I think
dobey has made a number of good points in this thread, including the tacit observation that one often can simplify otherwise overly complicated-looking mathematical formulas, as was done with the Speed (MPH) formula he mentioned sourcing from (page 90 of) the
Auto Math Handbook:
| | | quote | Originally posted by dobey:
This is the only formula you need, to calculate what speed you should have:
code:
gear ratio = ratio of current transmission gear x final drive ratio
rpm x tire diameter
mph = -----------------------------
gear ratio x 336
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In the spirit of simplifying things, like the Speed (MPH) formula
CowsPatoot listed for his helpful on-line calculator, look at the three constants within that formula. Taken together, those three constants comprise one multiplier: 31.415/1056/10. In tiurn, that somewhat complicated-looking multiplier reduces to one
divisor, which --- rounded to the nearest integer --- is simply 336 (i.e., the 336 in 1/336, which is the same 336
dobey used in the easier-to-remember formula he cited).
That aside, however, a key benefit of
CowsPatoot's on-line calculator is that it also provides an estimate (expressed in inches) of a tire's outer
diameter if one inputs to that calculator a Fiero's rear tire
size (e.g., 24.16 inches in diameter for a 215/60-14 rear tire's size).
dobey makes another good point, one which although simple, people nevertheless sometimes forget:
| | | quote | Originally posted by dobey:
...it's easy to get the wrong output by using the wrong inputs.
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To that I'd add that although it may seem counterintuitive to some,
it's also easy to (periodically) get the correct output from a Speed (MPH) formula even when one ignores inputting the gear ratio of the transmission gear that the car is in. However, at other times, when one ignores inputting the gear ratio of the transmission gear that the car is in, the output of a Speed (MPH) formula can be very, very wrong --- even though you may have used successfully in the past a Speed (MPH) formula that
didn't call for any transmission gear input, and that formula seemed to work just fine without that before --- maybe for several years.
Why? How can
that happen?
Well, there actually is no magic, nor even any real mystery involved here --- provided that the gear ratio of the transmission gear one's car is in, happens to be 1.00:1. --- as it is when I'm in the 3rd gear in my '86 Fiero with an automatic TH-125, and as it is when I'm in the 3rd gear of a different car, a `74 with a TH-400 automatic transmission. Two keys points are in order here:
- Within the Speed (MPH) formula highlighted above, inputting a transmission gear ratio of 1.00 --- has absolutely no effect on the formula's output, which is the identical result produced when ignoring a 1.00:1 transmission gear ratio.
- Conversely, ignoring a transmission gear ratio other than 1.00 easily can produce Speed (MPH) output results that are very, very wrong.
