Actual Speed Change from New Tire Size and Gear Change Calculator - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 11-29-2009
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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.
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Old 11-29-2009
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Get you a tuner so you can change that so it's correct. Plus you'll get some power. Win win Haha
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Old 11-29-2009
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Quote:
Originally Posted by KLC View Post
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.)
If you made those two changes together, your speedometer would read more than 12% higher than before.

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.
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Old 11-29-2009
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Check out this tire/wheel converter. DML Tire and Wheel Calculator
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Old 11-29-2009
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ok were the 255 stock tires? if so can any one answer if my speedo would be off, having 4.10 gears and 255/70/16 from the factory. the reason i ask it seems driving down the interstate at 75 mph everyone is passing me. i knock it up to i'm driving slower.
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Old 11-29-2009
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look on your door sticker for the factory tire size, usually ranger speedos tend to read fast from the factory

currieenterprises.com has gear and tire calculators
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Old 11-29-2009
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yeah my speedo was off before i put 33's on. then it was near corrected.
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Old 11-29-2009
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255's were the stock tires, and 3.73 stock gearing.

Quote:
Originally Posted by rwenzing View Post
If you made those two changes together, your speedometer would read more than 12% higher than before.

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.
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?
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Old 11-29-2009
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Quote:
Originally Posted by RangerNDog View Post
Check out this tire/wheel converter. DML Tire and Wheel Calculator
Thanks!
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Old 11-29-2009
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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?
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.
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Old 11-29-2009
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Quote:
Originally Posted by rwenzing View Post
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.
Okay, thanks alot for the information.
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Old 11-29-2009
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Quote:
Originally Posted by rwenzing View Post
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.
That's some awesomely useful information right there. I see a future sticky in the making.
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Old 11-29-2009
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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.
  • 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.]
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  #14  
Old 12-02-2009
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this calculator is much easier to use...

Tire Size Calculator - tire & wheel plus sizing
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