Enter gearbox ratios and final drive ratio below, either by either selecting a Subaru gearbox code or adding the ratios by hand into the boxes. The tyre size of the driving wheels are also needed to plot a graph for road speed against engine speed for each gear. Entering the engine “red line” rev limit will also calculate the maximum speed in each gear, and specifying a gear change rev point will plot the gear change points on the graph to show the vehicle speed when the gears are change.
All Subaru transmissions are fitted with addition gear sets as part of their design. In the majority of transmissions these have a ratio of 1:1 and therefore can be ignored in these speed calculations. However, in a small number of Subaru transmission designs these gear sets have been used to provide additional Reduction Ratios. For example, the three-speed automatic transmission fitted to certain Leone (L-series) models had an additional 1.026:1 reduction ratio, and more recently certain turbocharged Impreza models sold in Europe and the USA have a 1.100:1 reduction ratio in the transfer case. When calculating speeds for cars fitted with these transmissions, the ratio value input in the Final Drive box should be adjusted to take this into account. This is done by multiplying the ratios together, so for example, a transmission with a 3.545:1 ratio rear differential and a 1.100:1 ratio transfer case will have a Final Drive ratio of 3.900:1.
Also, many non-turbocharged Subaru models came fitted with Selectable Dual Range auxiliary gear ratios. When operating in “Hi Range” the auxiliary gear ratio is 1:1 so can be ignored in these speed calculations. If you wish to calculate the vehicle speed while operating in “Lo Range” the Final Drive value should be adjusted in the same way as above.
Calculation accuracy. While calculations of the gearbox ratios are precise, the road speed calculation needs to factor in the tyre size, which can vary depending on a number of conditions. The exact tyre size can change depending on a number of variables, including tyre air pressure and temperature, tread depth, rim width, axle load, and deformation due to rotational forces. Therefore, the road speed result output above should be treated as a best estimate under ideal conditions.