Cool RPM/Gearing/Tire Size/Speed Calculator

[pbrown]

I bought the Teleflex 82288P tach as a "close enough to stock" replacement in my 59 Hawk.

I recently got my tach up and running, and I found that I run 1800-1850 at 40 mph. My car has a different rear end than it had out of the factory, so I was curious what my rear end ratio might be to give me that RPM.

I haven't found the tag on my diff, so I thought I might be able to calculate the rear end ratio.

Since I knew my tire OD was 27.1" and I seemed to remember that my FOM ratio in 3rd gear is 1.0:1 (correct me if I'm wrong), I figured that there had to be a formula that would tell me the unknown quantity x that was my rear end ratio.

A Google search led me to this cool site:http://www.angelfire.com/fl/procrastination/rear.html



This would seem to be a great tool because it instantly answers not only "What effect would xxx rear end make?", but also things like "What effect would xxx overdrive transmission make?" and "What effect would tires of xxxxx OD make?"

[Chucks Stude]

What rpm is your car turning at 60 mph? Also, what size tires are you running?

[StudeRich]

That would be a P205X75R15.

http://www.ringpinion.com/Calculators/Calc_RPM.aspx

You would have to have a 3.73 to 1 Axle Ratio in your Automatic to run 1800 RPM at 40, and 2849 RPM at 60 MPH.
Was this a Drag Race Car? a Trailer toter or a stump puller?

[pbrown]

What rpm is your car turning at 60 mph? Also, what size tires are you running?

I have P205/75R15 tires which have an O.D. of 27.1"

Speed limit in my neighborhood is only 40, so I won't know the RPM at 60 mph for sure till I take a longer trip

Since I'm in top gear at 40 mph, I would assume it would be 1.5 times my RPM at 40 - am I wrong in that assumption?

[StudeRich]

Yes you are wrong, see post # 3.

[pbrown]

Rich, I will bow to your superior knowledge in general!

But if I'm wrong to say "Since I'm in top gear at 40 mph, I would assume it would be 1.5 times my RPM at 40" then I must be missing something on this ...

- isn't 1.5 times my 1800 RPM at 40 the same 2849 RPM at 60 MPHthat you listed?

[RadioRoy]

1.5 times 1800 = 2700. At least that's what my math skills, and my calculator tell me.

What am I missing here?

[pbrown]

1.5 times 1800 = 2700. At least that's what my math skills, and my calculator tell me.

What am I missing here?

I was doing 1.5 times 1850 or so and rounding in my head, so Rich's number seemed close. Now that I hit the calculator, I see that 2775 is the correct answer.

So Rich, are you telling us that the RPM change from 40 to 60 mph is not linear?

[j.byrd]

Hmmm, with my hand-held Autolite ratio computer, it shows about a 4:30 gear. Can that be right ? It shows gear ratios, tire diameter, speed in mph, and rpm, so I just rolled it to your numbers.

[pbrown]

Hmmm, with my hand-held Autolite ratio computer, it shows about a 4:30 gear. Can that be right ? It shows gear ratios, tire diameter, speed in mph, and rpm, so I just rolled it to your numbers.

4.3 seems high. The website I listed (http://www.angelfire.com/fl/procrastination/rear.html) says that I have like a 3.55 or 3.7.

My engine and tranny and rear end came from a 1964 Cruiser. I assume that those ratios were options for a 1964 Cruiser?

But then I guess my speedo could be wrong - which would throw all the calculations off. I'll check that with a GPS.

And while I doubt it, I guess that my tach could be out of calibration. It shows my idle speed at 5-600 RPM, which I think is correct, so the tach seems right

[StudeRich]

j.Byrd and pbrown, please refer back again to Post #3.
Your correct Tire Size, Speed, Trans. Type and Axle Ratio are all there, per the Linked Calculator.

OK, my number is 1899 for 40 MPH vs 2849 for 60, so that little bit could be what is making you think it is not linear maybe? Whatever.
It is what the Calc says it is, I think.

One thing you have to do in addition to CORRECT Trans. Type is use actual Studebaker Axle Ratios: 3.07, 3.31, 3.54, 3.73, 4.09, 4.27 or 4.54

[WCP]

You can use tire diameter in your calculations if your tires are made of steel (or wood). The rolling diameter of a 27.1" rubber tire is not 27.1". That is why manufacturers provide revs per mile ratings for their tires based on proper new tread depth and specified inflation pressures. The sidewall flex of radials is much greater than that of bias ply tires. Redo your calculations with the revs/mile for your tire and a GPS reading of your speed for speedometer correction.

[pbrown]

You can use tire diameter in your calculations if your tires are made of steel (or wood). The rolling diameter of a 27.1" rubber tire is not 27.1". That is why manufacturers provide revs per mile ratings for their tires based on proper new tread depth and specified inflation pressures. The sidewall flex of radials is much greater than that of bias ply tires. Redo your calculations with the revs/mile for your tire and a GPS reading of your speed for speedometer correction.

That is an excellent point! It didn't occur to me that rolling diameter would be different from OD.

You've got me thinking tho'...

Since I'm working backwards to get to choice between a predefined set of values (the available diff ratios) hopefully the difference would not skew things enough to move to the next available ratio.

But then again, my visual readings of my analog tach are so coarse that its error are probably enough to bury the difference between OD and actual rolling diameter.

The unknown in all this is the speedometer error. I'll look into that ASAP.


Revolutions Per Mile

[Mike Van Veghten]

Curious...

Question #1
Is this 27.1" diameter loaded or unloaded (running pressure?) ?
If it is unloaded, your figures are off.

Question #2
Where did the top of the tire measurement location come from ? Tire crown, the corner of the tread ?

Just wondering...

Mike

[pbrown]

If it is unloaded, your figures are off.

Agreed!

Where did the top of the tire measurement location come from ?

Published manufacturer figures. Unfortunately, they don't publish the revs per mile