Gear Ratio help and understanding
 

All;

Here is what I know so far after driving our new car across town.

At 55 mph it is about 3,000+ RPM. (what gear ratio do I have? Must be pretty low) After some searching on the forum I get the rear end ratio and how that affect's RPM.

I am confused about the "close" and "wide" tranny talk??

Here is my question.

If I have a Muncie "wide" tranny is there a way to calculated different RPMs for different rear end ratio's?

I though I saw a site to calculate this but not sure.

Whatever is in the car now I would like to change for a better mix of city and highway driving. That way if we want to participate in a gathering an hour or two away I don't have to rap the engine out or drive 50 mph...

Thank for any help

You're mixing apples and oranges. Internal gear ratios of a transmission are one subject, the gear ratio of a differential is another.

To simplify, only gears one through three of a four speed transmission are reduction gears, fourth is direct drive. Irrespective of whether a transmission is wide or close ratio, top gear is always 1:1.

Assuming you have have stock sized tires, RPM on the highway while in top gear is only affected by differential gearing.

Don,

1st step is to verify the tach accuracy. I don't doubt you may have a very low rear gear, but best to start with the basics. And I assume you are talking about being in top gear (4th) while running 3K at 55 mph.

A wide ratio trans has a wider span between the gears (as opposed to a close ratio has a narrow span between them). Typically a close ratio was installed with low rear end gears, and minimized the rpm drop between gears. But for street (cruising) purposes, a higher rear gear with a wider span between the gears is better (IMO).

I use an old DOS program called "Car Test" to tell me gear speeds, etc., etc. I am sure there are newer programs, but it works for me. You input the gear ratios, tire size, etc., and let it do the work. If you can run DOS, I can email the program.

Plasticman

Or just look on the bottom of the differential for the 2 letter stamped code.

Sounds like at least 4.11 to me. You can measure your tire height (should be about 26") and calculate it here:

http://www.richmondgear.com/101032.html

If you want to do your own calculations, the equation that relates mph, rpm, tire size and gear ratio is:

mph=(rpm/rgr)(TD/336)

where rpm is engine speed, rgr is the final drive ratio, TD is loaded tire diameter in inches. The final drive ratio is the rear end ratio (when in 4th gear), or if you are in one of the other three gears multiply the trans gear ratio (of the gear you are in) by the rear end ratio. 336 is a correction factor (inches/miles and minutes/hours).

You can measure your rear tire diameter, and assuming your tachometer is correct, you can calculate your rear end ratio. You probably won't get the exact number, but you will be close enough to know what it is.
1:cool:60

BUT this assumes that you know what the internal nylon drive gear is in the transmission. If you don't then your indicated mph is conjecture:ack:

I would assume that (based on the OP narrative, his final drive is 4.11 because very few 4.56 rears were factory installed, but I would find a highway odometer check and verify his speedo/Odo, and also check the code on his rear end (as mentioned)

Thanks so much everyone.

This does make more sense now.

OK, I get it:D 4th gear is a 1:1

So using the Richmond calculator posted I get the following.

Using 4.11 as a starting point

Speed of 55 mph

tire size of 27.5" (currently have 6.70 X 15)

Yields an RPM of 2,762

This seems about right for what I was seeing as I drove.

I'm thinking that a 3.55 ratio would be a much better compromise for city and highway driving.

Using the same as above a 3.55 would be only 2,386 RPM at 55 mph

Thoughts?

3:36, 3:55 and 3:70 are very popular ratios for highway driven cars.

My stock '67 300hp has a 3.36 axle and the wide-ratio 4-speed (2.56:1 1st gear), and it makes an excellent all-around combination. 70mph on the highway is 2900 rpm (27.2" tires), and the low 1st gear makes for smooth starts from a stop around town with no clutch-slipping. :thumbs:

I have 3.36 in mine with 205 75r15 tires (just over 27") and it is OK. In reality, you drive much faster on the highway and the rpms kind of get up there. I ended up putting a 5-speed TKO-600 with a .64 overdrive in mine to keep the highway rpm down and still have decent acceleration. LOTS of discussion on this topic....

Sounds pretty low to me. I like the formula above. I have 26" tall tires and 3.55 and I'm at 63ish for 3000rpm. If you want to get dirty, jack it up, have someone hold one tire, mark the driveshaft, have someone turn the other tire around once and count the truns of the driveshaft.

:iagree:......:thumbs:
i think that combo with the wide box and 336 rear is the best all around gearing, not just for small blocks but big blocks too....jmo.....
the theory of 10-1 first gear ratios are just for very spirited driving or drag racing but some folks like it for the street too....to each his own...:cheers:

I would follow JohnZ's advice above and change BOTH the final drive ratio and the transmission to the wide ratio box and keep (ESSENTIALLY) the same leverage (torque multiplication) in 1st, 2nd, and 3rd gear, but have the advantage of the longer rear axle ratio in 4th gear for cruising.:thumbs:

Smart guys, great advice!

Harry

Reading with factory parts as new {tires}
 

My experience is a 4.11 at 70 MPH, the engine is spinning 3500 RPM exactly 1/2 the MPH`s on the other hand the 4.56 ratio increase the RPM another 500 to 4000 at 70.....

OK, Got the car in the air and did confirm it is a Muncie tranny (m-20)

This is a close ratio unit. Unfortunately it has to come out due to a shift problem. :eek: The rear end was leaking as well but that is OK because I want to change the gear in that anyway.

I'm on the bubble between a 3.36 or a 3.55 but will keep doing some research. When the tranny is out I can get all the numbers off it.

i want to confirm that 1st gear ratio and confirm what John Z is saying because his combo sounds good.

if you go 336 over the 355 you will be happier.....two hundred rpm difference in 4th gear which doesn't sound like much but it is.....jmo

I'm gonna second JohnZ's recommendation for the 3:36 rear gear, I have it in one of my cars and it's perfect IMO for all-around driving. :thumbs: I have the 3:08 in my coupe and it's great for highway driving, but a little more difficult to master smooth starts in town with the 3:08.


Ironcross has exactly the same experience with the 4:11 gears as I had with my '63 with 4:11's. You could look at the tach and double the number that the tach needle was sitting on (30 for example, representing 3,000 rpm's) and get the highway speed. I usually drove that car by the tach, seldom looked at the speedometer for my speed.

:thumbs:

This may help


Transmission/Rear gear calc ===> click here


George