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Gear ratios, tire size...scratching head by PatrickTRoof
Started on: 11-18-2013 04:43 PM
Replies: 47 (895 views)
Last post by: PatrickTRoof on 12-29-2013 07:43 PM
PatrickTRoof
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Report this Post12-23-2013 07:45 PM Click Here to See the Profile for PatrickTRoofClick Here to Email PatrickTRoofSend a Private Message to PatrickTRoofEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by georgie:

Find a place in your town where the cop's have a (your speed is) and it will tell you how fast your going! It will tell you if your speedo is off .


LOL, there's an idea.
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jimmo
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Report this Post12-24-2013 07:21 AM Click Here to See the Profile for jimmoClick Here to Email jimmoSend a Private Message to jimmoEdit/Delete MessageReply w/QuoteDirect Link to This Post
Try this, its easy math. Forget about formulas and tire sizes for a minute. Mark your tire and the ground. Roll the car so the tire makes one revolution and mark the ground again. Now measure the distance in inches between the marks and you have the tire circumference. lets assume its 77 inches. We will assume your 4th gear is 1:1 and your axle ratio is 3.65:1 just for the example.
One mile = 63360 inches. 77 goes into 63360.....822.857 times. Your tire needs to roll 822.857 times to go exactly one mile. When you drive 60 mph you are driving one mile per minute. Engine revolutions are measured (per minute). So at 60 mph your tire is turning 822.857 times per minute. Now multiply that by your axle ratio (3.65) and you get 3003 rpms @ 60 mph.
The formula woul be ...

63360/(tire circumference)x(axle ratio)=rpm@60mph.

As long as you can accurately do 60 mph via gps or accurate speedo you can compare that number to what your tach is showing. Ill bet your tach is off. GM tachs that are 30 years old just arent reliable anymore.
I also found these. I am going to try it because my tach is way off.
http://www.fieros.de/en/articles/tach.html
http://www.howtoalmanac.com...tomotive/tachfix.htm

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PatrickTRoof
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Report this Post12-25-2013 08:39 PM Click Here to See the Profile for PatrickTRoofClick Here to Email PatrickTRoofSend a Private Message to PatrickTRoofEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by jimmo:

Try this, its easy math. Forget about formulas and tire sizes for a minute. Mark your tire and the ground. Roll the car so the tire makes one revolution and mark the ground again. Now measure the distance in inches between the marks and you have the tire circumference. lets assume its 77 inches. We will assume your 4th gear is 1:1 and your axle ratio is 3.65:1 just for the example.
One mile = 63360 inches. 77 goes into 63360.....822.857 times. Your tire needs to roll 822.857 times to go exactly one mile. When you drive 60 mph you are driving one mile per minute. Engine revolutions are measured (per minute). So at 60 mph your tire is turning 822.857 times per minute. Now multiply that by your axle ratio (3.65) and you get 3003 rpms @ 60 mph.
The formula woul be ...

63360/(tire circumference)x(axle ratio)=rpm@60mph.



I'll try this, thanks. I don't know if I want to hack up the PC board just yet, but I do want to clear up this mystery and make sure my engine isn't over-revving.
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edfiero
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Report this Post12-26-2013 12:05 PM Click Here to See the Profile for edfieroClick Here to Email edfieroSend a Private Message to edfieroEdit/Delete MessageReply w/QuoteDirect Link to This Post
Not to highjack, but having Tach issue on my 84 - 4 speed.
I've verified the Speed-o is accurate both with GPS and Mileposts. Tach previously read about 3000 at 60mph, now reads closer to 3500. Stock Size Tires.

What are common issues effecting the Tach on a Duke?
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dobey
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Report this Post12-26-2013 02:52 PM Click Here to See the Profile for dobeySend a Private Message to dobeyEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by PatrickTRoof:


I'll try this, thanks. I don't know if I want to hack up the PC board just yet, but I do want to clear up this mystery and make sure my engine isn't over-revving.


Unless your trans or tires are slipping, it's impossible for the engine to be over-revving at any given speed, as the gear ratios and tire size determine what speed the engine will be rotating at. If the engine speed increases, so will your vehicle speed.
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PatrickTRoof
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Report this Post12-27-2013 05:32 PM Click Here to See the Profile for PatrickTRoofClick Here to Email PatrickTRoofSend a Private Message to PatrickTRoofEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by dobey:


Unless your trans or tires are slipping, it's impossible for the engine to be over-revving at any given speed, as the gear ratios and tire size determine what speed the engine will be rotating at. If the engine speed increases, so will your vehicle speed.


I know that. I was just trying to make sure the PO didn't so something weird like put in a different final drive.

I connected a Sunpro mini tach directly to the coil and found that the in-dash tachometer is showing about 400-500 rpm high.

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project34
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Report this Post12-29-2013 01:14 PM Click Here to See the Profile for project34Send a Private Message to project34Edit/Delete MessageReply w/QuoteDirect Link to This Post
I think dobey has made a number of good points in this thread, including the tacit observation that one often can simplify otherwise overly complicated-looking mathematical formulas, as was done with the Speed (MPH) formula he mentioned sourcing from (page 90 of) the Auto Math Handbook:

 
quote
Originally posted by dobey:
This is the only formula you need, to calculate what speed you should have:

code:
gear ratio = ratio of current transmission gear x final drive ratio

rpm x tire diameter
mph = -----------------------------
gear ratio x 336


In the spirit of simplifying things, like the Speed (MPH) formula CowsPatoot listed for his helpful on-line calculator, look at the three constants within that formula. Taken together, those three constants comprise one multiplier: 31.415/1056/10. In tiurn, that somewhat complicated-looking multiplier reduces to one divisor, which --- rounded to the nearest integer --- is simply 336 (i.e., the 336 in 1/336, which is the same 336 dobey used in the easier-to-remember formula he cited).

That aside, however, a key benefit of CowsPatoot's on-line calculator is that it also provides an estimate (expressed in inches) of a tire's outer diameter if one inputs to that calculator a Fiero's rear tire size (e.g., 24.16 inches in diameter for a 215/60-14 rear tire's size).

dobey makes another good point, one which although simple, people nevertheless sometimes forget:

 
quote
Originally posted by dobey:
...it's easy to get the wrong output by using the wrong inputs.

To that I'd add that although it may seem counterintuitive to some, it's also easy to (periodically) get the correct output from a Speed (MPH) formula even when one ignores inputting the gear ratio of the transmission gear that the car is in. However, at other times, when one ignores inputting the gear ratio of the transmission gear that the car is in, the output of a Speed (MPH) formula can be very, very wrong --- even though you may have used successfully in the past a Speed (MPH) formula that didn't call for any transmission gear input, and that formula seemed to work just fine without that before --- maybe for several years.

Why? How can that happen?

Well, there actually is no magic, nor even any real mystery involved here --- provided that the gear ratio of the transmission gear one's car is in, happens to be 1.00:1. --- as it is when I'm in the 3rd gear in my '86 Fiero with an automatic TH-125, and as it is when I'm in the 3rd gear of a different car, a `74 with a TH-400 automatic transmission. Two keys points are in order here:
  • Within the Speed (MPH) formula highlighted above, inputting a transmission gear ratio of 1.00 --- has absolutely no effect on the formula's output, which is the identical result produced when ignoring a 1.00:1 transmission gear ratio.

  • Conversely, ignoring a transmission gear ratio other than 1.00 easily can produce Speed (MPH) output results that are very, very wrong.
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PatrickTRoof
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Report this Post12-29-2013 07:43 PM Click Here to See the Profile for PatrickTRoofClick Here to Email PatrickTRoofSend a Private Message to PatrickTRoofEdit/Delete MessageReply w/QuoteDirect Link to This Post
A lot of great information in this thread. I'm not going to worry about the tach for now, now that I've figured this out.
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