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Re: My Mercury Pickup Truck

Posted: Sun Apr 16, 2017 1:01 pm
by theholycow
InlinePaul wrote:Additionally I read somewhere that the auto companies like to make the speedometer biased to read higher than actual speed to avoid liability.
As described by Rope-Pusher, when speedometers were mechanically driven they couldn't be adjusted perfectly; you change out the speedometer cable drive/driven gear set for different ratios, but often you couldn't hit the exact ratio for perfection. It would be a disaster for people to get speeding tickets if the manufacturer chose the ratio to err in that direction, so of course they would err in the other direction.

Your Ranger has an electronically signaled speedometer, with either infinite or at least much finer adjustments. It was set accurately at the factory for whichever size tire came on the truck, neither fast nor slow.

I've verified many speedometers with redundant GPS phones/devices (as well as "Your speed is: " roadside radar displays, mile markers, and RI's famous "Measured Mile" on the highway) and found them all as close to perfect as I can observe.

Re: My Mercury Pickup Truck

Posted: Sun Apr 16, 2017 1:06 pm
by theholycow
Rope-Pusher wrote:Tire revolutions per mile follow the trend of tire circumference, but not directly. For the same circumference, different tire brands will have different revs per mile due to tire construction differences. Still, calculating the circumference gets you into the ballpark (on opening day).
You are, of course, talking about circumference calculated using the tire's size...actual circumference (mount on vehicle, inflate to normal, put a thin line of paint on and drive until the wet paint leaves two lines on the pavement...or just wrap a tape measure) would be a different story. That is, unless I'm missing something about differences between constructions/materials that differently affect high-speed hysteresis, thermal expansion, or something like that. It would be crazy to try for that kind of precision though!

Re: My Mercury Pickup Truck

Posted: Sun Apr 16, 2017 2:21 pm
by Rope-Pusher
theholycow wrote:
Rope-Pusher wrote:Tire revolutions per mile follow the trend of tire circumference, but not directly. For the same circumference, different tire brands will have different revs per mile due to tire construction differences. Still, calculating the circumference gets you into the ballpark (on opening day).
You are, of course, talking about circumference calculated using the tire's size...actual circumference (mount on vehicle, inflate to normal, put a thin line of paint on and drive until the wet paint leaves two lines on the pavement...or just wrap a tape measure) would be a different story. That is, unless I'm missing something about differences between constructions/materials that differently affect high-speed hysteresis, thermal expansion, or something like that. It would be crazy to try for that kind of precision though!
Happy Easter Cow! Aren't you glad the Ham is more the traditional meat at Easter than the Beef?

Speakin' of beefs (beeves?), yes, even using the actual measured circumference, a fudge factor is required to determine the actual Revs per Mile....and even then it will vary some depending on the amount of weight on the axle, the rim width, and whether it is a driven or non-driven wheel. One, or two might even find some difference due to the difference in road-load power. Maybe towing a trailer, or driving a big, blunt SUV might produce a different revs per mile than with the same tire mounted on a slippery sedan.

At what point would this difference be discernable? HaH! with proper instrumentation one, or two might measure the difference, but considering that a typical revs per mile might be in the 800's, even 8 revs per mile difference is only 1%.

Did I ever mention that I spent a summer at GM Tire and Wheel Engineering and four of the projects I worked on were related to revs per mile. One was in developing a more accurate formula for calculating Revs per Mile on "normal" tires, another was to study revs per mile on compact spare tires (they are different enough to need their own revs per mile prediction formula), the third was to look at the speed sensitivity of revs per mile (apparently, in the days of bias-belt tire construction, there was a significant difference at say 25 mph ("Twenty-Five Miles per Gallon") vs highway speeds.), and the forth was load sensitivity of compact spare tires for Revs per Mile.

Re: My Mercury Pickup Truck

Posted: Sun Apr 16, 2017 4:54 pm
by ClutchFork
Yep. I forgot the working radius of the tire is less than the stated size because of the squat factor. I remember putting a full load of wet elm wood in my 1984 F150 (econo truck with highway gears and soft springs). The differential looked to be only a few inches off the ground and the tires were very distorted at the ground. Must have taken an inch off the radius.

However, even with the squat, the tread does have it's circumference, so I think the true working radius is somewhere between the squat radius and the calculated radius.

OTOH, I might just be full of beans! :lol:

Re: My Mercury Pickup Truck

Posted: Sun Apr 16, 2017 6:07 pm
by Rope-Pusher
InlinePaul wrote: OTOH, I might just be full of beans! :lol:
I an't payin' one thin dime for a free bunch of holaes.


So, some guy from another GM division calls us up and asks for the rolling radius of the tires used on a cop-car. We had to ask him if he wanted to know the revs per mile or if he wanted to know the ride height. It ended up he wanted to know the revs per mile, so if we had given him the rolling radius (distance from center of axle to the ground) and he had calculated a theoretical revs per mile from that it would have not been as accurate as using our measured revs pre mile data.

Re: My Mercury Pickup Truck

Posted: Mon Apr 17, 2017 12:49 am
by ClutchFork
Rope-Pusher wrote:
InlinePaul wrote: OTOH, I might just be full of beans! :lol:
I an't payin' one thin dime for a free bunch of holaes.
What if I pay you to take them? The honey wagon will come by and do just that.

Re: My Mercury Pickup Truck

Posted: Fri Jul 23, 2021 10:03 pm
by ClutchFork
theholycow wrote: Sun Apr 16, 2017 1:01 pm
InlinePaul wrote:Additionally I read somewhere that the auto companies like to make the speedometer biased to read higher than actual speed to avoid liability.
As described by Rope-Pusher, when speedometers were mechanically driven they couldn't be adjusted perfectly; you change out the speedometer cable drive/driven gear set for different ratios, but often you couldn't hit the exact ratio for perfection. It would be a disaster for people to get speeding tickets if the manufacturer chose the ratio to err in that direction, so of course they would err in the other direction.

Your Ranger has an electronically signaled speedometer, with either infinite or at least much finer adjustments. It was set accurately at the factory for whichever size tire came on the truck, neither fast nor slow.

I've verified many speedometers with redundant GPS phones/devices (as well as "Your speed is: " roadside radar displays, mile markers, and RI's famous "Measured Mile" on the highway) and found them all as close to perfect as I can observe.
I once had a tranny rebuild in a '71 Ford. Something went wrong in the speedo gear department because then the car would read 55 mph but be going 70. A buddy and I headed for Colorado and we got pulled over doing 55 (in the days of the national 55 limit) and the cop said he clocked us at 70. We knew it was off but figured we had a good excuse, but the cop was not buying it from a couple of long-hairs. He actually made us follow him back 30 miles east to the last town to deposit a cash fine in an envelope into a dropbox.

One way to evaluate the odometer on a vehicle, and I suppose the speedometer too, is to compare with the mile markers on the highwayside. I would go for a longer distance, at least 10 miles or more, to minimize error from markers that maybe are not precisely located. Its a tough and dirty ballpark method for those of us who don't have GPS.

Re: My Mercury Pickup Truck

Posted: Sat Jul 24, 2021 12:15 pm
by IMBoring25
We always used section line road to section line road, as those were apparently laid out more precisely than the highway mile markers. Not all parts of the country are laid out in a grid, though.