Bronco City 
Specializing in the restoration and customizing of Ford Bronco's.
 But we service all makes and Models.
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    TIRE/ GEAR SIZE CHART        

 


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     Other Bronco Related Resources~ Tech Calculators and Databases~
       • BroncoZone     • RPM / Speed / Gear Ratio Calculator
       • BroncoHQ     • Another RPM/Gear Ratio/Tire chart
       • Bronco.com     • Compression Calculator
       • Classic Broncos     • Dyno Estimator
       • Early Bronco Registry     • Horsepower Estimator
           • Ford Engine Types
       • Northwest Vintage Broncos    
       • Early Bronco Web Ring    
        
   

       Free Answers and Technical Assistance~

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Gear Ratio Guide For Larger Tires

When ever you change tire size from the original size to another there will be a direct effect on power and drivability. We have developed this chart to help you select the optimum gear ratio to put your performance back where you want it.

If you are interested in a change that will give more power or better fuel mileage use the color coding as a guide.

A simple formula

The most simple formula for the correct gear ratio is this:

 

New Tire Size x Old Gear Ratio

New Gear Ratio =

――――――――――――――――

 

Old Tire Size

Example:

The original tire size is 30" and the original gear ratio is 3.91. You plan on moving up to 38". Then the formula would be calculated as: (38*3.91)/30 giving you 4.95. Not all gear ratios are available so you have the choice between 4.88 and 5.13, 4.88 being the closest.

Choose a little bit lower gear ratios if you do a lot of off-road driving and a little bit higher if you do more long distance highway trips.

This chart is based on  65 MPH and a 1:1 gear ratio, on manual transmission (4th gear).  Please NOTE: If your vehicle is equipped with an automatic transmission the RPM wil be higher than shown on the chart, because some slippage will occur in the transmission and torque converter.  With overdrive transmission, your RPM in overdrive will be 15 - 30% less than indicated, depending on the vehicle.

RPM Chart

 

Highway Driving, Good Fuel Economy

 

Daily Driver, Best Overall Performace

 

Towing and Power, Reduced Fuel Economy

 

Gear Ratio

Tire
Size

 

2.73

3.07

3.55

3.73

4.11

4.27

4.56

4.88

5.13

5.29

5.38

5.71

28"

2129

2395

2769

2909

3206

3331

3557

3806

4001

4126

4196

4454

29"

2056

2312

2674

2809

3095

3216

3434

3675

3863

3984

4052

4300

30"

1987

2235

2584

2715

2992

3109

3320

3553

3735

3851

3917

4157

31"

1923

2163

2501

2628

2896

3008

3213

3438

3614

3727

3790

4023

32"

1863

2095

2423

2548

2805

2914

3112

3331

3501

3610

3672

3897

33"

1807

2032

2349

2489

2720

2826

3018

3230

3395

3501

3561

3779

34"

1754

1972

2280

2396

2640

2743

2929

3135

3295

3398

3456

3668

35"

1704

1916

2215

2328

2565

2664

2845

3045

3201

3301

3357

3563

36"

1656

1862

2154

2263

2493

2590

2766

2961

3112

3209

3264

3464

37"

1611

1812

2095

2202

2426

2520

2692

2881

3028

3123

3176

3370

38"

1569

1784

2040

2144

2362

2454

2621

2805

2948

3040

3092

3282

39"

1529

1719

1988

2089

2303

2391

2554

2733

2873

2962

3013

3198

40"

1491

1676

1938

2037

2244

2331

2490

2664

2801

2888

2937

3118

41"

1454

1635

1891

1987

2189

2275

2429

2599

2733

2818

2866

3042

42"

1420

1596

1846

1940

2137

2220

2371

2538

2668

2751

2798

2969

43"

1387

1559

1803

1894

2087

2169

2316

2479

2606

2687

2733

2900

44"

1355

1524

1762

1851

2040

2219

2263

2422

2546

2626

2670

2834

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