There are tire calculators available that allow you to see the difference in your tire sizes and engine rpm's at certain speeds. Your car having an automatic or a manual might be a factor. But likely most important is the rear ratio.  If you are in the 3.40, 3.55 area you can go with somewhat taller tires. If you are in the 3.00 and sub ratios you likely don't want taller tires. Comparing the rpm's of your current tire size and a proposed tire on "paper" is a start. BTW, the Firestone dealer stating 225-40-15 tires is odd.  Following the factors that equates similarly to roughly a 205-60-15" tire which is much more common.


I think that 205-60-15" tires aren't that bad a size..., ratios permitting. And the speedo can be re-calibrated closer with an appropriate speedo gear. It may not be perfect, but you can get it close. If the tires you received hold air try them out temporarily to get an idea of how the car responds with the 225-60-15".


In summary, gear ratios don't mean much and tire sizes don't mean much but tire size and gear ratios combined mean everything.  BTW, have you looked to the Tire Rack for tires?  They even deliver to the many "associate" tire dealers in your area and you just show up with your old tires/rims for the mount and balance.

205 is slightly narrower than 225. 195 is even narrow. The bigger the number the wider the tire (very basically, it is a bit more involved). These numbers are millimeters. Width also depends on what size rim/wheel you put them on.

The other thing is going to be a problem because these cars NEVER came with 15 inch tires. So there is no CORRECT 15 inch size. You will just need to get tires there and have them mounted somewhere else most likely.

here's a decent explanation of tire sizes
https://www.justtires.com/en-US/tire-basics/tire-size

There are as Wittsend said above online tire size calculators for both width/height and the difference in speed.
For speed, all I do is follow my wife with her driving different speeds, and I look at my speedometer and see what the difference is and hopefully remember that number. On the 79 I will be using a GPS speedometer, so gears/tires/etc makes no difference.

Russell did a good job of explaining it. Because you are going to 15" wheels yes, you do want a relatively short 15" tire.  Ideally as close to the original size as possible. I ran a "guessed" 185-70-13' tire as the original through this calculator with a "new size" 15" wheel https://tiresize.com/calculator/ and you can see the range that is acceptable in green. It doesn't mean you can't deviate from that slightly.  As 2.34me stated above he runs 195-55-15" tire and that does fall within the green (acceptable) range.

Plug some number into the calculator and see what you get. You can always order tires on line and then get them mounted elsewhere. As I stated above the Tire Rack has "associated" other brand tire stores that work with them. They will ship to the desired store and you just arrange an appointment.  The bulk of a tire stores profits likely come from the mount and balance far more than the tire itself. So, while a local store would also prefer to sell you the tires they are likely content to take the bigger chunk of profit on the mount and balance.

ford man

thanks for posting the link for the gears !
Need some !

PROBLEM SOLVED!
    Boy am I dumb. I have been fighting with lying firestone for 3 months. They lied again today. They told me there was only one tire they could source in my needed size. I called another firestone and asked for options in my size. They had loads. I was going to use this information to pressure them to deal fairly. Then I thought, why not get the new store to honor my tire guarantee.
    They were wonderful. They gave me $110 ea. credit on my 7 month old $115. tires. I need to paint Russ's rims, so installation is next week. Thank you all for advice and ideas


Bill

Here is a side by side comparison of tires that came with the car and what I am getting on Tuesday.
 OEM  195/70/13       Hight  23.7   Width 7.7    Speed 60mph
195/55/15                         H 23.4   -1/8”  W 7.7        Speed 59.2
This is the tire Firestone said was the only tire that would fit, and no longer made.
225/40/15                          H 22.1    -1 ½    W 8.9  +1 1/8       Speed  55.9

I will post photos when it is all done.
Thanks again Russ
 
 

I ran a set of Fox LX wheels on my 75 Bobcat, with 225-60-15 for years with a 1/4 inch spacer. Started without, backs wedged against the springs. The spacer took care of the problem.

They sound like 15x7s with 4.5" backspacing, which is common for Fox3s. I have some 10-hole wheels off of an '86 T-bird I was going to use, but didn't want to fuss with spacers. Then I went 5-lug anyway with 15x7 slot mags. 


I went with 205/50-15s to keep the height close to stock. It's slightly shorter so the speedo reads is just a tad high, but that's better than too low in my opinion. One thing to be aware of is that short sidewalls like mine will make you feel the road noticeably more for better or for worse.