Tutorials/Tips 2

Gearbox calculator spreadsheet

Author: PressureLine
Source: Oovee

Things have changed a little since v6, so here’s a quick rundown of how things work:

Data Entry:

  • The only vales that you will need to change are in the Green boxes.
  • Depending on your regional settings you may need to use ‘,’ as your decimal separator.




  • Stall RPM: I generally use the idle rpm for this, but any value of around 500-800 rpm should be ok for this one.
  • Max Torque RPM: The engine speed that the engine generates it’s maximum torque at. Can sometimes be difficult to find for older engines.
  • Max Power RPM: The engine speed that the engine generates it’s maximum power at. This is usually readily available.
  • Max Torque: The maximum torque the engine can produce. Can sometimes be difficult to find for older engines.
  • The units for max torque can be selected via a drop-down list (currently Nm, lbft and kgm)
  • Apart from the Stall value these figures shouldn’t be too difficult to find, power and torque are regularly given as power/torque@rpm.

Torque Converter:

  • Yes/No dropdown for if the gearbox has a torque converter.
  • Selecting ‘Yes’ triggers a slightly different calculation for final torque, which helps compensate for the higher gear ratios most automatic gearboxes have.

Speed Reduction Factor:

  • Decreases the AngVel values across the gearbox without affecting torque.
  • The maximum AngVel value for any gear is 32.00, if you try to use a value of more than 32.00 SpinTires automatically reduces it to 32.00.
  • If when you look on the ‘Calculations’ or ‘Output’ tabs you see that one (or more) of the AngVel values is over 32.00 increase this number until all the AngVel values are less than 32.00.

Wheel Radius:

  • Radius (as defined in the wheel xml file) of the smallest drive wheel that the vehicle uses.
  • This number is used in the calculation of at the speeds given on the output tab, and is used to give a bit of a torque boost for smaller vehicles (SpinTires simply does not handle low torque very well)

Gear Ratios:

  • Ratios for gears 1 through 16 and Reverse.
  • Ratios are usually given in a form like “4.56:1”, take away the “:1” and enter into the sheet.
  • These ratio values can often be quite difficult to find. It is best if you know or can find out the exact model of gearbox you want and try to find that.
  • Sometimes you will only be given a list of max speeds. If you aren’t afraid of maths you can work out the ratios from this, it’s not all that easy though (if anyone gets really stuck with this, flick me a PM, I should be able to help)

Axle Ratio:

  • Just another name for the differential ratio. Usually not too hard to find.
  • You can input Hub and Differential ratios separately and it will multiply them together automatically to provide the overall ratio (ie for portal axles)
  • If you only have a single reduction step, or a total ratio simply input that as one of the values, and leave the other one as 1.00. If you leave one of the cells blank, or as 0.00 your gearbox will not work.

Transfer Case:

  • Much the same as the gearbox ratios. The calculator uses the ‘L‘ value for all the regular forwards gears and reverse. ‘H‘ is used for 1+.
  • Much the same as gearbox ratios, these can be difficult to obtain. Try and get a model number/reference and work from there.
  • If the vehicle in question doesn’t have a switchable transfer case, make ‘L‘ equal 1.00 and ‘H‘ = 2nd divided by 1st (it will then equal the ratio difference between 1st and 2nd, making 1+ equal to 2nd)
  • Some trucks have two speed differentials, in this case use the two differential ratios as the transfer case L and H ratios, and set the Axle Ratios to 1.00.



  • Useful for checking that you haven’t mis-entered any values on the Data Entry tab.
  • Do not change any values directly on this tab.



  • Copy the valid entries (any that aren’t ‘#DIV/0!’) starting with cell A1 (this will give you the proper indenting to paste directly into the truck xml) and paste into your truck xml, ensuring to preserve the end of section.
  • Users in the EU may need to either change their regional settings, or otherwise be prepared to change the ‘,’ used as a decimal separator to a ‘.’ otherwise the copied values will not work in SpinTires.
  • The speeds shown on the output tab are not maximum speed in that gear, they indicate the zero-slip ground speed that you will have at maximum torque for that gear.

Output Sample from values shown in picture above:

		<Motor MaxDeltaAngVel="0.025" Torque="93142">	
			<ReverseGear AngVel="0.46" /> <!-- 3.7kph/2.3mph -->
			<HighGear AngVel="3.64" /> <!-- 7.4kph/4.6mph -->
			<Gear AngVel="0.44" /> <!-- 3.6kph/2.2mph -->
			<Gear AngVel="3.18" /> <!-- 6.5kph/4mph -->
			<Gear AngVel="5.69" /> <!-- 11.6kph/7.2mph -->
			<Gear AngVel="8.87" /> <!-- 18.1kph/11.3mph -->
			<Gear AngVel="13.03" /> <!-- 26.6kph/16.6mph -->



1960-1966 GMC Trucks:

As always, questions, comments and suggestions welcome. Enjoy!


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