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No, more like 3400 RPM @ 60 by my calculations. But, you check me out.
Let's take the case of a truck with LT235/75/15 tires, which typically are 29" in diameter, 3.55:1 gears, a 7-toothed drive gear and a 16-toothed driven gear.
Tire revolutions/mile = 5280 ft/mile x 12in/ft divided by the tire's in/rev = 2185 rev's/mile
Driveshaft revolutions/mile = tire revs/mile x axle ratio = 7756 revs/mile
Speedometer shaft revs/mile = driveshaft revs/mile x drive gear divided by driven gear = 3393
So, if you are driving 60 MPH you'll traverse 1 mile in 1 minute. And if the speedo shaft has to turn 3393 revolutions in 1 mile or 1 minute it is turning 3393 RPM.
Or, how about a case where you have LT235/85/16 tires, which are 31.5" in diameter, 3.55 gears, an 8-toothed drive gear, and a 17-toothed driven gear. In that case the speedo shaft would be turning at 3360 RPM, or 99.03% of the original speedo's speed. IOW, the speedo would read 59.4 MPH when the previous setup read 60 MPH.
NOTE: The above is WRONG as I used the tire's diameter instead of the circumference. So, the two answers are off by Pi, meaning 3.14159. In other words, the first answer should be 1080 RPM, and the second should be 1070. But, the ratios stay the same, so the second example would still read 59.4 MPH when the first one read 60 MPH.
Last edited by Gary Lewis; May 2, 2016 at 07:00 AM.
Reason: Mistake!
Uh oh, I already see an error - I used the tire's diameter instead of circumference! So, my calcs are going to be high by a factor of Pi, meaning 3.14159. So, in the first example the speedo RPM should be 1080.
Gary, what if you embedded a spreadsheet that had two tabs in it. One for "Simple Calculations" and one for "Thorough Calculations". That would give visitors an option depending on how much they knew about their vehicle.
Chris - That's a good idea if I can show tabs. Otherwise I may just embed two spreadsheets, one simple and one thorough. I think I can play with that a bit today and we will see what happens. Thanks!
Gary,
That odd ball figure of 1080 bugged me.
Not sure of the rules/regulations in the States but here auto makers err with their speedo settings so no one gets pinged for speeding.
In the example we crunched the 1080 gives a speedo reading of 64.8 if the speedo was set up for 1000 at 60 MPH. The mighty makers covering their rear.
7/17 gears would pull it down to 61/62 MPH indicated
Cheers Steve
All - I need your thoughts regarding the spreadsheet idea. The question is how useful it will be, and to how many people?
I ask because it appears that the only way to give you a truly interactive calculator is for me to spend some money to buy some software to convert the spreadsheet to HTML. But, I don't want to spend the money if it isn't REALLY going to be beneficial to a number of people. And, I don't want to spend the money if there is already a similar calculator out there to which I can link.
Now, just in case one of you know of a way to do it easily/cheaply, let me tell you what I've found. Basically there are three ways to get a spreadsheet onto a web page:
Static: I can show you either a picture of my spreadsheet or put the results of my spreadsheet out such that they get updated when I change any parameter on the page. But, that doesn't do you much good.
Controlled Dynamic: This is the only way I see making it useful, and that involves buying software that becomes part of Excel and generates an HTML file that mimics the spreadsheet. I can lock any cells I don't want the user to play with, like the calculations themselves, and leave the variables open to edit - either free-form or via drop-down. So each user can play to his/her heart's content, dialing in tire sizes, axle ratios, drive gears, and driven gears.
Wide-open Dynamic: In this one I give the user a link to my spreadsheet, in Google Sheets format on my Google drive, and the user can change anything. Yes, I can protect cells, like the calculations, but any user with experience can un-protect them and make any changes s/he desires - changes that impact future users. So, while I'm sure FTE'ers wouldn't do that, I'm confident someone would ruin it for everyone.
So, please let me know your thoughts regarding how useful the following would be, and bearing in mind that the same software would allow other dynamic spreadsheets:
Simple: Given your current driven gear and indicated speed at some true/GPS speed, what would the indicated speed be with another driven gear? (This would assume your tire size, axle ratio, and drive gear stays the same.)
Thorough: This one would require the old and new tire size, axle ratio, drive gears, and driven gears, and would give the result in a percentage change from old to new.
Steve - I suspect you are right that 1000 RPM is the target, but I don't know that's the case. After all, 1080 is only a .8% error, and as you said at 60 MPH that's about 5 MPH, which is said to be the max error the Fed's allow. Further, as you've pointed out to me elsewhere, the 29" diameter I used for the tire may well not be the true rolling diameter, but unless you know the exact manufacturer and model number of the tire you won't know the rolling diameter. In fact, this snippet shows that Ford changed the driven gear for the same size of tire depending on the manufacturer, with BF = B.F. Goodrich. F = Firestone. GE = General, GY = Goodyear, U = Uniroyal:
Ok, I think I can answer my own question to some extent. I've found several sites that will calculate either a drive or driven gear, meaning the simple calc I mentioned. But, some of these sites require you to know the # of speedo RPM's to hit 60 MPH. And it looks like, with Steve's help, we've settled on 1000 RPM for our speedo's. But, that left me wondering. Hmmm, I have two speedo's laying on the bench (thanks again, Jonathan). A bit of playing had one of them sitting in the lathe at 1150 RPM:
Let's see, 1150/1000 x 60 = 69 MPH if the speedo expects 1000 RPM @ 60 MPH. And, while that pic may be hard to tell due to parallax, that's as dead-on 69 as you could get. But, not being content with one reading I dropped the lathe to 700 RPM. 700/1000 x 60 = 42 MPH, and that's exactly where the speedo was.
So, I think we have off-site calculators for drive or driven gears. And we know the speedometers expect 1000 RPM to show 60 MPH. All that leaves is a way to calculate "old" vs "new" setups, and I can offer on my web site the ability to download my spreadsheet if someone wants it.
Gary: thanks for putting the calculators on the speedometer page.I plugged in my info and it tells me I need an 18 tooth driven gear to correct my speedometer. That helps a bunch.
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