Effective Gear Ratios

[ReQ]

This is purely for curiosity sake first off, I know how to calculate gear ratios but one this has been bugging me, I don't care about engine RPMs or how it will throw off my speedo etc., I just want a specific number that I cannot seem to get to

the "effective" ratio depending on tire size, if you search around online you will find several calculators that will tell you how your effective gear ratio will change when you change tire size (old tire / new tire x gear ratio) etc., but what I want to know is what my current effective gear ratio is with the size tires already installed?

Let me explain further, my car has 2.73 gears, TH350 with 2.52 low gear, 27.4 diameter tires, 2.73x2.52=6.8796 how do I calculate the tire size into this, to see how it will effect that 6.8796 final, or more appropriately how do I calculate the tire size with the 2.73 rear, to see what it will effectively make the 2.73 when the rubber meets the road, as it were

its probably going to be so simple that I will hate myself.. but for some reason I simply cannot figure this out and its driving me nuts, furthering my frustration is this, if I follow the equation for "effective gear ratio changes" and just randomly use an "old" tire size such as 26.9 / 27.4 x 2.73 I get a 2.68 rear gear, so that would make the final 6.7536 effectively correct?

but if you change the "old tire" size to something else like 24.5, you will end up with a 2.44, so this "effective" gear ratio is just how the ratios will change depending on 2 different tire sizes? still seems wrong, how do you calculate what your actual ratio is with the tires you have now? what are the 27.4" tires doing to my 2.73 gear?


maybe I am missing something here

 

[pontiacgp]

:?:

 

[bill]

I will throw in my explaination....for what its worth. The taller the tire, the less the motor has to work to move the car at speed, but the more the motor has to work to get the car moving from a stand still. to help the engine keep up the revs and use up the torque it makes, we change gear ratios to a higher numerical, lower speed gear ratio. If you lower the diameter of the tire, you increase the rpms per mile, but you also increase the power put down to the street....because theres less diameter to turn per rpm. So, if you want a quicker take off w/your tires spinning all the way around the track, use a smaller tire, but if you want more speed....use a taller tire....hope this helps.

 

[bill]

Well....i did some calculations on this and heres what I came up with....
For example: If your drive your car 100 ft in low gear, w/your current set up, no spinning tires, your engine will turn 301 rpms to cover that distance. Now...if you change the tire size up or down, the rpms required to cover that distance will go up or down depending on the increase or decrease in size. If you decrease size by one inch, the motor will turn 312 rpms to go the same distance. If you increase the size by one inch, the motor will turn 290 rpms. Safe to say, one inch equals about 10 rpms per 100 ft traveled.

 

[ReQ]

I understand all of this, and this is exactly what I am talking about more or less... just a somewhat useless factor of it


I am struggling to explain what I mean and keep it from being extremely convoluted, I know that the gears will not physically change depending on the size of the tire, the axles will always make the same amount of revolutions with one turn of the driveshaft depending on the gears installed, and different size tires will travel different distances with one turn of the driveshaft as a result of the tire sizes, just for the sake of example if you had 4.0 gears with a 25" tire, one turn of the driveshaft will spin the tire 4 complete revolutions, that should equate to roughly 316" of travel, but if you cut the gear inhalf to a 2.0 and redo the test, you should get half the distance traveled correct? so what happens when you leave the 4.0 gears and cut the tire size in half? you should get the same distance you get with the 2.0 gears except you still have 4.0 gears, taking the engine and transmission completely out of the equation here

so one tire+gear combination can effectively preform the same as a different combination, that is more or less common knowledge, so you can basically say a 15" tire with a 4.0 gear effectively makes that 4.0 a 2.0 (for the sake of simple examples) I assume all of this is correct though the math may be wrong... and if you still follow me then maybe I am getting somewhere now :lol:


basically, when you change the size of the tire your gears are turning, you have the effect of making the tire turn faster or slower, so it feels different, how do you calculate that "feel" into a number? if we just assume the tires are directly altering the ratio of the gears themselves, what are the sizes changing the gears into? the 15" tires are changing the 4.0 gears into 2.0 if you only look at the distance traveled, so is it possible to do such a calculation without a baseline to compare too?

 

[ReQ]

this is the part that has me confused, if you use a "effective gear ratio calculator" and use my gears (2.73) and the other tires I run on my car as the "old" tire which are 26.9 diameter, and my other tires which are 27.4 for the "new" tire,it says I effectively have a 2.68 gear in the rear now, so that makes my new finale 6.7536

but what is my effective finale with the 26.9 tires? if you reverse that order you get 2.78 which is a 7.0056 finale, however if you change the "old" tire size and leave the "new" alone.. you will get a completely different number which will yield a different final gear ratio, I can't seem to grasp this concept apparently

what is my final with a 27.4" tire and 2.73 gears running low (2.52)

 

[pontiacgp]

:?:

 

[bill]

first of all...I think you have your ratio relationships backwards. A 2.73 gear ratio is the amount of driveshaft turns per revolution of the wheel....not the other way around. A 4.0 gear would mean that the drive shaft turns 4 revolutions to turn the tire once. That may be why you are confused...just sayin.....rear gears/transmissions dont multiply engine rpms, they divide them to make torque in low gear ranges....

 

[ReQ]

first of all...I think you have your ratio relationships backwards. A 2.73 gear ratio is the amount of driveshaft turns per revolution of the wheel....not the other way around. A 4.0 gear would mean that the drive shaft turns 4 revolutions to turn the tire once. That may be why you are confused...just sayin.....rear gears/transmissions dont multiply engine rpms, they divide them to make torque in low gear ranges....

you are correct, I did have it backwards in that explanation I gave, I was missing something important lol

so let me think about this again, the transmission divides the engine RPMs, so one revolution of the crank turns the input shaft one full turn, but in low gear the driveshaft will not make one full turn, 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?

 

[jrm81bu]

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.