ricosan Posted June 2, 2017 Share Posted June 2, 2017 Here is a photograph of my speedometer. As you can see it has RPMs indicated at various speeds. Were these numbers reliable figures or were they just a set of numbers that looked good when purchasing a car but really didn't directly correspond to the MPH/RPMs? ricosan Link to comment Share on other sites More sharing options...
Guest BillP Posted June 2, 2017 Share Posted June 2, 2017 Being a derivative based on the function of road speed, tire diameter and gear ratios it is not as accurate as a an electric or mechanical tachometer, but it is a useful guide. Note that it is only useful when in high gear. Link to comment Share on other sites More sharing options...
46 woodie Posted June 2, 2017 Share Posted June 2, 2017 If nothing changes, the figures will be correct. If you calculate RPM, gear ratio, tire height and speed, you will get the numbers on your MPH/RPM gauge. If you change something like tire size or gear ratio the numbers will be incorrect. There are formulas that will calculate each, RPM, MPH, tire height etc. If you look at your gauge the manufacturer is saying that at about 40 MPH your engine is turning 1680 RPM if you didn't change the tire size. 1 Link to comment Share on other sites More sharing options...
Frantz Posted June 2, 2017 Share Posted June 2, 2017 Of course anything you change would change BOTH speed and RPM on the gauge reading. Check out the gauge on my old Farmall ('63 460). They extended the needle and give calculations for each gear, show the RPM, ground speed, and PTO speed (540 for normal implement use). The same measurement is even used to calculate the hours. 1 Link to comment Share on other sites More sharing options...
Guest BillP Posted June 2, 2017 Share Posted June 2, 2017 So the engine speed was geared down by a ratio of 3.25 (1755/540) to run the power take off. At risk of hyperbole, I would say the slide rule, and the ability to use it, was a civilization-building instrument. Link to comment Share on other sites More sharing options...
JACK M Posted June 2, 2017 Share Posted June 2, 2017 UUgghhh, Slide rule, approximate at best. As per my college instructor. I never could get the thing figured out. Link to comment Share on other sites More sharing options...
capngrog Posted June 2, 2017 Share Posted June 2, 2017 9 minutes ago, JACK M said: UUgghhh, Slide rule, approximate at best. As per my college instructor. I never could get the thing figured out. Depending on the particular slide rule, slide rules could be very accurate. I used a slide rule until portable electronic calculators became affordable in the mid 1970s. Cheers, Grog Link to comment Share on other sites More sharing options...
mike6024 Posted June 2, 2017 Share Posted June 2, 2017 (edited) 3 hours ago, ricosan said: Were these numbers reliable figures Give us your tire outside diameter in inches and your final drive ratio and we can figure it out, ie how many rpm's you should have at, say, 40 mph and see if it is, in fact, about 1680. Supposing top gear for the transmission is 1 to 1. You take the miles per hour and convert that to inches per minute. Then divide by the tire outside circumference to get revolutions per minute. That is revolutions of the wheel per minute. Then multiply by the final drive ratio to get revolutions of the drive shaft per minute. And if the transmission is in direct 1 : 1 top gear ratio that is the engine rpm also. Or if it is in overdrive you need to multiply again. Edited June 2, 2017 by mike6024 (see edit history) Link to comment Share on other sites More sharing options...
JACK M Posted June 2, 2017 Share Posted June 2, 2017 1 minute ago, capngrog said: Depending on the particular slide rule, slide rules could be very accurate. I used a slide rule until portable electronic calculators became affordable in the mid 1970s. Cheers, Grog Fortunately for me I gave up the mechanical engineering major. I couldn't draw very well. And yes, calculators came along not to long after that. The down side of that is that calculators are allowed in schools and students these days cant do math. I asked one of my daughters to calculate my MPG on a recent trip and she had no clue. She also has trouble with analogue clocks. Times are a changin !! Link to comment Share on other sites More sharing options...
Guest Posted June 2, 2017 Share Posted June 2, 2017 Tire outside diameter is not always/ever correct in these calculations. To be 100% correct you need the rolling circumference. My 550 x 19 tires have a 3" difference between the circumference when laden or un-laden. Even the difference between four scrawny and four stout passengers makes a difference of 1.5" in the rolling circumference. Three different make of 550 x 19 tires had three different un-mounted circumferences. I bought the one with the largest circumference. Almost like getting overdrive. Link to comment Share on other sites More sharing options...
46 woodie Posted June 2, 2017 Share Posted June 2, 2017 If anyone would like to play with the numbers these are the formulas. MPH= RPM x Tire diameter, divided by Gear Ratio x 336. RPM= MPH x Gear Ratio x 336 divided by Tire Diameter. Gear Ratio= RPM x Tire Diameter divided by MPH x 336. Tire Diameter = MPH x Gear Ratio x 336 divided by RPM. For example at 2500 RPM with 28" tire and a gear ratio of 3.75 to 1 you would be going about 55 MPH. It's fun to see how the rear end ratio and tire height affect speed and RPM. Link to comment Share on other sites More sharing options...
Rusty_OToole Posted June 3, 2017 Share Posted June 3, 2017 As accurate as your speedometer if you have stock size tires and stock rear axle gears. Link to comment Share on other sites More sharing options...
ricosan Posted June 7, 2017 Author Share Posted June 7, 2017 Hey Guys, Link to comment Share on other sites More sharing options...
PFitz Posted June 7, 2017 Share Posted June 7, 2017 (edited) Tire diameter will get you close, but depending on tire brand, size, construction, and air pressure it can be off quite a bit in one mile. What is used for better accuracy is the tire's "rolling diameter". And it's easy to check on a flat surface. Use a tire on the axle that is connected to the speedometer drive. Make a mark on the tire exactly at the 6 o'clock position. You can find that position on the tire and the ground easily with a plumb-bob - hanging a weight from a string held in the center axis of the axle. Roll the car until the mark is back exactly at 6 o'clock. Then measure the distance from mark to mark on the ground. That is exactly the rolling diameter of one revolution of that brand and size/construction tire, at that air pressure. Paul Edited June 7, 2017 by PFitz (see edit history) Link to comment Share on other sites More sharing options...
Guest Posted June 7, 2017 Share Posted June 7, 2017 Measuring one revolution gives you the "rolling circumference". Diameter is the width across a circle not around it. Link to comment Share on other sites More sharing options...
PFitz Posted June 7, 2017 Share Posted June 7, 2017 33 minutes ago, Tinindian said: Measuring one revolution gives you the "rolling circumference". Diameter is the width across a circle not around it. True. It can be called rolling circumference, too. But often tires are measured by their diameter. And because it's not as easy to measure the circumfreance, many owners measure the diameter and multiply that times 3.1415 to get circumference. That diameter changes because of the vehicle's weight on the tire as it rolls and that change in diameter changes the circumference. Paul Link to comment Share on other sites More sharing options...
Guest Posted June 7, 2017 Share Posted June 7, 2017 The distance from the mark to the mark on the road is still a circumference measurement. It is NOT a diameter. The distance across a tire is a diameter. The distance across a loaded tire is still a diameter. As soon as you measure the distance a wheel/tire assembly travels in one revolution you are measuring circumference. Link to comment Share on other sites More sharing options...
SC38dls Posted June 7, 2017 Share Posted June 7, 2017 Divide the circumference by 3.14 and you get the diameter! Then you can get the speed using your formula. Hope that helps Link to comment Share on other sites More sharing options...
PFitz Posted June 7, 2017 Share Posted June 7, 2017 48 minutes ago, Tinindian said: The distance from the mark to the mark on the road is still a circumference measurement. It is NOT a diameter. The distance across a tire is a diameter. The distance across a loaded tire is still a diameter. As soon as you measure the distance a wheel/tire assembly travels in one revolution you are measuring circumference. The weight of the car changes the diameter as the tire rolls. That then changes the actual circumference of the tire per revolution. We can pick nits all night about what to call it, but my point is don't use the tire's standing, or unweighted diameter for calculating. Paul Link to comment Share on other sites More sharing options...
Guest Posted June 8, 2017 Share Posted June 8, 2017 Paul, that is exactly what I said in post #10. Just using the marked size on the tire for the calculation does not give you a 100% correct answer. The RPM's verses MPH will vary depending on how many people and what load is on the vehicle. Calculating RPM/MPH is only a general figure. The only accurate way is to have a calibrated speedometer and tachometer. Those dual faced speedos are pretty and an aid to driving in the most general way. Link to comment Share on other sites More sharing options...
padgett Posted June 8, 2017 Share Posted June 8, 2017 I just calculate speeds in gears using a spreadsheet, makes it easy to change ratios and tires. BTW Tirerack specs gives revs per mile for tires and another good source for modeling is here. Scroll down for revs per mile. This takes into account rim width. " The only accurate way is to have a calibrated speedometer and tachometer. " - the GPS in my phone is pretty accurate for steady state. Link to comment Share on other sites More sharing options...
ricosan Posted June 8, 2017 Author Share Posted June 8, 2017 Hey Guys, Sorry I've been awol this past week. Lots happening here. Thanks to all of you for your input. Using woodies formula, I came up with 2570 rpms at 60 mph. My speedometer says 2520 so I think that's close. I was concerned that I might be pushing the engine too hard cruising at 55 - 60 mph. This eases my mind somewhat. Thanks again Link to comment Share on other sites More sharing options...
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