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GEAR RATIO, TIRE SIZE, CRAWL RATIO, ETC. CALCULATORS
This calc can be used to find any (one) missing value.
Solve for your MPH.
Solve for your RPM.
Solve for your Differential Ratio.
Solve for your Tire Diameter.
Solve for your Transmission Ratio.
Solve for your Transfer Case Ratio.
Solve for your Crawl Ratio.
If your tires are bigger than stock you probably, like myself, have spend a lot of time trying to WAG (wild ass guess) your actual MPH when going down the road. If you know your gear ratio and tire size you can get a fairly close idea here. Fill in all entries except the one you want an answer to (in green section).
If you'd like to find your overall crawl ratio, enter the Diff/Trans/TransfCase ratios. Remember, the yellow section (crawl ratio) is only for output only, do not type anything in this box.
Clicking on the Transmission or TransferCase links will jump to bottom of this page.
Formulas used
and
These calculators are color coded by function 
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Popular Transmission Ratios
Manual Transmissions:
AMC SR4 (BorgWarner) Jeep 

1st 
2nd 
3rd 
4th 

4.07 
2.39 
1.49 
1.00 
SM 420 

1st 
2nd 
3rd 
4th 

7.00 
3.60 
1.70 
1.00 
SM 465 (MUNCIE) GM, Chevy 

1st 
2nd 
3rd 
4th 

6.55 
3.58 
1.57 
1.00 
T176 Jeep 

1st 
2nd 
3rd 
4th 

3.52 
2.27 
1.46 
1.00 
T177 Jeep 

1st 
2nd 
3rd 
4th 

3.82 
2.29 
1.46 
1.00 
T178 Jeep 

1st 
2nd 
3rd 
4th 

3.00 
2.08 
1.46 
1.00 
Mazda 5spd Ford, Mazda? 
1st 
2nd 
3rd 
4th 
5th 
3.40 
2.20 
1.50 
1.00 
0.79 
NV3500 Chevy 
1st 
2nd 
3rd 
4th 
5th 
4.02 
2.32 
1.40 
1.00 
0.73 
NV4500 Chevy 
1st 
2nd 
3rd 
4th 
5th 
5.61 
3.04 
1.67 
1.00 
0.73 
ZF S6650 Chevy 
1st 
2nd 
3rd 
4th 
5th 
6th 
5.79 
3.31 
2.10 
1.31 
1.00 
0.76 

Automatic Transmissions:
TH 350 Chevy 

1st 
2nd 
3rd 


2.52 
1.52 
1.00 

TH 400 GM, Chevy, Jeep 

1st 
2nd 
3rd 


2.48 
1.48 
1.00 

TH 700R4 GM, Chevy 

1st 
2nd 
3rd 
4th 

3.06 
1.62 
1.00 
0.70 
4L80E Chevy 
1st 
2nd 
3rd 
4th 

3.10 
1.48 
1.00 
0.75 

Allison 1000 Series Chevy 
1st 
2nd 
3rd 
4th 
5th 
3.10 
1.81 
1.41 
1.00 
0.71 
C4/C5/C6 Ford 

1st 
2nd 
3rd 


2.46 
1.46 
1.00 

Ford AOD Ford 

1st 
2nd 
3rd 
4th 

2.40 
1.47 
1.00 
0.67 
TORQUEFLITE 727 Jeep, Dodge 

1st 
2nd 
3rd 


2.45 
1.45 
1.00 




Popular Transfer Case Ratios
Dana 300 Jeep 

Lo 
Hi 


2.62 
1.00 

Borg Warner 1356 Ford 

Lo 
Hi 


2.69 
1.00 

DANA/Spicer 18 Jeep 

Lo 
Hi 


2.46 
1.00 

DANA/Spicer 20 Jeep, IH (TC145), GM, Ford 

Lo 
Hi 


2.00 
1.00 

NP 203 Chevy, Ford, Dodge 

Lo 
Hi 


2.00 
1.00 

NP 205 Chevy, Ford, Dodge, IH 

Lo 
Hi 


1.98 
1.00 

NP 207/208 Jeep, Chevy, Ford, Dodge 

Lo 
Hi 


2.61 
1.00 

NP 231 Jeep (YJ) 

Lo 
Hi 


2.72 
1.00 

Toyota (6774) Land Cruiser 

Lo 
Hi 


2.31 
1.00 


TIRE SIZE CHANGE CALCULATOR
This calc can be used to find a new gear ratio.
Bigger tires can help you crawl up those slick rocks, look cool, and rob you of power. One way to compensate for this is to swap out your differential gears. The left side of the calculator requires all three entries. The right side will show the results. The New Gear box will show the ideal gear ratio (getting as close to stock as possible). If you want a bit more power, for rock crawling, go with something closer to the Lo Gear number. Also, the Effective Ratio box shows your adjusted gear ratio if you choose to keep your original gears.
Formula used
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METRIC TIRE TO DIAMETER (INCHES) CALCULATOR
This calc converts a metric tire to inches.
Solve for Section Width (P or LT).
Solve for Aspect Ratio (width).
Solve for Rim Diameter (inches).
Solve for Tire Diameter (inches).
Solve for Section Width/Height (inches).
Most of my calculators will ask for your tire diameter, if you have metric tires (most of us do) you'll need to get an estimate of how tall they are. Or you can just pull out the ole tape measure.
Most of the formulas dealing with gear ratios will want a tire diameter (measured in inches). This formula is a quick way to get the tire diameter of those metric tires that are common on just about everything stock. For example a LT265/75R16 would be around 31.6 inches tall and 10 inches wide. Enter any three of the numbers into this form to solve for the fourth. "LT" means Light Truck and "P" means Passenger tire. The bigger number (on the left) is the Section Width. The number to the right of the slash ("/") is the Aspect Ratio (percent of width). The "R" means Radial tire and the last number, far right, is the rim diameter (in inches!).
Formula used
Width in inches = section width / 25.4
Section Height in inches = Width in inches X Aspect Ratio (%)
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POOR MAN DYNOMETER CALCULATOR
This calc can be used to guestimate your horsepower.
Solve for your 1/4 Mile Top Speed in MPH.
Solve for your 1/4 Mile Elapsed Time (E.T.).
For those curious about Horsepower Output, here's what I label a "poor man's Dyno."
Solve for HP by MPH : Plug in your top trap speed (1/4 mile) in MPH and your vehicle weight (include your own weight). Press the button and whala... your estimated HP. I did a little reverse math and also solved for the 1/4 E.T. as well.
Solve for HP by E.T. : Plug in your quickest ET in seconds and your vehicle weight (include your own weight). Also solved is your estimated 1/4 mile top speed in MPH.
Formulas used
ET = CubeRoot of (weight/hp) * 5.825
HP = Weight/(ET/5.825)³
HP = (mph/234)³ * weight
MPH = CubeRoot of (hp/weight) * 234
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RAMP TRAVEL INDEX (RTI) CALCULATOR
This calculator estimates the Ramp Travel Index (RTI) without having access to a "real" ramp.
Ramp Travel Index is based on climbing a 20 degree ramp with one tire while keeping the trailing
tire in line. At the point where any tire begins to lift into the air is where it's calculated.
Measure from the bottom of the ramp to the center of the bottom of the tire
(use a plumb from the hub to find center). This distance traveled in inches is divided by
the wheelbase in inches and multiplied by 1000. What this gives you is a percentage of travel
up the ramp in relation to your wheelbase. Basically if your wheelbase is 100 inches, and you
travel 75 inches your RTI will be 750 (75% or .75 X 1000).
Since this is based on math we can use a formula to estimate your RTI.
The trick is to get one tire off the ground while the other three remain touching the ground. Then
measure from the bottom of the lifted tire to ground.
I've seen forklifts being used, climbing up a steep rock face, even climbing stairs.
I suppose you could climb different things and use the calculator to find the RTIs.
Then take the average of all. In any event, if you're curious I've provided a calculator
that will get you an idea of your RTI. Anything above a 500 is great, there are a few rigs
out there that can do perfect 1000's or better. How will you do?
Formula used
RTI = Tire Height/sin(20*PI/180)/Wheelbase*1000
Inches Traveled = Tire Height/sin(20*PI/180)
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4LO.COM: Gear Calculator
RING & PINION CALCULATOR
A simple gear ratio calculator.
Okay, so you've torn your pumpkin apart and have the ring gear and pinion gear teeth count.
This calculator will give you the gear ratio, just plug in the numbers.
There are other ways to guestimate your gear ratio.
Another popular method is to mark your tire and driveshaft,
then carefully roll the vehicle so the tire turns one full revolution.
While you do this, count the number of turns (down to the tenth) of the drive shaft.
This will get you a close idea of your gear ratio.
Of course you can always look for a tag attached to the differential.
Formula used
Ring Gear Teeth Count / Pinion Gear Teeth = Gear Ratio
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SPEEDOMETER CHECK CALCULATOR
Speedometer check calculator.
Solve for Original Tire Diameter.
Solve for New Tire Diameter.
Solve for Original Speedometer MPH.
Solve for New Speedomter MPH (adjusted).
Ever wonder how far off your speedometer is with your new bigger tires? I'm using a simple ratio calculation, so this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assumimg nothing but tire size has changed). If you know specific information about your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator, it's probably a little more accurate.
Formula used
(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH
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ENGINE LITERS TO CUBIC INCH DISPLACEMENT (CID) CALCULATOR
A simple liters to cubic inch displacement converter (also converts to cubic centimeters)
Fill in any one blank to calculate the other two
Not much to this little calculator, just for those curious just how big you engine is... in cubic inches! "There is no substitute for raw cubic inches." In the early days you had 396's and 454's, now we have 5.7 and 8.0 liters. The conversion was simple so I threw in the conversion to cubic centimeters as well.
Formula used
1 liter = 61.02 cubic inches
1 liter = 1000 cubic centimeters
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BLOOD ALCOHOL CONTENT CALCULATOR
This calc will estimate B.A.C.
Use this calculator to get an idea of your alcohol tolerance by weight.
Use this calculator at your own risk, it's not meant to be an excuse to get sloppy drunk.
Females have a lower tolerance for alcohol so assume the estimated BAC is too low.
Based on the quantity of beverages consumed, the alcohol percentage in each drink,
the person's weight, and the time spent consuming the drink.
FLUID OUNCES BY DRINK
Beer 12 oz.  Wine Glass 4 oz.  1 Shot 1.5 oz
ALCOHOL PERCENTAGE BY DRINK
Beer 44.5%  Wine 1520%  1 Shot 3050%
Ounces of alcohol is self explanatory, % of alcohol is either half of the
proof, or marked on the label on wines/liquors. Most beer is 4%.
Formula used
BAC = ((ounces of alcohol * % of alcohol by volume * 0.075) / persons
weight)  (hours of drinking * 0.015)
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