The Down Range Forum
Member Section => Down Range Cafe => Topic started by: billt on October 12, 2016, 09:35:47 AM
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Here's the deal. I need some help with the math involved with this. My truck came equipped with P-235 X 15 tires from the factory. I don't know the rolling circumference of that tire. After they were worn, (around 22,000 miles), I replaced them with 31 X 10.5 X 15 tires. Obviously these tires have a larger rolling circumference that has effected both odometer and speedometer accuracy. I have had that size tire on the truck ever since.
My truck now has 152,000 miles on it. Which means for the last 130,000 miles it has registered fewer miles, and a lower indicated MPH because of the larger diameter tires. Is there a way to calculate the difference, so I know with some degree of accuracy, how many actual miles my truck has on it?
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Tire manufacturer's have spec sheets on their tires giving the full dimensions of inflated tires. You can easily adjust for wear by deducting the amount of tread depth lost. Following that it is simple math to come up with percentage of change + or -. Be sure and use the actual manufacturer data, and not generic figures for most accurate figures.
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Ok...I guess we can assume your speedometer was calibrated to mark a mile driven by counting wheel rotations based on a "standard" or "assumed" wheel radius (which is used to calculate circumference)
Find out what that assumed/standard circumference and we can find out how many times the wheel rotates per mile. Or maybe the calibrated wheel rotations per mile is documented someplace.
With that, we can determine how many times your wheel rotated to get your current mileage and convert that to the miles of the old tires...
BUT we would need the radius (you can measure that with a ruler easily) of the old tires, both when they were new and when replaced...since that radius will change and an approximate average radius can be calculated.
With out those numbers, we can't do much to determine what the actual mileage is
This is all theory...I've never done it so have never verified it
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Since you have already changed the tires out you can't use the old General Motors standard calibration method:
On a long straight section of smooth road, put a chalk mark on the tire and the road, drive the car forward a minimum of ten tire rotations (the more the better) and stop with the mark at the exact 6:00 position, mark the road, measure the exact distance between the marks, and divide by the number of rotations. We did this to calibrate speedometers back when they had gears in the transmission. We could change out the driven gear, and there were several choices.
One of our mechanics was good enough that he could go down the road with his watch and the mile markers to get close before we started the exact. He would go out on the highway, ten minutes later the car was on the lift, he'd come in with the old gear and tell me he needed XXXXX (color designation for the different gears), and head back out. He knew the tooth counts by memory and could usually nail it in one or two tries.
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Mathematically..
You didn't mention "aspect ratio" so figuring a 235/75/15 tire is approximately Ø29" or about 7% smaller than the 31" tire, you can use that as your percentage of mileage calc.
130,000 x .93 = 120,900 add the 22K for the total.
Other than that, does it really matter? :)
edit.. the average beginning depth of a tire tread is between 11/32 and 13/32 and won't really have an appreciable effect on distance traveled over the life of the tread. Most states consider a tire worn out at 3/32 or about .093"..
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I don't want to think about the math right now, but I do have a question. If I understand correctly, you put a larger tire on, and your speedometer now says you're going slower than you really are? My truck basically has the metric version of 35" tires. My speedometer says I'm going somewhere between 2 & 4 mph faster (depends on speed) than I really am. That has been my experience with all larger sized tires.
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Have a cop clock you with his radar gun , f@#k the math. ;D
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Here's a question for you guys and gal..
My wife burned 30K of rubber off her tire one winter and never moved an inch...
How?
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Have a cop clock you with his radar gun , f@#k the math. ;D
Or, just drive around. Our locals have radar set up in school zones, construction, and just for the fun of it. Most of them match ... Damn! Just look at your navigation system ... If you have one. That's all I use for speed anymore.
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If Bill wants to determine an accurate as possible calculation for h is trucks actual mileage, he is gonna need a lot more info than an accurate speed reading for his current tires.
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Short answer, yes, easy, just need to figure out the delta in circumference. Reality, give how tire size. Hanges dramatically with wear, presure and tempure. Then there is the whole accuracy of the system itself. Its not really possible to get an accurate number.
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Sure you can calculate like has been described. I had those tires on an older Suburban. If I recall correctly the difference I found was 7% (grain of salt here). I took the mileage that the tires had been on the vehicle, and added that amount. Not that anyone cared, I just got to thinking one day... :D
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I don't want to think about the math right now, but I do have a question. If I understand correctly, you put a larger tire on, and your speedometer now says you're going slower than you really are? My truck basically has the metric version of 35" tires. My speedometer says I'm going somewhere between 2 & 4 mph faster (depends on speed) than I really am. That has been my experience with all larger sized tires.
I'll clarify. The larger the tire, the slower your speedo will read compared to your actual speed. (You'll actually be traveling faster than indicated). To help visualize, think the difference in rotational speed with monster truck tires going 60 MPH, compared to tires on a Toyota Camry. Right now if my speedo say's I'm going 60 MPH, I'm actually traveling faster.
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I'll clarify. The larger the tire, the slower your speedo will read compared to your actual speed. (You'll actually be traveling faster than indicated). To help visualize, think the difference in rotational speed with monster truck tires going 60 MPH, compared to tires on a Toyota Camry. Right now if my speedo say's I'm going 60 MPH, I'm actually traveling faster.
I believe yours is doing that because I have no reason to doubt you. I just find it strange because mine does just the opposite. With larger than factory tires, my speedometer says I'm going faster than I am. If it says I'm going 70 mph, I'm really going about 66 mph.
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Let's go back to the start here. Why do you care how many "actual" miles? For sale purpose you just report odometer. That is what is asked for, and if you stray from that you open a can of worms. For maintenance, etc., it is too minor to worry about.
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New cars and trucks, my 03 Ranger for example, have a speed sensor in the differential and compensates for tire diameter thru the control module. If the module has not been programmed to the new tire size it will skew the numbers.
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Let's go back to the start here. Why do you care how many "actual" miles? For sale purpose you just report odometer. That is what is asked for, and if you stray from that you open a can of worms. For maintenance, etc., it is too minor to worry about.
Curiosity mostly. I'll never sell the truck because now that I'm retired I don't drive enough to justify buying a new one. I've had my 2015 Jeep Grand Cherokee for a year and a half now, and I just turned 3,800 miles. And over 500 of that was a trip to Lake Havasu City, Kingman, and back. Barring theft or a collision, I won't be buying any more new cars.
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I use a GPS function. First with a handheld, now with my phone. You can compare mph at various speeds and if you have an accurate known track, maybe mile markers, you can compare distance.
In both Pathfinders it tells me that my odometer is reading about 3mph higher at 65mph, than my actual speed. Given the percentage difference, I can probably apply that to the mileage as well.
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You would think by now, the way cars have advanced, they would have a speedometer adjustment on them for different size tires. Aftermarket tires are so popular today.
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You would think by now, the way cars have advanced, they would have a speedometer adjustment on them for different size tires. Aftermarket tires are so popular today.
That is why I wonder if it was set for the specific tires ordered with the car or for an "average" of the available options
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My new Jeep has the U-Connect 8.4" computer screen. You can look up anything, from the price of gas in your area, to the best rated pizza joints. And most anything in between. It wouldn't be hard at all for them to have a page of all the different tire sizes that will clear the fender wells of a Jeep Grand Cherokee.
All you would have to do is touch the correct tire size you wish to install, then touch "APPLY", and the speedometer along with the odometer would instantly calculate the change. They could even factor in tire pressure, because it also displays the current tire pressure for each wheel in real time. I can watch my tire pressure rise as they heat up, from when I pull out of the garage on cold tires, as I drive on the freeway at 70 MPH.
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problem with owner adjustment of of the speedometer settings is it makes it easy and impossible to detect fraudulent mileage changes.
On an analog speedometer, bars would appear between the numbers if it was wound back...not sure if there is protection on modern digital ones anyway?
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I had to change some indicator lights on my dash some months back. The speedo, tach, fuel, oil pressure and amp gauges were all modular. I could have replaced any and all of them including the odometer.
Changing the mileage ain't that tough...at least on my beater truck...(http://uploads.tapatalk-cdn.com/20161013/2f842999e4d728bdd4a5b5dbd8fda34b.jpg)
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I had to change some indicator lights on my dash some months back. The speedo, tach, fuel, oil pressure and amp gauges were all modular. I could have replaced any and all of them including the odometer.
Changing the mileage ain't that tough...at least on my beater truck...(http://uploads.tapatalk-cdn.com/20161013/2f842999e4d728bdd4a5b5dbd8fda34b.jpg)
They let you drive that in Massachusetts? I figured you would either get thrown in jail or hauled to the state line. [emoji3]
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They let you drive that in Massachusetts? I figured you would either get thrown in jail or hauled to the state line. [emoji3]
LOL
No, they wouldn't let me drive that... Wifey's truck, she blew a rear brake line last week, thank God she was in a parking lot. It's easier for this old fart to remove the bed to access the line, do the shocks, cut off and replace the shackle mounts, shackles and do the back brakes. Might last me another 10 years that way. I did pretty much the whole front end back in April. It's only got 106K on a near 14 y/o feather hauler...
Gonna grind a bit o ferrous oxide off the chassis and put some Krylon on there as well..
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LOL
Gonna grind a bit o ferrous oxide off the chassis and put some Krylon on there as well..
Spray some Phoz (phosphoric acid solution) on first. That alters the surface rust and makes it an almost impermeable surface. Plus, it accepts paint and enhances adhesion. Should find it at Lowe's or Homer's.
FWIW
Crusader Rabbit
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Thanks Mr Rabbit! I'll look into that!
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You would think by now, the way cars have advanced, they would have a speedometer adjustment on them for different size tires. Aftermarket tires are so popular today.
I did change mine. When my truck was new, I bought a Hypertech Power programmer and one of the functions was a speedo calibration to match the larger tires that came as an option.
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Spray some Phoz (phosphoric acid solution) on first. That alters the surface rust and makes it an almost impermeable surface. Plus, it accepts paint and enhances adhesion. Should find it at Lowe's or Homer's.
FWIW
Crusader Rabbit
+1 to what Brother Rabbit said......
Also, for a few more bucks, look into painting it with Por-15. The frame will be basically impervious to salt and rust, and you'll not have to do it again (just don't coat threads with it ;D ).
http://www.por15.com/
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Is all that compatible with a plastic fuel tank and stock fuel lines?
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problem with owner adjustment of of the speedometer settings is it makes it easy and impossible to detect fraudulent mileage changes.
The way my truck is now, it's showing fewer miles than it actually has. Just by me running larger tires on it for the last 24 years. I've never touched the odometer.
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Is all that compatible with a plastic fuel tank and stock fuel lines?
I don't know for sure, but I'm fairly sure the paint would be OK. Most folks brush it on instead of spraying, so you could avoid contact with things you don't need coated. The stuff CR was talking about come in a plastic spray bottle (at least what I have used in the past has.....some may brush on) so if you are careful, you could keep it off certain areas.
It can be used on a variety of surfaces, from styrofoam to wood to concrete...I would think it would be OK on plastic, but wouldn't hurt to shoot 'em an email to verify.
My friend used to do complete frame-off restorations of 60's era British Land Rovers and he used the Por15 to coat the frames and suspension parts.
http://www.por15.com/POR-15-Technical-Information_ep_62.html
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Thanks Peg but I went with the "I'm lazy" version and knocked off the big chunks and sprayed it down with some anti-rust stuff...
It's still a pretty solid chassis despite it's length in the salt capital of the east..
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Thanks Peg but I went with the "I'm lazy" version and knocked off the big chunks and sprayed it down with some anti-rust stuff...
It's still a pretty solid chassis despite it's length in the salt capital of the east..