Calling on Math Whizzes....

635 Views 10 Replies 8 Participants Last post by pimpin88, May 19, 2007

N0DIH

Discussion Starter · May 18, 2007

Ok, I am not the brightest on math anymore, I let too much slip over the years...

A friend of mine showed me that if a car in 1:1 ratio gear (say 3rd on a OD auto, or 4th on most sticks) the engine rpm @ 80 mph is the axle ratio with the decimal place moved over.

So like my Cad with 3.08's now runs 3060-3080 rpm @ 80 mph in 3rd. When I had 3.42's, it ran 3400 rpm. My 99 Burb with 3.73's runs 3700 rpm.

So it gives you a good reasonabally accurate axle ratio without counting ring gear and pinion teeth.

Ok, why??? Another friend says this can't possibly be because tire sizes are different.

But it works 100% on every car I have tried....

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RunningOnEMT

· #2 · May 18, 2007

tire sizes are usually chosen to match fairly well with OEM gear ratios so as not to put TOO much stress on the driveline...

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N0DIH

Discussion Starter · #3 · May 18, 2007

I am looking to see why the math works this way, not why they chose what tire size.

This is the question: Why does my rpm/1000 @ 80 mph = Axle Ratio?

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codewize

· #4 · May 18, 2007

In a gear that provides 1:1 ratio.

The reason the math works is simply the math. The question is why are the cars designed like that.

N0DIH said:
I am looking to see why the math works this way, not why they chose what tire size.

This is the question: Why does my rpm/1000 @ 80 mph = Axle Ratio?

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N0DIH

Discussion Starter · #5 · May 18, 2007

Why 80? why not 70? Cosmic coincidence?

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fleetwood76

· #6 · May 18, 2007

Hello!

Have you tried this formula with cars with very different sizes of wheels.

I think that the speed "80 mph" must alter depending on wheelsize.

80 mph is the right speed for cars with average wheelsizes 15" normal tires or 16" and larger with lowprofile tires.
The total size of those wheels schould alter only up to 10% and the reading of a rpm meter and speedo meter can probably make it hard to get exacly numbers, all this combined do that the formula seems accurat enought, in those cases.

I don't think this formel would work with 80 mph if you compared a Mini Cooper with 12" tires with a raised pick up truck with 33" monster mudders.

I have been off school to long to do the math tho.

jolle

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Ranger

· #7 · May 18, 2007

So if I read you correctly, at 80 MPH I should turn 3100 RPM (3.11 final drive)? That seems high as I turn 1950 at 70 MPH. Oh wait, that's in OD. I'll have to take it to 80 in 3rd and see what I get. Interesting if it is true in every case. I'll have to test it on my wifes Bonneville (3800).

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N0DIH

Discussion Starter · #8 · May 18, 2007

Yes, my Suburban has 30-31 in tires (245 75 16's) and my Cad has 235 70 15's. Even in my niece in law's Toyo Matrix at 80 mph in 3rd (screaming!) it showed an reasonably accurate 4100 rpm for 4.10's.

So tire size doesn't matter, as the speedo's are accurate that seems to be the common denominator.

Even my 80 T/A with non OD and 275 60 15's came in at around 3100-3200 rpm @ 80 with a non lockup converter.

So let me know what you find! I am very curious on this one...

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Red_October_7000

· #9 · May 18, 2007

That's neat... I gotta try that some time. This is a good way to find out, on test drives, what axle ratio the car has. So I guess Toyota is still putting in 3+OD+OD boxes, eh? My '88 MR2 has 3 gears and two overdrives, labeled as 1 through 5. 3 is "top gear" and 4 and 5 are both overdrives, neither of them particularly high. At 70-80 MPH in 5th is's running around 4,000, I think. Then again it redlines way up at 7,650!

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JimHare

· #10 · May 18, 2007

N0DIH said:
Why 80? why not 70? Cosmic coincidence?

More or less. It had to be something. Don't attach any significance to the value. Now, if it had come out to something like your social security number divided by your age times the number of nose hairs in Jesda's left nostril, less the number of times Gurl has faked an orgasm, well, then you'd have some kinda odd number...

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pimpin88

· #11 · May 19, 2007

JimHare said:
More or less. It had to be something. Don't attach any significance to the value. Now, if it had come out to something like your social security number divided by your age times the number of nose hairs in Jesda's left nostril, less the number of times Gurl has faked an orgasm, well, then you'd have some kinda odd number...

Points!!!

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