I think
dobey has made a number of good points in this thread, including the tacit observation that one often can simplify otherwise overly complicatedlooking 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


In the spirit of simplifying things, like the Speed (MPH) formula
CowsPatoot listed for his helpful online 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 complicatedlooking 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 easiertoremember formula he cited).
That aside, however, a key benefit of
CowsPatoot's online 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 215/6014 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.


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 TH125, and as it is when I'm in the 3rd gear of a different car, a `74 with a TH400 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.