Calculating expected rpm/speed for 3.23 stick gears

[Bleep Bleep]

It is simple linear math.

If you’re at 50 mph at 2500 rpm w/ a 26” tire and want to know the rpm with a 27” tire, multiply 2500 by 26 and divide by 27.

If you’re at 50 mph at 2500 rpm and want to know your rpm at 80 mph, multiply 2500 by 80 and divide by 50.

Makes perfect math logic. BUT, hardly as much as fulfilling as factoring through a polynomial equation…

Ok, ok its not actually polynomial. But I did get that rare chance to use the abacus again.

In all seriousness, mainly for giggles I’m solving for the rare chance the stock 3.23 gears have been swapped out. No solid reason to suspect. Just a brand new car to me and I expected lower revs at highway speeds. Again, just curiosity.

 

[RemCharger]

Jack it up and count the revolutions to figure out the rear gears

 

[BSB67]

Simple yes, but getting the number I was after includes the simple math with tranny gear ratios of 2.66, 1.78, 1.30, 1.00, 0.80, 0.63 included in the mix. I’m not building my car for the 1/4 mile.

It’s all the same math and in the equation I posted. It has nothing more to do with the 1/4 mile than it does for what you want

 

[71 Runner]

OK, you win!

Since I am changing every nut bolt and screw. Adding full QA1 front with 2” drop & rear 4 link suspension. Switching from auto to manual, 8 3/4 3.23 to S60 and haven’t decided on ride height yet. Let alone tire size. It is nice to have something to use to figure out the different effect the choices have on gear ratio compared to tire size/wheel size.

12.4” wide
315/35/17 = 25.7” / 4.3” tire thickness 786 revs/mile
315/40/17 = 26.9” /5” / 750
315/45/17 = 28.2” / 5.6” / 717
315/50/17 = 29.4” / 6.2” / 686

11.6 wide
295/35/17 = 25.1” / 4.1” tire thickness 803 revs/mile
295/40/17 = 26.3” / 4.6” / 767
295/45/17 = 27.5”/ 5.2” / 735
295/50/17 = 28.6”/ 5.8” / 786

10.8 wide
275/35/17 = 24.6” / 3.8” tire thickness 821 revs/ml
275/40/17 = 25.7” / 4.3” / 786
275/45/17 = 26.7” / 4.9” / 754
275/50/17 = 27.8” / 5.4” / 725

Yes, the choice of wheel size does make a difference in actual tire size if you’re changing wheel size while trying to keep the same gear ratio.

275/55/14 = 25.9”
275/50/15 = 25.8”
275/40/17 = 25.7”

That’s what the link is great for, avoiding very expensive mistakes.

 

[Nutso]

This gear ratio calculator is helpful if you know your transmission gearing and axle ratio…but you could reverse engineer it to figure out your rear gear ratio. I’m not sure if this was what you were using in your first post. Gear ratio calculator

 

[gtxrt]

with an automatic non lock up torque converter you need to figure the slippage of the converter anywhere from 4% to 8% or more depending on the stall of the converter. average 4%- 5% with stock stall. so 25 mph would really be more like 24 mph.

 

[BSB67]

with an automatic non lock up torque converter you need to figure the slippage of the converter anywhere from 4% to 8% or more depending on the stall of the converter. average 4%- 5% with stock stall. so 25 mph would really be more like 24 mph.

The formula posted above takes converter slippage into account at 5.5%. Adjust as needed.

 

[gtxrt]

Yes, there is a simple formula. Manual transmission or automatic?

Mph x 355 x final gear ratio x transmission gear ratio / tire diameter = rpm.

Use 336 if it is a manual transmission.

so one converter i have is a tight 4.6% what numbers do i use ? i always just use the 336 formula and take that final number and times it by 95.4%

 

[BSB67]

so one converter i have is a tight 4.6% what numbers do i use ? i always just use the 336 formula and take that final number and times it by 95.4%

355.

You do realize that tire wear and tire growth, tire pressure, all variables that are not accounted for, have a larger impact on the results than missing the converter slip by a couple %.

 

[gtxrt]

355.

You do realize that tire wear and tire growth, tire pressure, all variables that are not accounted for, have a larger impact on the results than missing the converter slip by a couple %.

my converter is a xtm 12" one from Hughes transmission has 4.6% they have less slip for cooler running. my other car has a 11" has 5.6% slip.

 

[gtxrt]

355.

You do realize that tire wear and tire growth, tire pressure, all variables that are not accounted for, have a larger impact on the results than missing the converter slip by a couple %.

your right 1% either way no big deal

 

[Bleep Bleep]

Yes, there is a simple formula. Manual transmission or automatic?

Mph x 355 x final gear ratio x transmission gear ratio / tire diameter = rpm.

Use 336 if it is a manual transmission

I should have asked for clarity. In the formula, where does the 336 constant (it’s a manual) come from? I get the 5% converter slip but curious. Thanks!

 

[BSB67]

I should have asked for clarity. In the formula, where does the 336 constant (it’s a manual) come from? I get the 5% converter slip but curious. Thanks!

(5280 x 12)/(3.1416 x 60) = 336

5280 ft/mile
12 inches/foot
Pi=3.1416
60 minutes/hour

 

[Bleep Bleep]

(5280 x 12)/(3.1416 x 60) = 336

5280 ft/mile
12 inches/foot
Pi=3.1416
60 minutes/hour

Ok. If an automatic that’s where multiplying the 336 x1.05 equates to 355, representing the 5% slip for the manual?

Not to go down a rabbit hole, but what is the actual final unit of measure the 336/355 represents? I tried reducing the equation to a final calculated measure but get lost in the math.

Just curious is all. Thanks!