Actual Speed Change from New Tire Size and Gear Change Calculator
#1
11-29-2009
Actual Speed Change from New Tire Size and Gear Change Calculator
Does such a calculator exist?
255/70R16 to 285/75R16 (Speedometer in the vehicle will read less than the actual speed that is being traveled.)
and
3.73 to 4.56 (Speedometer in the vehicle will read more than the actual speed that is being traveled.)
I'm hoping because of the tire and gear changes the speedometer change will be negligible, but wondered exactly what that change is.
255/70R16 to 285/75R16 (Speedometer in the vehicle will read less than the actual speed that is being traveled.)
and
3.73 to 4.56 (Speedometer in the vehicle will read more than the actual speed that is being traveled.)
I'm hoping because of the tire and gear changes the speedometer change will be negligible, but wondered exactly what that change is.
#3
11-29-2009
The speedometer change from 255/70R16 to 285/75R16 would be almost exactly cancelled by going from 3.73 to 4.10 axle ratio. Less than 1% error.
#5
11-29-2009
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#6
11-29-2009
#8
11-29-2009
255's were the stock tires, and 3.73 stock gearing.
Thanks!
Just to verify, you are saying that my speedometer will read 12% higher than I am actually traveling? So if the speedometer says I am going 50mph I will actually be going about 44mph?
Just to verify, you are saying that my speedometer will read 12% higher than I am actually traveling? So if the speedometer says I am going 50mph I will actually be going about 44mph?
#10
11-29-2009
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 3.73 to 4.56 change alone would make the speedometer read 1.223 or 22.3% too high and the 255/70R16 to 285/75R16 alone would make the speedo read 0.918 or 91.8% of actual.
Multiply the 2 together and you get 0.918 * 1.223 = 1.123 or 12.3% higher than the original indicated speed.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Changing from a 3.73 to a 4.10 ratio instead with the same tire change:
0.918 * (4.10/3.73) = 1.009 or 0.9% higher than stock - an error of less than 1/2 MPH at 50 MPH.
#11
11-29-2009
That's the right idea. (This assumes that the factory speedometer reading was exactly right but it usually reads 1~2 MPH fast.)
The 3.73 to 4.56 change alone would make the speedometer read 1.223 or 22.3% too high and the 255/70R16 to 285/75R16 alone would make the speedo read 0.918 or 91.8% of actual.
Multiply the 2 together and you get 0.918 * 1.223 = 1.123 or 12.3% higher than the original indicated speed.
Changing from a 3.73 to a 4.10 ratio instead with the same tire change:
0.918 * (4.10/3.73) = 1.009 or 0.9% higher than stock - an error of less than 1/2 MPH at 50 MPH.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 3.73 to 4.56 change alone would make the speedometer read 1.223 or 22.3% too high and the 255/70R16 to 285/75R16 alone would make the speedo read 0.918 or 91.8% of actual.
Multiply the 2 together and you get 0.918 * 1.223 = 1.123 or 12.3% higher than the original indicated speed.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Changing from a 3.73 to a 4.10 ratio instead with the same tire change:
0.918 * (4.10/3.73) = 1.009 or 0.9% higher than stock - an error of less than 1/2 MPH at 50 MPH.
#12
11-29-2009
That's the right idea. (This assumes that the factory speedometer reading was exactly right but it usually reads 1~2 MPH fast.)
The 3.73 to 4.56 change alone would make the speedometer read 1.223 or 22.3% too high and the 255/70R16 to 285/75R16 alone would make the speedo read 0.918 or 91.8% of actual.
Multiply the 2 together and you get 0.918 * 1.223 = 1.123 or 12.3% higher than the original indicated speed.
Changing from a 3.73 to a 4.10 ratio instead with the same tire change:
0.918 * (4.10/3.73) = 1.009 or 0.9% higher than stock - an error of less than 1/2 MPH at 50 MPH.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 3.73 to 4.56 change alone would make the speedometer read 1.223 or 22.3% too high and the 255/70R16 to 285/75R16 alone would make the speedo read 0.918 or 91.8% of actual.
Multiply the 2 together and you get 0.918 * 1.223 = 1.123 or 12.3% higher than the original indicated speed.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Changing from a 3.73 to a 4.10 ratio instead with the same tire change:
0.918 * (4.10/3.73) = 1.009 or 0.9% higher than stock - an error of less than 1/2 MPH at 50 MPH.
#13
11-29-2009
Tire calculators are convenient but here's a trick (courtesy of John Griggs):
If you need to know a tire diameter and you don't have a resource like the internet available, you can use a conversion factor (1270) to simplify the calculation.
Examples:
285/75R16:
(285 x 75 / 1270) + 16 = 32.8" diameter
235/75R15:
(235 x 75 / 1270) + 15 = 28.9" diameter
325/35VR19:
(325 x 35 / 1270) + 19 = 28.0" diameter
The above will work on any tire that is sized by three numbers in the commonly used metric/English format like 235/75R15, for example. The format may be called P-Metric, Euro-Metric, LT-Metric, etc.
[Note that the 1270 conversion factor cannot be used for “inch size” flotation tires which have the approximate (but usually slightly exaggerated) diameter embossed right on the sidewall (like 31x10.50R15 or 33x12.50R15). If you need a more accurate diameter spec for these tires, consult the manufacturer's literature or website.]
If you need to know a tire diameter and you don't have a resource like the internet available, you can use a conversion factor (1270) to simplify the calculation.
- Multiply the first two numbers in the tire size together
- Divide the result by 1270
- Add the third number to get the tire diameter in inches
Examples:
285/75R16:
(285 x 75 / 1270) + 16 = 32.8" diameter
235/75R15:
(235 x 75 / 1270) + 15 = 28.9" diameter
325/35VR19:
(325 x 35 / 1270) + 19 = 28.0" diameter
The above will work on any tire that is sized by three numbers in the commonly used metric/English format like 235/75R15, for example. The format may be called P-Metric, Euro-Metric, LT-Metric, etc.
[Note that the 1270 conversion factor cannot be used for “inch size” flotation tires which have the approximate (but usually slightly exaggerated) diameter embossed right on the sidewall (like 31x10.50R15 or 33x12.50R15). If you need a more accurate diameter spec for these tires, consult the manufacturer's literature or website.]
#14
12-02-2009
Join Date: Oct 2009
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