TIRES - GEARS - RPM - MPH

 
Changing to larger than stock tires will alter a vehicles speed, rpm, effective gear ratio, acceleration, fuel economy.

Tire size, gear ratio, mph, and rpm, form a mathematical pattern of  performance.
Change one, and all are affected; knowing any of the three, the fourth can easily determined.


TIRE DIAMETER = MPH x GEAR RATIO x 336
                              RPM


RPM = MPH x  GEAR RATIO x 336
              TIRE DIAMETER


GEAR RATIO = RPM x TIRE DIAMETER
                MPH x 336


MPH = RPM x TIRE DIAMETER
            GEAR RATIO x 336


If you're considering a tire size change and know your rearend gear ratio use the table below as a quick reference.
This will help you figure your engine rpm at 60 mph cruising speed. Likewise, if you measure your tire size, and observe rpm, and
mph, you can calculate what gears are in your axles.
G.R. vs T.D.
24"
25"
26"
27"
28"
29"
30"
31"
32"
33"
34"
35"
36"
38"
40"
42"
44"
2.56:1
1,971
1,892
1,820
1,752
1,690
1,631
1,577
1,526
1,478
1,434
1,391
1,352
1,314
1,245
1,183
1,126
1,075
2.73:1
2,102
2,018
1,940
1,869
1,802
1,740
1,682
1,627
1,577
1,529
1,484
1,441
1,401
1,328
1,261
1,201
1,147
2.94:1
2,264
2,173
2,090
2,012
1,942
1,874
1,811
1,753
1,698
1,646
1,599
1,552
1,509
1,429
1,358
1,294
1,235
3.08:1
2,372
2,277
2,189
2,108
2,033
1,963
1,897
1,836
1,779
1,725
1,674
1,626
1,581
1,498
1,423
1,355
1,294
3.21:1
2,472
2,373
2,282
2,197
2,119
2,048
1,977
1,914
1,854
1,798
1,745
1,695
1,648
1,581
1,483
1,412
1,348
3.31:1
2,549
2,447
2,353
2,266
2,185
2,110
2,039
1,973
1,912
1,854
1,799
1,748
1,699
1,610
1,529
1,458
1,390
3.42:1
2,833
2,528
2,403
2,341
2,257
2,179
2,107
2,039
1,975
1,915
1,859
1,806
1,756
1,683
1,580
1,505
1,436
3.54:1
2,726
2,817
2,516
2,473
2,336
2,258
2,181
2,110
2044
1,982
1,924
1,869
1,817
1,772
1,635
1,558
1,487
3.73:1
2,873
2,757
2,651
2,553
2,462
2,377
2,298
2,224
2,154
2,089
2,027
1,969
1,915
1,814
1,723
1,641
1,587
3.90:1
3,003
2,883
2,772
2,689
2,574
2,485
2,402
2,325
2,252
2,184
2,120
2,059
2,002
1,897
1,802
1,716
1,638
4.10:1
3,157
3,031
2,914
2,806
2,706
2,613
2,526
2,444
2,368
2,296
2,228
2,185
2,105
1,994
1,894
1,804
1,722
4.27:1
3,288
3,158
3,035
2,923
2,818
2,721
2,630
2,545
2,466
2,391
2,321
2,255
2,192
2,077
1,973
1,870
1,793
4.56:1
3,511
3,371
3,291
3,121
3,010
2,906
2,809
2,718
2,633
2,554
2,478
2,408
2,341
2,218
2,107
2,008
1,915
4.88:1
3,758
3,607
3,469
3,340
3,221
3,110
3,006
2,909
2,818
2,733
2,652
2,577
2,505
2,373
2,255
2,147
2,050
5.29:1
4,073
3,910
3,760
3,621
3,491
3,371
3,259
3,154
3,055
2,982
2,875
2,793
2,716
2,573
2,444
2,328
2,222
5.38
4,143
3,977
3,824
3,682
3,551
3,428
3,314
3,207
3,107
3,013
2,924
2,841
2,762
2,616
2,486
2,367
2,260
5.71:1
4,397
4,421
4,058
3,908
3,769
3,639
3,517
3,404
3,298
3,198
3,104
3,015
2,931
2,777
2,638
2,512
2,398
6.17:1
4,715
4,581
4,385
4,223
4,072
3,932
3,801
3,678
3,653
3,455
3,354
3,258
3,167
3,001
2,851
2,715
2,591
7.17:1
5,521
5,300
5,096
4,907
4,732
4,589
4,417
4,274
4,141
4,015
3,897
3,786
3,681
3,487
3,313
3,155
3,011

 

CALCULATING ACTUAL SPEED

 
With the change to taller tires, your speedometer will read "slower" than the actual vehicle speed. To determine
the percentage of speedometer error, the formula is a simple relationship between old, and new tire diameters:

ACTUAL SPEED = NEW TIRE DIAMETER x INDICATED SPEED
                              OLD TIRE DIAMETER

EXAMPLE: You've replace your 28 inch OEM rubber, with 35 in. tires,
you can calculate your actual speed at a 60 mph speedometer reading.


35 x 60
28
 
Which = 75 MPH


Another way of looking at this relationship, would be to figure what the indicated speed would be
if your were actually going 60 mph. In this case, the tire diameter relationship is flip-flopped to:

 

INDICATED SPEED = OLD TIRE DIAMETER x ACTUAL SPEED
NEW TIRE DIAMETER

Using the previous example, your speedometer reading at an actual 60 mph is:

28 x 60
35
 
Which = 48 MPH

GEAR SELECTION 

Using the above tire change as an example, let's say that your vehicle is currently running a 3.40:1 final drive gear set. Now that you have changed to a taller tire, you want to determine the actual, or effective, final ratio. This can be figured by dividing the old tire diameter by the new, and multiplying by the current gear ratio (:1): 28 x 3.40
                                                                35        Which = 2.72:1

Dropping from a 3.40:1 to a 2.72:1 ratio will reduce off the line responsiveness, and severely affect slow speed trail capabilities. If your new 35 inch rubber is just what you want, but you now need to restore your vehicle's low end, the following formula will allow you to determine what gear set (equivalent) ratio should be installed to compensate, and bring back the low end:


EQUIVALENT RATIO = NEW TIRE DIAMETER x ORIGINAL RATIO OLD TIRE DIAMETER

Or, in this example: 35 x 3.40
                                                              28           Which = 4.25:1
By installing a gear set in the range of 4.25:1, you will not only restore your vehicle's low end responsiveness, you will likewise restore your speedometer's accuracy.


 

FIGURING GEAR RATIOS

 
Knowing what gears are in a given axle is a must when considering that axle for a swap.
The actual ratio/reference code,will normally be on a tag attached to a bolt, or stamped in the axle housing.
 If not found, there's a simple method to mathamatically determining the ratio for an axle below



1). Raise both wheels of the axle, with the transmission in Neutral (Use safety stands and block tires).
2). Make a reference mark on the driveshaft, and on the differential housing.
3). Next, without rotating them, make a mark on both tires, and there respective fender wells.
4). With a friend watching the driveshaft, carefully rotate both tires at the same time, exactly one revolution.
5). The number of turns the driveshaft makes indicates the ratio.
RESULT:   If the driveshaft rotates 4 1/2 turns, for instance, the axle ratio is roughly 4.5:1.
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