gear change

junior barns · 07-19-2021, 05:52 PM

Is there a formula that tells the rpm drop at the finish line between different rear gears ? 5.11 to 4.88 to 4.56 etc.

JeremyDuncan · 07-19-2021, 05:55 PM

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

L.Fite · 07-19-2021, 06:25 PM

Quote:

Originally Posted by JeremyDuncan

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

I did one similar to that in excel...
not as neat as that one but usable.

ss3011 · 07-19-2021, 08:14 PM

The only iffy part of the calculation is what you use for converter slip . The higher you rev the engine the less slip you get with the torque converter .

Adger Smith · 07-20-2021, 06:40 AM

old fashion way

4.88 divided by 5.13 =.9513
4.87% difference

4.56 divided by 5.13 = .8889
11.11% difference

Original RPM + or - the % difference
The real life number change may vary due to converter slip and/or tire growth.

Ray Gottry · 07-20-2021, 07:27 AM

RPM X C
_______ = MPH

G X 1056

MPH X G X 1056
______________ = RPM

C

MPH X G X 1056
_____________ = C

RPM

RPM X C
____________ = G

MPH X 1056

RPM = REVS PER MINUTE

C = TIRE CIRCUMFRUNCE (UNLOADED)

G = GEAR RATIO

MPH = MILES PER HOUR

You can move the numbers around in the formulas to tell differences of changes. Has worked great for me.

Not real scientific but fun.

Bowtieguy1967 · 07-20-2021, 11:34 AM

MPH X gear ratio X 336 / tire height (this only works assuming the trans final drive ratio is 1-1)

Examples:

120 MPH X 4.56 X 336 ÷ 28" = 6556 RPM

120 MPH X 4.88 X 336 ÷ 28" = 7027 RPM

120 MPH X 5.13 X 336 ÷ 28" = 7387 RPM

Obviously converter slip is not factored into the equation.

Dave Noll · 07-20-2021, 03:37 PM

http://atiracing.com/tech/racing_calculators.html

This one @ ATI figures in converter slip.

junior barns · 07-24-2021, 10:30 PM

Thanks all! When making a gear change the mph will change. In general going from a 5.13 going thru the traps at 7300 rpm and changing to a 4.88 what rpm drop can I expect?

Adger Smith · 07-25-2021, 12:01 AM

7300 - 4.87% (355.5 RPM) = 6944.9
Providing the converter slip % stays the same.. (It usually doesn't)

07-19-2021, 05:52 PM	#1
junior barns Senior Member Join Date: Jun 2006 Posts: 539 Likes: 11 Liked 20 Times in 14 Posts	gear change Is there a formula that tells the rpm drop at the finish line between different rear gears ? 5.11 to 4.88 to 4.56 etc.

07-19-2021, 05:55 PM	#2
JeremyDuncan Member Join Date: Apr 2010 Posts: 254 Likes: 923 Liked 280 Times in 69 Posts	Re: gear change http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

07-19-2021, 08:14 PM	#4
ss3011 Senior Member Join Date: Feb 2002 Location: Southeast Michigan Posts: 885 Likes: 63 Liked 221 Times in 102 Posts	Re: gear change The only iffy part of the calculation is what you use for converter slip . The higher you rev the engine the less slip you get with the torque converter .

07-20-2021, 06:40 AM	#5
Adger Smith VIP Member Join Date: Dec 2001 Location: Texarkana Ark/TX Posts: 2,348 Likes: 434 Liked 676 Times in 260 Posts	Re: gear change old fashion way 4.88 divided by 5.13 =.9513 4.87% difference 4.56 divided by 5.13 = .8889 11.11% difference Original RPM + or - the % difference The real life number change may vary due to converter slip and/or tire growth. __________________ Adger Smith (Former SS) Last edited by Adger Smith; 07-20-2021 at 06:43 AM. Reason: sp

07-20-2021, 07:27 AM	#6
Ray Gottry Junior Member Join Date: Feb 2019 Location: Lake City, Fl. Posts: 55 Likes: 95 Liked 53 Times in 21 Posts	Re: gear change RPM X C _______ = MPH G X 1056 MPH X G X 1056 ______________ = RPM C MPH X G X 1056 _____________ = C RPM RPM X C ____________ = G MPH X 1056 RPM = REVS PER MINUTE C = TIRE CIRCUMFRUNCE (UNLOADED) G = GEAR RATIO MPH = MILES PER HOUR You can move the numbers around in the formulas to tell differences of changes. Has worked great for me. Not real scientific but fun.

07-20-2021, 11:34 AM	#7
Bowtieguy1967 Junior Member Join Date: Apr 2006 Posts: 7 Likes: 0 Liked 1 Time in 1 Post	Re: gear change MPH X gear ratio X 336 / tire height (this only works assuming the trans final drive ratio is 1-1) Examples: 120 MPH X 4.56 X 336 ÷ 28" = 6556 RPM 120 MPH X 4.88 X 336 ÷ 28" = 7027 RPM 120 MPH X 5.13 X 336 ÷ 28" = 7387 RPM Obviously converter slip is not factored into the equation.

07-20-2021, 03:37 PM	#8
Dave Noll Senior Member Join Date: Nov 2007 Location: Div. 6, Eastern, WA Posts: 694 Likes: 2,255 Liked 219 Times in 105 Posts	Re: gear change http://atiracing.com/tech/racing_calculators.html This one @ ATI figures in converter slip. __________________ Dave Noll, E/FS ,L/SA 6526

07-24-2021, 10:30 PM	#9
junior barns Senior Member Join Date: Jun 2006 Posts: 539 Likes: 11 Liked 20 Times in 14 Posts	Re: gear change Thanks all! When making a gear change the mph will change. In general going from a 5.13 going thru the traps at 7300 rpm and changing to a 4.88 what rpm drop can I expect?

07-25-2021, 12:01 AM	#10
Adger Smith VIP Member Join Date: Dec 2001 Location: Texarkana Ark/TX Posts: 2,348 Likes: 434 Liked 676 Times in 260 Posts	Re: gear change 7300 - 4.87% (355.5 RPM) = 6944.9 Providing the converter slip % stays the same.. (It usually doesn't) __________________ Adger Smith (Former SS)