Unlike most online "tire calculator" applications, this program was developed to help you assess the effects of changing tire sizes and gear ratios on the performance of your car, as measured by the MPH per RPM. Instead of calculating the percentage of speedometer error (at my age, doing math while driving 70 MPH is the equivalent of texting!), it gives a detailed assessment of the result of changing the revolutions per mile of your wheels by mounting a different sized tire.

Because tires are sized in two different ways, Metric (P-Metric and Euro-Metric) and Non-metric, there is a button at the top of the page to select which measurement you will be using for the tire size. The page defaults to Metric (for our purposes, there is no difference between P-Metric and Euro-Metric). If you are using racing tires sized in inches, then click on the "Non-metric" button and the inputs will be changed to accomodate that sizing format. You can switch back to the previous format if you select it in error.

It was originally written to help select the proper gear ratios based upon the track design on which the car would be run. The chart of MPH per RPM, from 1000 to your entered maximum, gives a quick evaluation of the drop in RPM when shifting up to the next higher gear, and the ultimate top speed of the car if it is limited by RPM rather than horsepower. Additionally, if you have the data on your torque curve, you can enter the torque values (for each 200 RPM up to your maximum RPM) and the program will calculate the best shift points in each gear. The best shift point is deemed to be the point at which the torque available in the next higher gear is greater than the torque at the shift point.

When a field is selected, the field prompt will be reduced in size, and the valid range for the entry is shown in a light grey font. The ranges for each field are:

Note for 928 owners: When I originally wrote this program, the 928 manual 5-speed was designed such that the first 4 gears transmitted power through 5th gear. If this is the case for your 928, then you must enter the gear ratios instead of the gear sets, using the procedure below (for 1st through 4th gear):

The application initially presents entries for first through fifth gear, with fifth gear being optional. To add a sixth gear, click on the "Sixth gear" button. This will add a sixth gear row to the gear entries section, and make the entries for fifth gear required. The button will then be deactivated. If you mistakenly added the sixth gear, you can just leave the sixth gear entries blank, and the program will ignore the fact that you added a sixth gear (but remember that fifth gear entries will now be required). If this is not what you wanted, you will need to refresh the browser page to go back to the initial state of the application. The same is true if you add a seventh gear; entires for fifth gear will be required, and the only way to go back to the original presentation is to refresh your browser page.

Your gear sets or gear ratios can be found in your owner's manual (at least, they were for my admittedly older Porsches). These were the stock gear sets when I first wrote the program:
Car 1st 2nd 3rd 4th 5th Final
1974 911 5-speed11:3518:3323:2927:2529:217:31
1975-79 911 5-speed11:3518:3323:2926:2628:238:31
1980-81 911 5-speed11:3518:3223:2926:2628:238:31
1989-93 911 Carrera 412:4217:3627:3935:3838:337:31
1976 930 Turbo (std)16:3623:3028:2532:21 9:38
1977 930 Turbo16:3623:3028:2532:20 9:38
1978-81 930 Turbo16:3623:3028:2532:20 9:38
1994-95 911 Carrera/4/RS3.8182.1501.5601.2421.0243.444
1977 924 4-speed10:3616:3425:3430:29 9:35
1980-82 924 Turbo10:3616:3424:3528:3137:279:35
1983-84 944 5-speed10:3616:3424:3530:2837:279:35
1983-84 944 5-speed10:3616:3424:3530:2837:279:35
1987-91 944S/S2/Turbo10:3517:3525:3529:3035:2912:42
1978-82 928 manual17:4422:3926:3429:2832:2312:33

Once all fields have been entered, and if you don't have or don't want to enter your torque curve, click on the "Calculate chart" button to produce the chart. Clicking on the "Clear these fields" button resets all the main form fields to empty entries (the torque entry form has its own clear button). If you have entered a field incorrectly, the chart will not be calculated, and an error message for the incorrect field will be shown. While the program will open the chart in a new browser window or tab, this behavior, as well as whether or not focus is shifted to the new window or tab, is determined by the personalized settings of your browser.

If you have the torque curve for your engine, you can enter it into the program by clicking on the "Enter torque curve" button. Note that this button is only active if you have already entered a valid maximum RPM. Another entry form will pop up over the main form, with spaces for torque entries from 2000 to 9000 RPM. You don't have to enter values into all the fields, but you do want to have all the values between 2000 RPM and your maximum RPM entered before saving the form. Note that if your maximum RPM does not fall on an even 200 RPM, you will need to place an entry in the next field to make sure running your engine to redline is evaluated. For example, if your redline is at 5500 RPM, then you will need to enter the torque value for 5600 RPM (extrapolate from your data, if necessary). The program will then interpolate your torque at maximum RPM by averaging the two entries lower and higher than the entered maximum. If you enter your torque curve, the values you entered will appear on the chart in parentheses alongside the RPM values. You can use any torque measurement unit you wish (English or metric), as long as all the entries use the same system, since the program is comparing relative torque rather than absolute values. You can see a sample chart with a torque curve entry by selecting "1975-76 914 2.0 w/torque" from the gear set selection list.

Note: your entries in the torque curve form are not saved until you click on the "Save entries" button on the torque entry screen, and receive a confirmation that the entries have been saved. If you close the window, either by clicking the "Close window" button, pressing the Esc key or clicking outside the entries window, without first saving your entries, a warning dialog will be displayed to remind you that there are entries in the form, but they have not yet been saved. If you click on the "Return to Form" button then the torque entry form will be redisplayed. If you intended not to save the torque entries, then the entries will not be used to calculate the chart.

Sample chart:

Performance Chart – Paul's 914

Tire size:  225/50-15  (868 revolutions/mile)

First gear:  11:34
Second gear: 18:34
Third gear:  23:29

Fourth gear: 27:25
Fifth gear:  31:22
Final gear:   7:31

RPM 1st 2nd 3rd 4th 5th 
 1000
   5.1
   8.3
  12.4
  16.8
  22.0
 1200
   6.1
   9.9
  14.9
  20.1
  26.4
 1400
   7.1
  11.6
  17.3
  23.5
  30.8
 1600
   8.1
  13.2
  19.8
  26.8
  35.2
 1800
   9.1
  14.9
  22.3
  30.2
  39.6
 2000 (85)
  10.1
  16.5
  24.8
  33.6
  44.0
 2200 (88)
  11.1
  18.2
  27.2
  36.9
  48.4
 2400 (89)
  12.1
  19.8
  29.7
  40.3
  52.8
 2600 (91)
  13.1
  21.5
  32.2
  43.6
  57.1
 2800 (92)
  14.1
  23.1
  34.7
  47.0
  61.5
 3000 (94)
  15.2
  24.8
  37.2
  50.3
  65.9
 3200 (96)
  16.2
  26.4
  39.6
  53.7
  70.3
 3400 (97)
  17.2
  28.1
  42.1
  57.1
  74.7
 3600 (98)
  18.2
  29.7
  44.6
  60.4
  79.1
 3800 (98)
  19.2
  31.4
  47.1
  63.8
  83.5
 4000 (98)
  20.2
  33.0
  49.5
  67.1
  87.9
 4200 (97)
  21.2
  34.7
  52.0
  70.5
  92.3
 4400 (96)
  22.2
  36.3
  54.5
  73.8
  96.7
 4600 (92)
  23.2
  38.0
  57.0
  77.2
 101.1
 4800 (90)
  24.2
  39.6
  59.5
  80.5
 105.5
 5000 (88)
  25.3
  41.3
  61.9
  83.9
 109.9
 5200 (82)
  26.3
  42.9
  64.4
  87.3
 114.3
 5400 (79)
  27.3
  44.6
  66.9
  90.6
 118.7
 5500 (76.5)
  27.8
  45.4
  68.1
  92.3
 120.9
      
Shift Points
  4600
  4400
  4400
  4200
 

The first thing you'll probably look at is the MPH at redline in top gear (either the one painted on your tach, or the one you think you can safely use!). You may need to scroll the window to show the bottom of the chart. This will be your top speed if it will be RPM-limited.

If you did not enter your torque curve into the program, check to see how much of an RPM drop you will experience as you shift up through the gears. To do this, look at the MPH at your shift point in 1st gear. Now, look for the same speed in the 2nd gear column, or close to it. The RPM of that row is the RPM you will drop to when you shift from 1st to 2nd gear. Ideally, you want the RPM drop to diminish as you go through the gears, with the drop from 4th to 5th being the smallest. Also, if you have a copy of the torque curve for your car (but didn't enter it in the program), then you can tell if you're too far off the torque peak when you shift.

You can determine your shift points (they're not necessarily at redline) by looking up the RPM drop in conjunction with the torque curve. You want to shift when the torque available in the higher gear after the shift is equal to or greater than the torque available in the current gear at that RPM.

Because this chart is displayed in a separate window or browser tab, you may produce several charts at one time. Clicking on the "Submit" button will produce a new chart in a new window or tab. You may use this feature to do side-by-side comparisons of different tire sizes and/or gear ratios. You may print a chart by using your browser's print function.

Adding Cars To and Deleting Them From the List

You may add, update and delete cars to and from the gear set selection list. This allows you to save your gear and/or torque entries so that they don't have to be re-entered each time you want to generate a chart. You cannot, however, update or delete the entries that are built into the application itself. Use the "Add/update this car" and "Delete this car" checkboxes to perform these functions. If you check the "Add/update this car" box, then, once you have made all your gear entries and, optionally, your torque entries, and calculated the chart, your entries will be saved to the list under the name you entered in the "Car Description" field. To update a car in the list, just select that car first, check the "Add/update car" box, and then modify the entries as desired. When you select an added car from the list, the program automatically updates the "Car Description" field to the name of the selection. If you want to create a new entry from the pre-filled or edited fields, you need to change the car description to something different.

To delete an entry, you may go directly to the car list pulldown and select the car you want to delete. Note that the "Delete this car" box cannot be checked unless you have selected an entry that is not one of the application-provided ones. When you have selected the entry, the "Delete car" button will be activated. Clicking on the button will first bring up a confirmation dialog, and to complete the deletion you must click on the "Delete Entry" button on the dialog. Once confirmed, the stored entries for that car, and the entry for the car in the car list, are removed from the program.

Note: The function of adding cars to the list utilizes your browser's local storage capability (not cookies), the capability of which varies depending on the browser and the device's operating system. While you should have enough room in local storage to save many cars to the list, there is no way to predict the capacity on any individual system. Also, note that these entries are being saved to that browser and device, so that if you run the web application from another browser on the same device, or even a synched browser (e.g., Chrome) on a different device, it is unlikely that the entries will be available on a device and browser other than the one in use when the entries were created.