The Ratios Thread

Coche Blanco
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Joined: July 16th, 2009, 2:22 am

July 5th, 2010, 6:35 pm #1



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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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)
Final drives:
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)
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.

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.


Tires General:

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.

175/70-13

A: 3.322 x 891 = 2960
B. 3.107 x 891 = 2768
C: 2.868 x 891 = 2555
D: 2.668 x 891 = 2377

Click the spoiler for other calculations.
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145/80-12:
A: 3.322 x 954 = 3169
B. 3.107 x 954 = 2964
C: 2.868 x 954 = 2736
D: 2.668 x 954 = 2545

155/80-12:
A: 3.322 x 927 = 3079
B: 3.107 x 927 = 2880
C: 2.868 x 927 = 2659
D: 2.668 x 927 = 2473
Larger diameter tires essentially lower the gear ratio down and in doing so, lower the cruising RPMs and increases fuel economy.

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.
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Last edited by Coche Blanco on May 6th, 2018, 1:41 pm, edited 61 times in total.
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Good bye
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Joined: December 3rd, 2008, 4:48 am

July 6th, 2010, 4:32 am #2

Very good, keep going. :)
This will help us all make decisions about tires and gears, since 3.52s are rare and 3.79s are very available. What size 13s do we need to buy to equal 3.52s. I already have 3.79s and 175/70-13s on my 91.
Thanks
Andy
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bogs
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Joined: October 6th, 2009, 4:57 am

July 6th, 2010, 8:01 am #3

Good start on the thread CB .

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Coche Blanco
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Joined: July 16th, 2009, 2:22 am

July 6th, 2010, 8:55 am #4

I'm going to sleep. I'd like some good criticism on this when I wake up. But be nice, it is 4 a.m. here.
1993 3/5 2dr Hatch 'The White One' (SOLD)
1995 4/5 4dr Sedan 'Gumby' (SOLD)
1993 3/3 Convertible 'Impulse' (SOLD)
1993 3/5 Convertible 'Impulse V2.0' (SOLD)
1998 4/5 2dr Hatch 'TBD'
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Woodie
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Joined: February 13th, 2009, 12:29 pm

July 6th, 2010, 9:40 am #5

Nicely done, this will be helpful indeed.

Here's your criticism, but it's minor quibbles and typos:

Up top, you should lose the 89-94 from this title - "Gear ratios for 1989-1994 US Swift/Metro 5-speed:"

Next paragraph, titled "Final drives", you should lose the wheel/tire size.

I was going to suggest four or five other names the car was sold as, but we are talking about North America, you probably chose best.

In the first sentence of "Tires General" you say "A popular replacement size is 144/80/12," you mean 155/80-12, simple typo.

Last edited by Woodie on July 6th, 2010, 11:53 am, edited 1 time in total.
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Coche Blanco
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Joined: July 16th, 2009, 2:22 am

July 6th, 2010, 3:36 pm #6

OH MY GOD!!!!!!! I nearly deleted the whole post...lmao

Thanks Woodie, that's what I needed. A fresh set of eyes. I've got to reword that last paragraph to make it a little easier to understand.
1993 3/5 2dr Hatch 'The White One' (SOLD)
1995 4/5 4dr Sedan 'Gumby' (SOLD)
1993 3/3 Convertible 'Impulse' (SOLD)
1993 3/5 Convertible 'Impulse V2.0' (SOLD)
1998 4/5 2dr Hatch 'TBD'
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mjspiess
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Joined: March 15th, 2009, 4:13 pm

July 6th, 2010, 3:54 pm #7

I think this fits in "The Ratio Thread" as well... I made this table for people who want to run different rims/tires, but would like speedo/odo stock or as close to stock as possible. Again, not all these tires exist or even fit the Metro, but in a perfect world would give you the closest OEM results. The combos in BOLD are dead on combos that will keep your speedo/odo reading correctly.

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Coche Blanco
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Joined: July 16th, 2009, 2:22 am

July 6th, 2010, 4:11 pm #8

I'd like someone to tell me what's the max size tire you can run on a stock height Metro...
1993 3/5 2dr Hatch 'The White One' (SOLD)
1995 4/5 4dr Sedan 'Gumby' (SOLD)
1993 3/3 Convertible 'Impulse' (SOLD)
1993 3/5 Convertible 'Impulse V2.0' (SOLD)
1998 4/5 2dr Hatch 'TBD'
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KY Metro
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Joined: December 12th, 2009, 3:17 pm

July 6th, 2010, 4:46 pm #9

You could make this much simpler. I am not good at explanations, but I think the whole revs/mile thing is unnecessary.

Look at your example. You have a 4.10 and you want a 3.79, right? 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.

Look at the Rims n Tires Calculator.

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.
Last edited by KY Metro on July 6th, 2010, 4:47 pm, edited 2 times in total.

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KY Metro
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Joined: December 12th, 2009, 3:17 pm

July 6th, 2010, 4:58 pm #10

BTW, for the 95+ to get XFI gearing just from the tires, 4.39 / 3.79 = 1.16, or 116%. So you would need tires 16% over 22.8", which would be 26.4" diameter. Thats a tall tire.

The closest I can find in production is a 165/80 r15, which is 25.4", or 11.6% taller. Effectively lowers the ratio to 3.93.

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