Explain rear end ratios to me....

I took my car down the track for the first time and did ok for being a driver and not anything special as far as engine/trans/rear end goes. It's a 1970 Monte Carlo with a 327 and TH350. Someone asked what gear I had in the rear end. I have no clue, but it gets decent mileage, doesn't wind at highway speed, and is slow off the line. I figured I would try to figure it out. It's not a posi because one tire turns one way and the other turns the other way. What I did was put it on blocks, blocked one wheel, counted the tire spins to the driveshaft spins. I rotated the tire 10 times and the shaft rotated about 12.75 times. Then took the formula I found online (shaft spins times 2, divide by 10) to get 2.55. The closest ratio to that is 2.56 for a GM. Did this formula and numbers look correct?

If it were a 4.10 ratio the tire would turn 10 times and the shaft 20.5 times according to the formula. If both a 2.56 and 4.10 were running the same RPM's, which tire would turn faster? I'm assuming the tire that turns faster would be faster off the line and thus be the 4.10, correct?

Would a 2.56 or 4.10 get better:
Gas mileage
launch on the line
lower RPM's going 65mph

They may be stupid questions to some, but my mind was wondering and thought I'd ask here.

I always rotate the tire one revolution and count how many revolutions the pinion yoke makes.

The 4.10 will increase your rpm's at 65 a bunch, your gas mileage will most likely go down, but it will be a lot more fun off the line. I think gm had a 3.42 gear ratio, that or even a 3.73 would be fine, the 3.42 would be my choice for a car that gets driven a lot. Add a posi trac too.

2.56 for better gas mileage

4.10 for better launch from stop

2.56 for lower engine RPM at 65

you can look at it this way, making a big jump up (NUMERICALLY) from 2.56 to 4.10 would be like getting an extra deep 1st gear, and giving up third gear. You are talking about a HUGE change.
The 4.10 will:

1) Get much worse gas mileage.
2) Will launch much harder off the line
3) Will be turn much more rpm at 65 mph.

And for everyday cruise driving, it will be fun up to about 50mph, notsomuch above that.

I do the same but rotate the tire exactly two full rotations and do the math, on the pinion yoke, driveshaft, if it rotates 2 and a little more than 1/2 turn it has 2.5+ ratio, if it rotates 3 and 1/4 it has 3.25 ratio and so on .
Cars with posi or spools you need to be able to have both tires spin and spin them one full rotation and count the rotation on the yoke or driveshaft , same deal 4 + 1/10 turn is 4.10 or 4.11, 4 and a little more than 1/2 turn are 4.55 or 4.56.
IHTHs

Gear ratios, in the rear and in the transmission, change the ratio of revolutions of the axles/wheels vs engine RPMs.

Just as your car accelerates better in first gear than third, changing the rear ratio has the same effect. Rear ratios are determined by a lot of different factors, and changing the gears is not a job for a novice.

Putting a 3.73 or a 4.10 in your car will make it quicker. But, try riding around in 2nd gear for a few days. That is how it will behave in 3rd with the new gear set.

Do you have a tach?

If you have a 2.56 gear set now a 4.10 will increase RPM by 1.6%

So if you are cruising 2000RPM at highway speed now you will increase that to 3200RPM with the 4.10

Not 1.6%……..60%!

In my mind I multiply the original rpm by 1.6 and get the correct outcome. Maybe I worded it incorrectly.

If you change the ratio you’ll need a posi unit too. Just changing to the posi will help the spin factor. Can’t remember if you said what engine you have but that has a big factor on gears so I’d look for something in the 3.20-40 range, best of both worlds. Then finding gears made for your rear (10 bolt-12 bolt) would be the next problem. Big ball of wax, maybe leave it till something falls in your lap.
Looked and saw you have the 327, they like to rev so maybe a 4.10 would not be tooo bad.

Before spending any money, make sure you identify what rear end you have. As I recall, 70 Monte Carlos could have an odd ball 'O' type rear end instead of the more easy to get parts for 10 bolt or 12 bolt.

Is it a 12 Bolt rear , if so try some 3.31s nice all around gear. 10 bolt 3.42.

I'm not wanting to switch rear ends, just curious as to the differences in performance and such. Thanks for all the replies. Like learning new things.

one thing that is commonly missed, is how efficient the engine is at xxxx rpm at ?? mph.
if the engine spins too fast, it will lose mpg.
if it spins too SLOW, it may be lugging, thus also loosing mpg.
also, it may be possible to loose mpg regardless of rpm, because the engine may be too small vs weight of vehicle.
in this instance, a larger engine may give better mpg because it isn't working as hard as the smaller engine.
all in all, it amounts to the total combination of engine, transmission, rear gear, tire diameter [and width], and vehicle weight, that needs to be "tuned" for maximum miles per gallon.
and we are not even talking, thinking about, or incorporating, aerodynamics. that can also increase or decrease the mpg of any combination. sometimes by a lot, or sometimes by an almost minuscule, undetectable amount.

This may have been stated but just in case.

The 2.56 gears would turn the tires faster at the same engine speed but the car would accelerate slower. Torque is the turning force an engine makes. When accelerating, the car engine uses it's torque to turn the tires.

Gear ratios can act like a torque multiplier. Gears take the energy from the number of turns and convert it to torque or turning force. The higher the number of turns in to the gears verses number of turns out, the greater the turning torque.

It's kind of like leverage.

Imagine a wrench that is 2.56 foot long and another that is 4.10 foot long. When turning a big bolt the 4 foot wrench has more leverage and can turn the bolt more easily. Replace the bolt with a tire and the same thing.

The 4.10 gears can turn the tires more easily than a 2.56 gear, so the car accelerates more quickly since the tires can turn more easily. Over simplified and a physics teacher would cringe but you get the point.

As for the gas mileage lots of things go into consideration but with 2.56 rear end gears and the transmission in 3rd gear (1 to 1 ratio), every time the engine turns 2.56 times the rear wheels turn 1 time.

with 4.10 rear end gears and the transmission in 3rd gear (1 to 1 ratio) the every time the engine turns 4.10 times the rear wheels turn 1 time.

Either way the tire moves the car the same distance with one tire turn but the number of times the engine turns is different.

Every time that engine makes 1 more rotation it uses more gas so the 4.10 gears cause the engine to turn more times and use more gas.

I hope that was helpful and didn't come across insulting. Sometimes I overthink the question.

Great explanation, I like it when it's simple to understand and relate to. Thanks.