Gearing calculation: someone check my math PLZ

[EricB]

Here goes:

My rear tires are 225/50R15 that comes out to a 23.86" diameter

I've got a loaner original 4spd from Victor Laury (thanks Victor!) so 4th gear is a 1.00 ratio. My rear end right now is a 4.11

At what feels like a slow freeway cruising speed and observing a steady 4000rpm I calculate I am actually travelling at 69mph. "Feels" right since everyone in L.A. including the CHP does at least 75-80mph on the freeway regardless of the 65mph speed limit.

(rpmxtire diameter)/(final ratioxtrans ratiox336)=mph

(4000X23.86)/(4.11X1.00X336)=69.xxmph

When I get my 81-3 5spd back with an 0.745 ratio 5th gear from rebuild I've been toying with the idea of putting in a 4.375 final drive. At the same revs my speed would actually increase to 87mph???

(4000x23.86)/(4.375x0.745x336)=87.14mph

Did I do the math right?

-e

[240ZX]

Hey Eric, it may be possible that the 4 spd you have came out of my 1970 240Z. A few months ago Victor drove up to my place and I traded the 4 spd for a case of Pacifico beer. He was pretty happy with the deal. Also, if you visit most any of the big tire outfit online, they have the calculation formulasto figure out your speed...just type in the info and it will calculate it for you.

[EricB]

I got the tire diameter from the info section on discounttires.com

I got the mph formula from a rock crawler website.

I just want to make sure I plugged all the numbers in the right way in the right units. It just seems crazy that the combination of the lower 4.375 (vs the 4.11) and the 5spd would yield a higher cruising speed for a given rpm.

If you asked for a case of Pacifico, any idea what Victor drinks so I can go drop off a case at his place? He gave it to me before Xmas and I lagged putting it on as my wife and I spent nearly every weekend for almost 6mo looking for our first house.

Needless to say we found it in the end.

-e

[TXZ]

Here's a link to a handy calculator specifically designed for the Z. It's great for trying our a variety of what-ifs with transmissions/tire/rear end/ and engine redline combinations.

http://www.geocities.com/z_design_studio/

[Alfadog]

Hi Eric,

The 5 speed will of course give you a higher cruising speed, but the higher-ratio diff will lower that!

Here is a neat little program that will calculate it all for you and you don't even have to get out your calculator!

gearcalc.zip

[EricB]

Hey TXZ thanks for the link, unfortunately it doesn't let me input non OE final drive ratios like 4.11 and 4.375

-e

[Z Kid]

http://www.albinsgear.com.au/formulae.html this one tends to work fairly well.

[Black Beaut]

On a similar vein to all this gearing calculation stuff, here's a little thing I knocked up that shows you what the equivalent diff ratio would be if you changed your tyre size. Handy for all those drag strip cheats

http://www.robshelley.btinternet.co.uk/240z/diff_calc/

Cheers,

Rob

[abas]

EricB

I agree with your maths

I use a formula for mph per 1000 rpm as

60000

-------------------------------

(diff ratio)x(wheel rpms per mile)

for a 1:1 top gear ratio

so for 0.745 you divide by 0.745

regards

Andrew

[TXZ]

EricB, if you highlight the ratio window in the calculator below and type in your own rear end ratio the calculator will give you the desired result. It's a good idea to double check with your own calculation, however.

http://www.geocities.com/z_design_studio/

[zguitar71]

I have the 83 5 sp and a 4:37 rear R200. I use the car for autox and street driving (mostly on the weekends). If it is your daily driver I would stay with the 4:11. The 4:37 is very low. I go through 1st very fast with 23.75 diameter tires. I think the solution for me is taller tires (25", 205/60/15 or 205/55/16) then I can have better gearing for the steet and still have the low gearing on the track with the autox tires. Right now my speedo is around 90 when I am going 65 becauise I have a 3.70 speedo cog and the smaller diameter tires. I think 75 is around 3700 RPM.