Effective Gear Ratios

[bill]

...... it will take 2.52 revolutions of the input shaft to equal 1 full turn of the output correct?

then it gets divided again at the rearend by 2.73, and 2.52x2.73=6.87, so this means it will take 6.87 turns of the engine to move the wheels one full revolution? this seems wrong to me but who knows.. its all voodoo to me right now

but all of that aside, how do i figure out what my actual final drive is including tires?

Yes....in order for the engine to move the weight of the car without bogging down, there has to be a gearing up of the torque the motor makes. Thats why when referring to diff ratios, the higher the number, the lower the gearing....meaning the more motor revs needed per single rev of the axleshaft...
In your situation, maybe the real issue is what size tire is best for the situation....if you never change gears, you are basically running the engine, the trans and the diff at the same ratio all the time all the way around the track, and the only variables are the engine rpms and the tire diameters. For example, if you are pulling away from every one out of the turns, but they catch you and pass you at the end of the straight, then maybe your gear is too low or your tire is too small.....Im not sure what problem youre trying to solve .... just throwing out scenerios.

 

[Intragration]

There is no number. The only time you can reference an effective gear ratio is with a comparison of two varibles for one constant. ei. two tire sizes and one gear ratio. It has to be a comparison.

x2 The first tire, your baseline (or if you're only calculating for one tire size) is 1:1. It's irrelevant until you calculate for a second tire size, which then becomes a factor of the first and allows you to calculate effective gear ratio with the second tire size as compared to the first.

 

[Mike P]

“......how do i figure out what my actual final drive is including tires?......”


Simple answer is you don’t. Your final drive ratio is determined by the gears inside the differential. The term effective ratio is normally used when using an overdrive unit of some type and is based on what the engine “sees” .....i.e. with a rear axle with a 4.10 ratio and a transmission with a .75 overdrive the ENGINE will effectively see a 3.07 final drive.




“......x2 The first tire, your baseline (or if you're only calculating for one tire size) is 1:1. It's irrelevant until you calculate for a second tire size, which then becomes a factor of the first and allows you to calculate effective gear ratio with the second tire size as compared to the first......”


Correct. You can calculate what the tire SIZE effect is, but only comparing one tire to another. If you have a 24” tall tire currently on the car and change to a 29” tall tire divide the 24 by 29 which results in .827 which is the percentage of change. Now multiply .827 times you current ratio say 2.73 which results in 2.257. The car will now respond like you had changed the rear axle ratio to a 2.257 gear set and left the 24” tires on it.

 

[G_Body_Enthusiast]

There is no number. The only time you can reference an effective gear ratio is with a comparison of two varibles for one constant. ei. two tire sizes and one gear ratio. It has to be a comparison.

x2 The first tire, your baseline (or if you're only calculating for one tire size) is 1:1. It's irrelevant until you calculate for a second tire size, which then becomes a factor of the first and allows you to calculate effective gear ratio with the second tire size as compared to the first.

X3 when i swapped my 200c transmission out for a non lock up th-350 i changed the effective gear ratio as i changed the gearing for the transmission itself. to complicate matters i also use a back tire that is taller than the original. with the transmission the gears are numerically lower making them taller (they act taller than the original gears in the 200c) which makes the gear ratio act more like a 2.29 than a 2.41. this only effects take off as both transmissions have the same final gear of 1:1. the taller tire makes it act even worse in final drive again making it act more like a 2.29. so it's a double whammy to performance. the upside is i havent had to have the transmission rebuilt twice or even once since it was put in. the weak and wheezy 3.8 made the old 200c puke parts twice and i gave up on having a third go around. the downside is performance is even more lackluster than it was before.

so the effective gear ratio can be effected in two different ways, both in final drive and in the first and second gears of the transmission. lets call them static and dynamic effective gear ratios.

my kingdom for 4.10 gears and a 2004r!

 

[ReQ]

I see what everyone is saying, I am not really trying to solve a problem per se, its just pure curiosity really, right now I have a 6.8796 final in second gear with 27.4" diameter tires, in my case my car turns 5700rpms before I lift, then it drops to about 2500 in the turns, which is about perfect for my setup

the real reason I am pondering this is the simple fact that I cant seem to come to terms with the "no number" concept, I know you have to have something to compare too, for example if I had 26.9" tires and switched to 27.4 (the size I currently run) with the 2.73 that would effectively change my 2.73 to a 2.68, which makes sense

the part that bugs me is that imaginary number that was the starting point with the smaller tires, the smaller tires had an effect on the gears, but you cannot put a number on that effect without another point of reference, that being a different size tire, the best you can do is say that your 2.73 gear acts like a 2.73 with 26.9" if you have no other point of reference

you can switch from a 26.9" tire to a 27.4" tire with 2.73 gears, if you do the math that gives you a 6.7536 final, but if you switch back to the 26.9" tire you will get a 7.0056 final, so is that the magic number I am looking for? is a 2.73 gear with 26.9" tires working like a 2.78 gear? no because you only came to that number by comparing it to the 27.4" tire, use a different tire you get a different result of course, I just don't understand why there NEEDS to be a point of reference, why can't you just calculate a theoretical value based on just one size tire and the gears

 

[pontiacgp]

:?:

 

[Intragration]

the real reason I am pondering this is the simple fact that I cant seem to come to terms with the "no number" concept, I know you have to have something to compare too, for example if I had 26.9" tires and switched to 27.4 (the size I currently run) with the 2.73 that would effectively change my 2.73 to a 2.68, which makes sense

I think you're making it more complicated than it needs to be, and I think you have your math backwards again. If 2.73 is your current gear, and 27.4" is your current tire size, then your current "tire factor", "zero number", whatever you want to call it, is 1:1 It's 1:1 because your CURRENT tire size is the same as your CURRENT tire size. Therefore, your current effective gear ratio is 2.73. While this doesn't give you a comparison to any other tire sizes, it's at least useful as Pontiacgp said, in that you have your current driving dynamics as a reference point. If you then go with 26.9" tires, their circumference is 1.86% smaller than the current tire size, and so your new "tire factor" is 1.0186:1, and so therefore your new effective gear ratio would be 2.78. If I'm not mistaken, with the 26.9" tires, you'd have to be up to 5,800 RPM instead of 5,700, and in turns you'd be at 2,550 instead of 2,500. I'm not a racer, except in a straight line, so Pontiacgp or anyone else please feel free to correct me if I've got something wrong.

 

[G_Body_Enthusiast]

I see what everyone is saying, I am not really trying to solve a problem per se, its just pure curiosity really, right now I have a 6.8796 final in second gear with 27.4" diameter tires, in my case my car turns 5700rpms before I lift, then it drops to about 2500 in the turns, which is about perfect for my setup

the real reason I am pondering this is the simple fact that I cant seem to come to terms with the "no number" concept, I know you have to have something to compare too, for example if I had 26.9" tires and switched to 27.4 (the size I currently run) with the 2.73 that would effectively change my 2.73 to a 2.68, which makes sense

the part that bugs me is that imaginary number that was the starting point with the smaller tires, the smaller tires had an effect on the gears, but you cannot put a number on that effect without another point of reference, that being a different size tire, the best you can do is say that your 2.73 gear acts like a 2.73 with 26.9" if you have no other point of reference

you can switch from a 26.9" tire to a 27.4" tire with 2.73 gears, if you do the math that gives you a 6.7536 final, but if you switch back to the 26.9" tire you will get a 7.0056 final, so is that the magic number I am looking for? is a 2.73 gear with 26.9" tires working like a 2.78 gear? no because you only came to that number by comparing it to the 27.4" tire, use a different tire you get a different result of course, I just don't understand why there NEEDS to be a point of reference, why can't you just calculate a theoretical value based on just one size tire and the gears

compare it to the stock tire height, THEN you'll have a better idea how your gear is acting with whatever size tire you're using. that's what i do, i compare it to the stock tire height to give me an idea how a taller or shorter tire/rear gear will act.

currently i have a th-350 and a 2.41 gear. if i multiply the first gear with the rear gear i get 6.07 however what the car originally had was a 200C which has a 2.74 gear which originally gave me a gear of 6.60. quite a difference huh? anyway i still have a final drive ratio of 1:1 x 2.41= 2.41 but with my slightly taller tires it acts more like a 2.29. it isnt just tires that change how the rear gear acts unless you never change the transmission.

your stock tires are your base line, your original transmission gears are your base line. at least then you have a frame of reference to compare your performance gains when you change your tires/transmission/rear gear. adding an OD gear changes how the rear gear acts too. 3.73 x .67 (OD gear for the 2004r) = 2.49 that's pretty close to my current gear (2.41) and how it acts in 3rd gear with my th-350. now if i change the tire size then i have to factor that in to tell me how the rear gear will act compared to the original tire height.

without a starting point you have nothing to compare to and that means you cannot compare anything as you only have 1 set of numbers. so you need a rear gear ratio, a tire height and a transmission gear. again your starting point shouldbe the original values of your stock tire height, your original rear gear and a transmission gear (1 or .67 for 2004r or .70 for 700r4)

you have a 2.73 rear gear and a 1:1 final gear in your th-350 which gives you a final drive ratio of 2.73. the original tires are 25.4" tall (based on 195/75/14) but you now have 27.4" tires so lets see how that changes how your rear gear acts:
2.73 x 1= 2.73. 25.5 / 27.4= .92. 2.73 x .92= 2.51. because the tires will turn slower your gear will act taller (numerically lower). your final number is based on the gear you end at after you get up to whatever speed you drive at, in this case 3rd gear. you're using the first gear and that's only used for off the line performance comparison, you want to use the last gear your transmission has to figure out final drive numbers.

now if you want to know off the line performance numbers you will want to multiply the first gear of your transmission with the rear gear then you can use the same equation as above to compare tire heights to see how that will make the gear act off the line at the track. but make no mistake, the two equations are not the same and should not be confused or interchanged.

for final drive it is (last gear in your trans.) x (rear gear) x tire height difference (original tire size divided by new tire size)
1 x 2.73 x (25.4/27.4) = 2.51

 

[ReQ]

A lot of great information here, but the vast majority of it I already know, the question I am asking is more or less superfluous

I am not really talking about any car in particular, I am just using what I have for an example and the gears I run in my racecar are what everyone runs at this track, the goal is to get as close to a 6.95 final as possible, I am a bit under that because of the tires I am running at the moment (225/70-15) the actual circumference is known to me due to a stagger gauge but irrelevant for this discussion, since I am only thinking theoretically

I may have figured out my own question by thinking about this in reverse, if you remove the rear end from a car, eliminate the transmissions ratio and the engines revolutions, and rather than the engine turning the gears which turn the tires, turn the tires instead, if you assume the tires have a circumference of 50" (for the sake of this example) and you have a 4:1 gear in the diff, one full revolution of the tires will turn the yoke 4 times and the tire will roll out to 50" (its circumference), if you change the gear to a 2:1 and leave the same tires and repeat the test, the yoke will only turn twice but it will still move 50 inches, so the 4:1 and the 2:1 are both moving the same distance albeit with a different amount of revolutions, this will always be true no matter what gear you use as long as the tire stays the same circumference

now if you change the tires circumference to say 80" and leave the 4:1 gear and roll the tire one complete revolution, it will move 80" and turn the yoke 4 times, same amount of mechanical reduction (4 turns into 1) but the distance is different, so if you compare the 50" tire to the 80" tire 50/80x4.00=2.50 with the 80" tire you are turning the same amount of rpms to cover a different distance, so that effectively transforms the performance of the 4.00 gears into a 2.50 ratio, BUT only when you compare it to the 50" tire and that has been my hang up, what was the effective transformation of the 4.00 gear with 50" tires? there is no way to get a value for this unless you have another value for comparison, that bugs me

maybe I just fail at the science of quantity

edit: keep in mind, all of that was meant to be in reverse functionality

 

[G_Body_Enthusiast]

you're making more out of it than it has to be. i have been guilty of doing this but believe me, what you want to figure out is simple and the math involved is simple.

your final drive ratio is your rear gear and last gear in your transmission, NOT the first gear which you are using. unless what you're looking for is the first number for torque multiplication which would be the first gear times the rear gear but it is not called final anything by any stretch of the imagination. there is no final in this equation as it uses the first gear not the last, last and final are used as synonyms. first and final are more like antonyms.

an example of this is the rule of thumb i had read many many years ago for good take off performance in terms of gearing is that for street cars about 9:1 and 10:1 for track cars. you get this by multiplying the first gear and your rear gear. mine is a paltry 6.07, 2.52 x 2.41= 6.07. so i need to go up about 3 points so i would need to use a 3.73 or shorter gear. 2.52 x 3.73 = 9.39. a good choice for track use in a general sense, it does NOT take into consideration how tall my tires are and what my cam specs are. i have to change tire sizes to more closely match the cam specs for its PEAK RPM performance. the rear gear puts you in the ballpark, the tire size tunes the RPM through the lights for optimum performance. taller lowers the RPM, shorter raises it.

7 isnt a very good number, it really isnt and running tires 2" taller than stock only make the 2.73 gear act worse than it already is. it's a gear used for fuel economy, not performance. it keeps RPMs down when you're in the final gear for your transmission. it's why an 86 monte ss can have a 3.73 gear and your 3 speed cant, it uses an overdrive gear that gives it a final drive ratio of 2.49. it acts like it has an economy gear but has far better acceleration gearing (2.72 x 3.73= 10.22) compared to your 2.52 x 2.73= 6.88. at cruising speed it even has a better final drive ratio than you do, 2.49 to your 2.73.

final ratio and acceleration gearing are two different animals as they serve two different purposes and do two different things but both effect performance.