Please, teach me-- Wheel size/Final Drive Gear.

I've heard some people on YouTube/Racing videos say something to the effect of, "I went with smaller wheels because it helped[or extended?] my final drive ratio."

I'm curious what (if any) effects of wheel/tire size matter to the final drive ratio.

Smaller tire diameter = higher final drive ratio

Larger tire diameter = lower final drive ratio

Thank you.

And, higher drive ratio is for more acceleration, and lower is for speed, correct?

What he said is right, but I'm not sure whether it clarifies

The total effect of gearing and tire size is the relationship between revolutions of the engine and distance traveled in whatever gear you're in.

We can simplify it by imagining the car's in fourth, which in many cases is 1:1, so now the engine and the driveshaft are turning at the same rate. Now let's say your differential is 3.73:1, just to pick a common one.

So now for every revolution of the engine, your rear wheels turn 1/3.73 of a revolution.

Let's now pick a starting tire size. Let's make it 20" in diamater, which since circumference (the distance rolled per revolution) is pi*diameter, would be a little under 63".

63 / 3.73 = about 16.9", so that's how far your car travels for every revolution of the crank.

Now, let's make that tire 5% smaller, or 19".

A little under 59" circumference now, and 59/3.73 = about 15.8"

I've introduced some rounding errors, but the upshot is that a 5% reduction in tire diameter is like being in a ~5% lower (numerically higher) gear. Or to put it another way, you'll rev that much higher at a given speed.

I think in giving the rough idea, I may have done that sloppy math thing where I use 5% like it's an actual value. Remember that 105% of 95% of something isn't exactly 100% of that thing...

Of course, if you take "final drive" strictly to mean the gear ratio in the differential, then of course tire size doesn't affect it at all

Wait, did I say I was trying to help clarify?

ransom wrote: What he said is right, but I'm not sure whether it clarifies The *total* effect of gearing and tire size is the relationship between revolutions of the engine and distance traveled in whatever gear you're in. We can simplify it by imagining the car's in fourth, which in many cases is 1:1, so now the engine and the driveshaft are turning at the same rate. Now let's say your differential is 3.73:1, just to pick a common one. So now for every revolution of the engine, your rear wheels turn 1/3.73 of a revolution. Let's now pick a starting tire size. Let's make it 20" in diamater, which since circumference (the distance rolled per revolution) is pi*diameter, would be a little under 63". 63 / 3.73 = about 16.9", so that's how far your car travels for every revolution of the crank. Now, let's make that tire 5% smaller, or 19". A little under 59" circumference now, and 59/3.73 = about 15.8" I've introduced some rounding errors, but the upshot is that a 5% reduction in tire diameter is like being in a ~5% lower (numerically higher) gear. Or to put it another way, you'll rev that much higher at a given speed. I think in giving the rough idea, I may have done that sloppy math thing where I use 5% like it's an actual value. Remember that 105% of 95% of something isn't exactly 100% of that thing... Of course, if you take "final drive" strictly to mean the gear ratio in the differential, then of course tire size doesn't affect it at all Wait, did I say I was trying to help clarify?

You've successfully confused me further. Math is essentially a foreign language to me. I took away from it that the smaller the wheel, the less distance the vehicle needs to make the rotation of the crank-- or something of the like.

Higher number = faster acceleration = lower top speed.

Lower number = slower acceleration = higher top speed.

We need a head scrathing emoticon.
What that translates to is for example. 20" tire in 4th gear at 60mph is 4000rpm, lets say. A smaller tire at the same speed of 60mph in 4th would be at a higher rpm.
Even simpler, yes smaller tire diameter will accelerate quicker but top out at a lower speed than a larger diameter tire.

What is scary is I actually understood what ransom said.
Guess the aeronautical and structural engineers I work with is rubbing off on me.

Now available in online calculator form!

http://www.apexgarage.com/tech/gear_ratios.shtml

When I joined GRM I was told there would be no math...

or.....

http://www.miata.net/garage/tirecalc.html

92CelicaHalfTrac wrote: Smaller tire diameter = higher final drive ratio Larger tire diameter = lower final drive ratio

That's the way logic works, but in practice a "lower ratio" is higher numerically, because it correlates to a lower gear. 1st is a lower gear than 2nd, even though it's a higher ratio.

So lower = lower speed, higher = higher speed. Also, "short gear" and "tall gear", which coincidentally enough have the same effects as short tires and tall tires.

(don't blame me, I didn't build the berking thing)

Knurled wrote:

92CelicaHalfTrac wrote: Smaller tire diameter = higher final drive ratio Larger tire diameter = lower final drive ratio

That's the way logic works, but in practice a "lower ratio" is higher numerically, because it correlates to a lower gear. 1st is a lower gear than 2nd, even though it's a higher ratio. So lower = lower speed, higher = higher speed. Also, "short gear" and "tall gear", which coincidentally enough have the same effects as short tires and tall tires. (don't blame me, I didn't build the berking thing)

Lower numbers = higher top speed

Higher numbers = lower top speed

Escort has a higher top speed with the 4.1 final drive than the 4.38 final drive i ended up using.

use my CRX for an example ... using 205 - 50 - 15's I end up with higher top end at CMP ... using 225 - 45 - 13's I end up with quicker lap times ... the variable here is how much the wider tire helps in the corners .... what I DO know is I'm at higher rpm's coming out of the corners with the 13's ... so the torque challenged CRX accelerates more quickly .. and if I run out of usable revs in 4 ( on the 13's) then I always have 5th to cruise on to the next corner

know shift points on the track ... I'm as much as 1 - 2 sec earlier going to 4th with the 13's .... does this help ?

wbjones wrote: use my CRX for an example ... using 205 - 50 - 15's I end up with higher top end at CMP ... using 225 - 45 - 13's I end up with quicker lap times ... the variable here is how much the wider tire helps in the corners .... what I DO know is I'm at higher rpm's coming out of the corners with the 13's ... so the torque challenged CRX accelerates more quickly .. and if I run out of usable revs in 4 ( on the 13's) then I always have 5th to cruise on to the next corner know shift points on the track ... I'm as much as 1 - 2 sec earlier going to 4th with the 13's .... does this help ?

Yes it does, thank you.

I'll add my simple thoughts.

As has been said above, the transmission alters the amount of revolutions that make it through to the driveshaft. Then the final drive alters it again.

Skip all that for a second.

All other factors being equal, let's say you change from a 25" tire diameter to a 27" tire diameter. Think about the circumference. It has increased. So, for every 1 revolution of the tire, the car now moves forward a greater distance than it did before.

Think of it like a 10-speed bike. The pedals and your feet are the engine. The sprockets are the transmission. In the case of a bike, the final gears are 1:1. As you shift up through the gears, your feet (the engine) moves slower. In the same way, changing to a larger diameter tire has the same effect as shifting to a higher gear.

They all work together with that math thing. Here is the net result: 1) the engine turns. 2) the transmission uses gears to reduce or add to that speed and sends it out to the driveshaft. 3) the differential gears take in the driveshaft speed and alter it like the transmission, but in cars there is just one fixed gear. 4) the tires propel the car X number of inches for each single revolution of the tire. I know you said math is a foreign language, but the bottom line is that all of the parts between the engine and the tires work to change the speed of components just like a 10-speed bike does. They all work together.

When someone says that their smaller tires help their differential ratio, they are correct. Smaller tires have the same effect as having lower gears.

Street cars with small tire diameters often have high gear ratios (which are represented by smaller numbers in the ratio). I'll give you two examples: Let's take two identical 1985 chevy pickups - same engine, transmission, same 28" tire diameter. Now let's take one of them and jack it up, put monster 38" mud tires on it. Take the other one and airbag it with 25" street tires. Now, say you want them both to have the same highway RPMs. You will have to put lower gears in the differential of the off-roader and higher gears in the differential of the street truck.

One last little tidbit... when people talk about lower and higher gears, they're talking about like the difference between 1st gear and 3rd gear. The actual mathematical ratios are opposite. A higher gear is numerically lower and a lower gear is numerically higher.

Take this example. A typical set of transmission ratios might look like this:

1st gear: 2.7:1 ratio
2nd gear: 1.8:1 ratio
3rd gear: 1:1 ratio
4th gear: 0.75:1 ratio (OD)

As you can see as the gears get higher, the ratio number that represents them gets lower.

what Curtis73 is saying .....take 2 identical cars... think a typical 4.11 rear end ... it will accelerate the car quicker but have a lower top speed than .. say .. a 3.73 rear end ....

not going to read though all this... but play around with this a bit only changing the tire size,

http://www.f-body.org/gears/

Just to confuse things, tire diameter is not the wxact way to estimate speed. The rolling radius, is a more accurate way. Figure the revs per mile by diameter and then see what the manufacturer says.

EdenPrime wrote: You've successfully confused me further. Math is essentially a foreign language to me. I took away from it that the smaller the wheel, the less distance the vehicle needs to make the rotation of the crank-- or something of the like.

I suck, but maybe I can redeem myself with a real-world example that ties right back to something you can feel.

Ever ride a bicycle with multiple gears? You know how you put it in an "easier" gear to go up hills, and a "harder" gear to go faster?

That's all about the relationship between how far you move the pedals and how far the bike goes. The "harder" the gear you're in, the further you go for one turn of the pedals.

Putting a smaller tire on your car is like putting it in an "easier" gear, because the smaller tire rolls less distance every time it goes around. Easier to accelerate, easier to go up hills, but the engine has to spin a bit faster at a given speed.

Make more intuitive sense?