Finding Proper Gear Ratio For Bigger Tires

[misweetrevenge]

BEFORE NOTE:
THIS IS FOR REFERENCE PURPOSES ONLY. I AM NOT TELLING YOU THIS IS WHAT GEARING YOU SHOULD HAVE. THIS IS FROM SOME RESEARCH I HAVE DONE AND HAVE ONLY COMPILED TOGETHER INTO ONE PLACE. THIS IS SUPPOSED TO GET YOU BACK TO CLOSE-TO-STOCK GEARING FOR YOUR BIGGER TIRES. I HAVE NOT DONE THIS TO MY RIG YET SO I CAN NOT SPEAK FROM PERSONAL EXPERIENCE AGAIN THIS IS RESEARCH I HAVE DONE AND COMPILED.

ok so from the research I've done this is what I have found out... before I decide what ratio to go with I want to know for sure that this is correct so if this is wrong PLEASE inform me...

What I have found:
1) I got the code of my rear differential off the axle itself which happens to be "GM2" (laying under the truck with feet at rear and differential above your head mine was on the bottom right.



2) Taking that code I went to this website: GM Axle Ratio Chart
According to the chart I have a 3.44:1 Gear ratio.

3) Find the sticker on your door and get the STOCK tire size. (mine happens to be 235/85/16)

4) Get the size of your current tires. (mine are 33/12.5/16.5)

5) Go online and find a tire size converter. I used this one: Tire Size Converter (the stock tires now become 31.7x9.25x16)

6) Find the percentage of difference between the tires. I did it as follows:
C is for current tire
S is for stock tire
D is for difference
P is for percentage

C - S = D then you take the difference you got and do this:
D / S = P

So using my numbers in the above formulas will look like this:
33 - 31.7 = 1.3
1.3 / 31.7 = 0.04100946 or 4%

If you'd like to check your work you can do this:
S + P = C
My numbers: 31.7 + 4.100946% = 33

So now that you have the percentage your tire has increased you can use this to find the gear ratio you should have TO BE CLOSE TO STOCK RATIOS

GM2 = 3.44
so we take the stock gear ratio and add 4% to it
3.44 + 4% = 3.5776

and there you have it. AGAIN PLEASE TELL ME IF THIS IS INCORRECT

 

[Hogback Fabrication]

6) Find the percentage of difference between the tires. I did it as follows:
C is for current tire
S is for stock tire
D is for difference
P is for percentage

C - S = D then you take the difference you got and do this:
D / S = P

So using my numbers in the above formulas will look like this:
318 - 235 = 83
83 / 235 = 0.353191489361702 or 35%

On a metric tire, the first number is width in mm, the second is the aspect ratio and the third is the wheel size.

You're comparing the two tire widths, not the tire heights.

Convert the 235/85/16 back to a height in inches and you'll find it's ~31-3/4" tall. Your 33's probably aren't actually much taller than that, if you measure them. That does sound tall for a stock tire though.

 

[Jester1]

Or if you have a smart phone you download free garage buddy app from turtle rock, punch in a few #'s and whamo it does the calcs for you.

Hot rod mag did an article of 101 things gear heads need. It's handy as he'll. Has all kinds of stuff like sizing carb cfm's for engines, metal thickness, conversion charts, yada yada.....

Everyone on this board should check it out.

 

[misweetrevenge]

On a metric tire, the first number is width in mm, the second is the aspect ratio and the third is the wheel size.

You're comparing the two tire widths, not the tire heights.

Convert the 235/85/16 back to a height in inches and you'll find it's ~31-3/4" tall. Your 33's probably aren't actually much taller than that, if you measure them. That does sound tall for a stock tire though.

Ok I'll look into that. thanks for the input. Other than that though everything there seem ok?

 

[misweetrevenge]

Or if you have a smart phone you download free garage buddy app from turtle rock, punch in a few #'s and whamo it does the calcs for you.

Hot rod mag did an article of 101 things gear heads need. It's handy as he'll. Has all kinds of stuff like sizing carb cfm's for engines, metal thickness, conversion charts, yada yada.....

Everyone on this board should check it out.

Sounds like a great thing to have. Is it for your computer or can you get it for your phones do you think?

 

[misweetrevenge]

On a metric tire, the first number is width in mm, the second is the aspect ratio and the third is the wheel size.

You're comparing the two tire widths, not the tire heights.

Convert the 235/85/16 back to a height in inches and you'll find it's ~31-3/4" tall. Your 33's probably aren't actually much taller than that, if you measure them. That does sound tall for a stock tire though.

Just did the conversion from metric to inches and you sir are correct.
235/85 turns into 31.7x9.25

So from this I should look at the percentage difference from the 31.7 to the 33s correct?
which would be about 4%
which means the gearing should go from stock at 3.44 to 3.5776 (or as close to 3.58 as you can get) to be back to stock gearing.

 

[chulisohombre]

Since the difference is 4 percent you want to multiply your gear ratio by 1.04 and that adds your 4 percent to it. So it would be 3.44x1.04. Also if you are going to regear, jump up in number to the next available ratio. In your case 3.73 would be a better choice to get past the bigger tires rolling resistance. You wouldn't see much difference from 342 to 355 ratio. I went from 373 to 410 and the ratio and power were way closer to stock than would have been 389 like the ratio would have been going From the calculator. That's my take and I've heard many people say the same thing.you also have to look at what transmission you have and what rpm the engine would be at on final drive ratio as well.

 

[K588]

On a non-od ,V8 equipped vehicle I always did tire height X .12. example 38 x .12 = 4.56 always got me pretty close.

 

[Carcinogen]

There are also charts all over the place online, superlift has a good explanation:

http://www.superlift.com/library/gears.asp

 

[sweetk30]

play with this its faster and more fun http://www.grimmjeeper.com/gears.html

 

[nvrenuf]

The chart and formulas available on www.4lo.com are a whole lot easier.

 

[Big Ray]

that difference is soooooo small....do you really plan to spend all that $$$ on changing the gears...(front and rear)...?

 

[Hogback Fabrication]

that difference is soooooo small....do you really plan to spend all that $$$ on changing the gears...(front and rear)...?

^True for sure!

Another thing to consider, the 73-87 Chevy's came with gears as tall as 2.56:1 and as deep as 4.56:1 stock (I've seen reference to factory 4.88 and maybe 5.13 but haven't seen them personally), with basically the same tire. And stock may not be ideal for your transmission/tire setup, or, your location (mountain trucks usually had deeper gears). So there's more to consider than just getting things back to "stock".

 

[misweetrevenge]

that difference is soooooo small....do you really plan to spend all that $$$ on changing the gears...(front and rear)...?

No I don't. Using this information I found the very small difference is not even close to being worth the $2000.

^True for sure!

Another thing to consider, the 73-87 Chevy's came with gears as tall as 2.56:1 and as deep as 4.56:1 stock (I've seen reference to factory 4.88 and maybe 5.13 but haven't seen them personally), with basically the same tire. And stock may not be ideal for your transmission/tire setup, or, your location (mountain trucks usually had deeper gears). So there's more to consider than just getting things back to "stock".

personally I know what considerations to take into account for gearing as I've had a few muscle cars and the first thing we do to them is regear. But this is good info for those that I put this up for so thanks.

 

[Big Ray]

 

[blazinzuk]

The one thing no one seems to take into account is static loaded radius of the tire. If you have a 35" tire, the distance from the hub up is going to be greater than the hub down.

Nothing to really do with your formula. But something to think about when you are figuring your rpms

 

[Big Ray]

I think that's why most tech articles suggest marking the tire and doing 1 revolution, measure & divide by pi...more accurate...


also, I've noticed most tire manufacturers state that distance (distance per revolution) for their particular tires. Of course this doesn't allow for wear.

 

[6.2Blazer]

These formulas and calculators are a good place to start but do not take into account other factors. All they do is tell you what gears are needed to give you the same RPM at some speed. They do not take into account rotational mass, rolling resistance, and aerodynamic drag caused by larger tires. Point being is you can run 30" street radials on aluminum rims and then switch to 44" Boggers on beadlocks and regear so you have the exact same RPM at 55 mph the truck is going to feel like it lost a bunch of power. In this case you need a lot lower gearing than what the calculator says to help compensate for it.

Again, these calculators are good tools but definitely not the be-all, end-all for regearing.

 

[misweetrevenge]

These formulas and calculators are a good place to start but do not take into account other factors. All they do is tell you what gears are needed to give you the same RPM at some speed. They do not take into account rotational mass, rolling resistance, and aerodynamic drag caused by larger tires. Point being is you can run 30" street radials on aluminum rims and then switch to 44" Boggers on beadlocks and regear so you have the exact same RPM at 55 mph the truck is going to feel like it lost a bunch of power. In this case you need a lot lower gearing than what the calculator says to help compensate for it.

Again, these calculators are good tools but definitely not the be-all, end-all for regearing.

thats why I said its for "close-to-stock" gearing. maybe you're right though maybe I should add "close-to-stock" RPM at "close-to-stock" speed on highway...



Or maybe I should just have this thing deleted since it seems its getting more criticism than it is helping newbies like myself that are SIMPLY LOOKING FOR STOCK RPMs AT A CERTAIN SPEED.

 

[chulisohombre]

It's a good start to a overall guide. The criticisms are to help the thread be more accurate and state the real world issues thàt come up when regearing a truck. It in no way is a poke at you but will help others when they search for this info and read all that was said here.