZ_Duramax

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Joined
Aug 18, 2011
Location
Benson, NC
I'm looking at an 05 z71 tahoe. It is lifted with 35" tires. He has had it regeared to 4.56. How do I figure the difference in these gears vs the larger tires? I want to find out how close this is to a factory ratio with factory size tires. Can anyone help?
 
Google something like "gear ratio calculator tire size". There are lots of online calculators where you can entire two tire sizes and a gear ratio or whatever and will calculate a difference... Just poke around and see which of them output what you actually want, because theres tons of them out there to choose from. As long as you know the factory ratio you're comparing to, you're good to go. You've got two ratios, and two tire sizes, so if all you care about is comparing shaft RPM or engine RPM or whatever, it's quite easy because nothing changed but the diff ratio so you can neglect everything else.

I usually just use Excel and some math and do it myself, but that depends on your comfort level with ...math.
 
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Fabrik8 said:
Google something like "gear ratio calculator tire size". There are lots of online calculators where you can entire two tire sizes and a gear ratio or whatever and will calculate a difference... Just poke around and see which of them output what you actually want, because theres tons of them out there to choose from. As long as you know the factory ratio you're comparing to, you're good to go. You've got two ratios, and two tire sizes, so if all you care about is comparing shaft RPM or engine RPM or whatever, it's quite easy because nothing changed but the diff ratio so you can neglect everything else.

I usually just use Excel and some math and do it myself, but that depends on your comfort level with ...math.
Click to expand...

Math wasn't my strong point. Lol

I looked through Google and couldn't find what I was looking for.
 
The first Google hit for "gear ratio calculator tire size" will give you a rough answer. It won't spit out a percentage difference, but it will tell you how bad it is.

New Ring and Pinion Gear Calculator (For Larger Tires)

If all you care about is percent difference, you only need tire sizes and rear ratios and nothing else. Transmission doesn't matter, because the difference in shaft RPM is all you need to know. If you care about engine RPM, that's a different story. You haven't stated what you actually want to know yet.
 
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Factory ratio should be 3.73.
Regeared to 4.56

Factory tire is 265/70r17
Tire on it now is 35-12.50

I just want to know if the lower gear offsets the increased tire size.

I'll try your Google link.
 
That would be a 31.6" Tire stock, so the math would be:
35/31.6=1.108
4.56/3.73=1.222

So your speedometer should be off by 1.108/1.222=.907.

In other words, youre geared about 10% lower than stock. If the speedo says you're going 60mph, you're actually going about 54mph.
 
jeepinmatt said:
That would be a 31.6" Tire stock, so the math would be:
35/31.6=1.108
4.56/3.73=1.222

So your speedometer should be off by 1.108/1.222=.907.

In other words, youre geared about 10% lower than stock. If the speedo says you're going 60mph, you're actually going about 54mph.
Click to expand...


The link said optimal ratio would be 4:10 for 35". I guess if I get it I would be ok to go to 36-37 if they will fit. It's just going to be a mall crawler.
 
For easy math:

31/35= .885
.885x3.73= 3.30

So with 35s on a 3.73, it would act like 3.30 gears. To get the equivalent gear, reverse the first part:

35/31=1.13
1.13x3.73= 4.21

So, 4.10s would be a little less than stock and 4.56s would help out with the extra weight.
 
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