A common 4x4 application is an upgrade to larger-than-stock tires.
Once completed, this change immediately alters vehicle speed at a
given rpm, rpm at a given speed, and effective gear ratio, which in
turn affect both acceleration and fuel economy. Tire size, gear ratio,
mph and rpm weave an intricate pattern of performance. Changes
one and all four are affected.; knowing any of the three, the fourth
can be easily determined. The following four formulas illustrate the
Once completed, this change immediately alters vehicle speed at a
given rpm, rpm at a given speed, and effective gear ratio, which in
turn affect both acceleration and fuel economy. Tire size, gear ratio,
mph and rpm weave an intricate pattern of performance. Changes
one and all four are affected.; knowing any of the three, the fourth
can be easily determined. The following four formulas illustrate the
If you are contemplating a tire size upgrade and know your rearend
gear ratio,how to calculate 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.
How To Calculate Actual Speed: With the change to bigger tires,
your speedometer will real "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.
gear ratio,how to calculate 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.
How To Calculate Actual Speed: With the change to bigger tires,
your speedometer will real "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.
Example: You’ve replaced your 28-inch OEM tire with a new set of
35-inch new tire and you want to know your actual speed when the
speedometer reads 60 mph:
35*60/28 =75mph
Speedometer ratio adjustment calculation.
Ever wonder how far off your speedometer is with your new bigger
tires? By using a simple ratio calculation, this info will only
approximate your corrected speed and depends on the accuracy of
tire size diameters (assuming 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.
35-inch new tire and you want to know your actual speed when the
speedometer reads 60 mph:
35*60/28 =75mph
Speedometer ratio adjustment calculation.
Ever wonder how far off your speedometer is with your new bigger
tires? By using a simple ratio calculation, this info will only
approximate your corrected speed and depends on the accuracy of
tire size diameters (assuming 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.
Another way of looking at this relationship would be to figure what
the indicated speed would be if you were actually going 60 mph. In
this case, the tire diameter relationship is flip-flopped to:
the indicated speed would be if you were actually going 60 mph. In
this case, the tire diameter relationship is flip-flopped to:
Formula used
(New Tire Dia /Old Tire Dia) * Speedometer MPH=Actual
MPH
Using the previous example, your speedometer reading at an actual
60 mph is:
28*60/35 =48 mph
Gearing up: Using the above tire change as an example, lets 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*3.40/35 = 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.
60 mph is:
28*60/35 =48 mph
Gearing up: Using the above tire change as an example, lets 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*3.40/35 = 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.
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