The table is meant to be used to calculate RPMs easily. If your car has a 4.10 and it spins 3,000 RPMs at 60mph and you want to know what it would spin with a 4.39 simply find the number (1.07 in this case) and multiply it by current RPM. 3,000*1.07 = 3,210 RPMs at the given speed.
The original post for this has been hidden in the spoiler tag below.
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These are OEM numbers. Your car may or may not have had a transmission swap in its lifetime, so this will not be 100% accurate for everyone.
Gear ratios for US Swift/Metro 5-speed:
1st = 3.416 (3.583 1998+ 1.3L/4 cylinder)
2nd = 1.894
3rd = 1.280
4th = 0.914 (O/D)
5th = 0.757 (O/D)
79:18 = 4.389 (1995+ w/1.0L & Convertible)
78:19 = 4.105 (GT/GTi & 1989-1994 3 cylinder)
72:19 = 3.789 (1995+ w/1.3L & XFI)
74:21 = 3.524 (pre-1995 w/1.3L)
Tall vs. Short Gears:
A "taller" final drive is better for MPGs but hurts acceleration. i.e. putting a 3.79 in place of a 4.39.
A "shorter" final drive is good for acceleration but worse for MPGs. i.e. putting a 4.39 in place of a 4.10.
The non convertible Geo Metros from 1989 to 1994 all had 145/80-12 tires OEM. These tires diameter are 21.1 inches. I do not recommend you stay with the 12" rim, tires are more expensive/rare for that size. A popular replacement size is 155/80-12, so I will include that tire into the calculations too.
The 1995+ "Metros" all had 155/80-13 OEM. These tires diameter is 22.8 inches. I do recommend the 13" rims on ALL Geos, but I do not recommend the OEM size on them. I would run 175/70 (diameter=22.6 vs 22.8), they hurt MPGs a little but they make up for it in traction and ride comfort. I will not include OEM size in my calculations because they are so similar in size (OEM is 100.5% the size of the 175).
Convertibles had some weird tire size that was around the same diameter as the 145/80-12
Tires effect on gear ratio:
For all the examples we will use 5th gear, 0.757:1.
Below I am multiplying 5th gear by the final drive. That number is the amount of times the engine has to turn to rotate the tire once.
Final x 5th gear = engine revs:tire revs
A: 4.389 x 0.757 = 3.322:1
B: 4.105 x 0.757 = 3.107:1
C: 3.789 x 0.757 = 2.868:1
D: 3.524 x 0.757 = 2.668:1
Now I am multiplying the times required for the engine to run the tires once, by the amount of rotations it requires to go 1 mile. This will show me the number of times the engine has to turn to go one mile. The bold numbers are the RPMs @ 60mph in 5th gear.
A: 3.322 x 891 = 2960
B. 3.107 x 891 = 2768
C: 2.868 x 891 = 2555
D: 2.668 x 891 = 2377
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A: 3.322 x 954 = 3169
B. 3.107 x 954 = 2964
C: 2.868 x 954 = 2736
D: 2.668 x 954 = 2545
A: 3.322 x 927 = 3079
B: 3.107 x 927 = 2880
C: 2.868 x 927 = 2659
D: 2.668 x 927 = 2473
How to DIY:
First, go to http://www.miata.net/garage/tirecalc.html
Enter into the first set of boxes your tire size.
Take the number that says "revs/mile" and call it (P)
Now multiply that number by A,B,C, or D (your final drive)
The number you get will be your 60mph RPMs.
Tire Size for Lower Ratio DIY:
*credit to KY Metro for helping my sore brain.
"I have the 4.10 and I want it to rev like the 3.79 ratio. " Divide it out:
4.10 / 3.79 = 1.08 (108%, or 8% 'longer legs' on the XFI). So you need an 8% taller tire. Since your tire is 21.1" tall, you need to go to 22.8". That just happens to be the standard height OEM for 95+Metros. Go to the Miata link above, punch in your starting tire on one side, and play around with the second tire. It does most of the work, as it has the diameter, revs/mile, %faster, etc.