Computing Gear Ratio & RPM Change

Racer Roy · Post by **Racer Roy** » Sun Sep 17, 2006 4:27 pm

Hey gang:
I have been doing some thinking and have a situation that does not make sense.
I would love some input about how to figure out what my RPM change will be when I change gears.
I am currently running a 3.73 rear in second gear in a TH350, which is 1.52. This gives: 1.52x3.73=5.66 total. I am turning 5200 this way.
Another car is using 2.73 rears in the 2.52 FIRST gear(TH350) for 2.52x2.73=6.87 He is turning 6000 this way. (same tires too)
Now the confusion comes from my "ARC Industries" gear calculator slide rule. It says the RPM difference between a 6.87 total to a 5.66 should be 1500 RPM.
Does anyone have a more accurate way to find out what my RPM will be?
I am trying to see what I would gain by going to a 4.11 final drive with the 1.52 second gear(6.24 total) My slide rule says that should be approx. 800 rpm

Thanks for any suggestions.
RR

bill jones · Post by **bill jones** » Sun Sep 17, 2006 8:25 pm

http://wahiduddin.net/calc/calc_speed_rpm.htm

-This page has a simple calculator where you need to know the radius of the tire, then just insert 1 for the transmission ratio, and then enter your final drive ratio.
-It gives you MPH as the answer---so you'll have to change the other parameters to keep the MPH the same as you change final gear ratios---and see what rpm you'll have to enter to get the same MPH.

Greg D · Post by **Greg D** » Sun Sep 17, 2006 9:29 pm

I use this really simplistic spreadsheet to give me a rough idea what the MPH per gear will be at a specific RPM. I also use it to determine RPM drop between gear change.

http://members.cox.net/gadavis1/RPM%20M ... 20Gear.xls

Racer Roy · Post by **Racer Roy** » Sun Sep 17, 2006 10:54 pm

Thanks, guys, both will be helpful.

RR

Cobra · Post by **Cobra** » Sun Sep 17, 2006 11:05 pm

Try this: multiply rear end ratio, (3.73), by gear ratio, (1.52), which gives you 5.6696. Do the same with the 4.11 rear end, =6.2472. The 3.73X1.52 combo equals 5200, so merely divide 6.2472 by 5.6696 = 1.1019X5200=5729.7587 or rounded 5730 . Or just divide 4.11 by 3.73 = 1.1019 X 5200 = 5729.7587 or 5730 rounded.

Racer Roy · Post by **Racer Roy** » Mon Sep 18, 2006 12:40 am

Cool, that is what I was hoping for, as 6000 is a little ragged with these cars.

thanks

RR

mactheknife · Post by **mactheknife** » Mon Sep 18, 2006 2:13 am

I use the gear split software. Its pretty powerful. Lets you play with the transmission gears as well as the rear end gears, altitude and many other variables. It graphs so you could see the changes you make.

BillyShope · Post by **BillyShope** » Tue Sep 19, 2006 10:10 am

Racer Roy wrote:Hey gang:
I have been doing some thinking and have a situation that does not make sense.
I would love some input about how to figure out what my RPM change will be when I change gears.
I am currently running a 3.73 rear in second gear in a TH350, which is 1.52. This gives: 1.52x3.73=5.66 total. I am turning 5200 this way.
Another car is using 2.73 rears in the 2.52 FIRST gear(TH350) for 2.52x2.73=6.87 He is turning 6000 this way. (same tires too)
Now the confusion comes from my "ARC Industries" gear calculator slide rule. It says the RPM difference between a 6.87 total to a 5.66 should be 1500 RPM.
Does anyone have a more accurate way to find out what my RPM will be?
I am trying to see what I would gain by going to a 4.11 final drive with the 1.52 second gear(6.24 total) My slide rule says that should be approx. 800 rpm

Thanks for any suggestions.
RR

A tip when making calculations: Always round to the nearest significant digit. It should have been 5.67 and 6.88 for the final ratios.

And, if your data is correct, your friend with the 2.73 rear is turning 6300. (Assuming same tire size.)

As Cobra pointed out, always use ratios to make these calculations. You don't need any fancy spreadsheets; just your calculator.

Cobra · Post by **Cobra** » Tue Sep 19, 2006 11:39 am

Good luck with your new gear!

Racer Roy · Post by **Racer Roy** » Tue Sep 19, 2006 11:47 am

This is all great info, guys.
My pal is running my car from last year, and the tach is dead on 6000, but it could easily be 2-300 rpm off. I would anticipate a slight variation from "on paper" calculations and the real world, but to be within a few hundred RPM is very close indeed.

Thanks everyone.

RR

BillyShope · Post by **BillyShope** » Tue Sep 19, 2006 1:32 pm

Cobra wrote:In making scientific calculations you can significantly change results by rounding wrong figures. Using 6.88/5.67 would give us 1.2134X5200= 6309.7002, which is incorrect. Multiplying figures is a prime example of this case.

Again, assuming the tires are identical in size, the 6300 figure is correct.

3.73X1.52 = 5.6696, which is essentially 5.67, NOT 5.66

2.73X2.52 = 6.8796, which is essentially 6.88, NOT 6.87

We can't trust tachometers unless they've been calibrated. Either the tires aren't the same or one (or both) of the tachometers are in error.

jacksoni · Post by **jacksoni** » Tue Sep 19, 2006 2:03 pm

Does convertor slippage differences between the two cars factor into this somewhere as well?

BillyShope · Post by **BillyShope** » Tue Sep 19, 2006 5:25 pm

jacksoni wrote:Does convertor slippage differences between the two cars factor into this somewhere as well?

I can't imagine it being worth a 5% difference. Now, if one was a stick and the other had a high stall converter, it could get pretty close to making up the difference.

But, I'm only guesstimating. Does someone out there have a torque converter performance curve? Maybe I can Google something up.

Racer Roy · Post by **Racer Roy** » Tue Sep 19, 2006 6:32 pm

To my knowledge, the cars have nearly identical powertrains.
350, flat tops, iron in/out, same trans/ converter, etc.
The only diference is mine has 882 heads, and the other has 305 heads.
Could the way the converters react to the gears they are in be something?
Of course, the car in first gear has a lot more engine braking than mine does.
Thanks again

RR

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Computing Gear Ratio & RPM Change

Computing Gear Ratio & RPM Change

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