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Discussion Starter · #1 ·
Help me understand just what the heck happenswhen im tooling down the highway at 3000 rpm and doing 70 and every gear ration calculator says my axle ratio is 3:55 or better.

Is the TH350c that terrible and wasting energy?


I can flash my stall convertor to 1800-1900 rpm

Make sme wonder about installing 354 or some mid gear in the rear axle.
 

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Here is my non-mathematician analysis. ( I am the least qualified person on DC to do this :laughing:)

Let's say your tire is 24" in diameter (this is the weakest link in my analysis, because I didn't want to go outside and measure)

That would mean a radius of 1'. The circumference of a circle is 2 times pi times the radius, so the circumerence of your tire would be 6.28 ft meaning you would move that far every revolution. Now if you went 70 miles in one hour, you would go 369,600 feet. If you divide that by the circumference of your tire, you would need a little less than 59,000 revolutions of your tire during that hour or 983 revolutions per minute. If the tailshaft of your tranny was 1:1 with your engine in high gear ( a big if ), you would need 983 X your axle ratio to get your engine RPM or you could reverse it if the RPM were known and divide the RPM by 983 to get your axle ratio. If you are doing 3000 RPM at 70 mph in this setup, the axle ratio would be 3.05.

To make this work for your car, you need to know the distance from your axle center to the ground to determine your tire radius and you need an idea of whether your output is near 1:1 with your engine.

Dang, I should have been an engineer!:laughing: OK, now all of the real engineers can point out where I screwed up. :rolling:
 

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Discussion Starter · #4 ·
TExdentist, LOL My dad owns a dental lab, when I was a kid I used to polish crowns as a high school job. Godbless your profession I have a lot of respect for dentist.

This th350C must be loose. there are tons of 200 and 700 transmissions on CL I may just convert.

Here is my non-mathematician analysis. ( I am the least qualified person on DC to do this :laughing:)

Let's say your tire is 24" in diameter (this is the weakest link in my analysis, because I didn't want to go outside and measure)

That would mean a radius of 1'. The circumference of a circle is 2 times pi times the radius, so the circumerence of your tire would be 6.28 ft meaning you would move that far every revolution. Now if you went 70 miles in one hour, you would go 369,600 feet. If you divide that by the circumference of your tire, you would need a little less than 59,000 revolutions of your tire during that hour or 983 revolutions per minute. If the tailshaft of your tranny was 1:1 with your engine in high gear ( a big if ), you would need 983 X your axle ratio to get your engine RPM or you could reverse it if the RPM were known and divide the RPM by 983 to get your axle ratio. If you are doing 3000 RPM at 70 mph in this setup, the axle ratio would be 3.05.

To make this work for your car, you need to know the distance from your axle center to the ground to determine your tire radius and you need an idea of whether your output is near 1:1 with your engine.

Dang, I should have been an engineer!:laughing: OK, now all of the real engineers can point out where I screwed up. :rolling:
 

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Mmmm, yeah, probably.

Texdentist, your dead-on. You get an Honorary Engineering Degree:thumbsup:, (but you make more money as a dentist, trust me, I am an Engineer.)
My Dad was an electrical engineer for the phone company for 40 years. I started college as an engineering major, but math more complicated than the simple stuff above...uh...pointed me toward another career. :laughing:
 

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Help me understand just what the heck happenswhen im tooling down the highway at 3000 rpm and doing 70 and every gear ration calculator says my axle ratio is 3:55 or better.

Is the TH350c that terrible and wasting energy?


I can flash my stall convertor to 1800-1900 rpm

Make sme wonder about installing 354 or some mid gear in the rear axle.

Transmission efficiency isnt really a factor because in high gear you have a 1:1 locked input & output shaft.
Its also pretty unlikely that it is slipping in high gear at a steady cruise, or it would be obvious to you.
What is the most likely thing that is happening is that your lock up converter isnt very efficient, and it isnt locking either.
Based on a stock 27" tall tire with a 2.87 gear and a locked converter you should be going 84 mph at 3000 rpm.
If you have shorter tires then that calculation is off some.
 

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Discussion Starter · #7 · (Edited)
Transmission efficiency isnt really a factor because in high gear you have a 1:1 locked input & output shaft.
Its also pretty unlikely that it is slipping in high gear at a steady cruise, or it would be obvious to you.
What is the most likely thing that is happening is that your lock up converter isnt very efficient, and it isnt locking either.
Based on a stock 27" tall tire with a 2.87 gear and a locked converter you should be going 84 mph at 3000 rpm.
If you have shorter tires then that calculation is off some.
I got something wrong then. Maybe my tach is off? I'll check.

My lock up is not engaged as it is disconnected
 

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Discussion Starter · #10 ·
I have been reading about how to attach a toggle switch to the lock up.

How much rpm drop should I expect?
 
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