doing the math. tire size vs speedo error correction
03-19-2010, 12:18 AM
#32
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Year: 96
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Engine: 2.5l I-4
Anyone take into account the lower rolling resistance of a bigger tire vs. a smaller one? (At the same width) That's, when you're trying to calculate mpg, of course. Maybe then you oughta take bigger mass (inertia) of a bigger tire/wheel into account, also.
Last edited by SHO_91; 03-19-2010 at 02:38 PM.
03-19-2010, 12:29 AM
#33
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Year: 96
Model: Cherokee
Engine: 2.5l I-4
some people have been talking about the gas mileage error that occurs with oversize tires. so, here's the calculation for the people that wanna benefit from it:
WARNING: you need to know basic geometry for this.
first... tire size... cherokee stock tire size according to my door sticker is 215/75 R 15...this means that the thickness of the tire in mm is 215mm. the sidewall is 75% of the width. converting it into inches 215/25.4=8.4645669in.
sidewall is above and under the rim means 150% (75% above +75% below). in other words 1.5 times the width... 8.4645669*1.5=12.69685in.
add the rim diameter to it 12.69685+15=27.69685in.
so the stock cherokee tire height is 27.69685in.
now the circumference of this tire is the ratio of 22/7 to the height (just a rule in math). so it's 27.69685*22/7=87.047244 inches. convert it to feet, that makes 87.047244/12=7.253937
one mile is 5280 feet so in order to travel one mile a tire needs to make 5280/7.253937=727.88059 ROTATIONS.
are you with me so far????
so, say your new tire is 31 inches tall. let's do the math for that... 31*22/7=97.42857 inches. so 97.42857/12=8.11904 feet.
now, if the current tire turns as many rotations as the stock tire it will make 727.88059*8.11904=5909.69717 feet. how many miles: 5909.69717/5280=1.11926 miles.
HENCE, YOUR MARGIN OF ERROR FOR 31 INCH TALL TIRES IS 1.11926 MILES PER EVERY ONE MILE ON THE SPEEDOMETER.
In other words, when your speedometer says you've driven 100 miles, you have actually driven 111.926 miles.
WARNING: you need to know basic geometry for this.
first... tire size... cherokee stock tire size according to my door sticker is 215/75 R 15...this means that the thickness of the tire in mm is 215mm. the sidewall is 75% of the width. converting it into inches 215/25.4=8.4645669in.
sidewall is above and under the rim means 150% (75% above +75% below). in other words 1.5 times the width... 8.4645669*1.5=12.69685in.
add the rim diameter to it 12.69685+15=27.69685in.
so the stock cherokee tire height is 27.69685in.
now the circumference of this tire is the ratio of 22/7 to the height (just a rule in math). so it's 27.69685*22/7=87.047244 inches. convert it to feet, that makes 87.047244/12=7.253937
one mile is 5280 feet so in order to travel one mile a tire needs to make 5280/7.253937=727.88059 ROTATIONS.
are you with me so far????
so, say your new tire is 31 inches tall. let's do the math for that... 31*22/7=97.42857 inches. so 97.42857/12=8.11904 feet.
now, if the current tire turns as many rotations as the stock tire it will make 727.88059*8.11904=5909.69717 feet. how many miles: 5909.69717/5280=1.11926 miles.
HENCE, YOUR MARGIN OF ERROR FOR 31 INCH TALL TIRES IS 1.11926 MILES PER EVERY ONE MILE ON THE SPEEDOMETER.
In other words, when your speedometer says you've driven 100 miles, you have actually driven 111.926 miles.
Last edited by SHO_91; 03-19-2010 at 01:48 AM.
03-19-2010, 12:32 AM
#34
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LOL 84wagoneer beat me to it. Correct me, if I'm wrong, but doesn't the vehicle speed sensor pick up wheel speed? Gearing would then have no bearing on the speedometer being off, but it would change engine speeds (rpm). Like, when you have your car in neutral (tranny disconnected) or disengage the clutch, you still get a speedo readout, as long as the car is moving. The only thing that provides a speedometer signal in that case are the rolling wheels. I bet, you could even lift the front of the car and spin a wheel and get a speedometer readout. I've done it on other cars.
Last edited by SHO_91; 03-19-2010 at 02:36 PM.
01-26-2011, 05:24 AM
#35
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I absolutely loved your explaination! It is a little hard for me to understand the math part of it but I comprehend to overall idea. I did have a question for you. Can I gain MPG's out of my cherokee by going to a taller tire? OR let me put it to you like this, I want more mpg, I want the performance, and I want my speedo to read correctly!
I have 3.55's now with 265/75/16(almost 32)... I know you said 4.10's would make the speedo read correctly with 32's....if I went with a 3.73 or 3.90 could I gain mpg over a stock setting and then corrected the Transfer Case gear?? maybe loose a little torque power over stock, but gain mpg with a soft foot.
If so....what t-case gear with a 3.73 or 3.90 and 32's
Thanks for any suggestions you may have!!!
I have 3.55's now with 265/75/16(almost 32)... I know you said 4.10's would make the speedo read correctly with 32's....if I went with a 3.73 or 3.90 could I gain mpg over a stock setting and then corrected the Transfer Case gear?? maybe loose a little torque power over stock, but gain mpg with a soft foot.
If so....what t-case gear with a 3.73 or 3.90 and 32's
Thanks for any suggestions you may have!!!
Last edited by caspers-flight; 01-26-2011 at 05:28 AM.
01-26-2011, 05:32 AM
#36
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one point just because a tire says 31 dont mean there 31 inchs tall? what about when you change tire brands you might get different sizes just a thing to ponder .lol
01-26-2011, 08:29 AM
#37
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Well, with the math since I have a 2.5L and stock 4.10's I need to go up to 4.53's when i go to 31's, anybody droolin' over getting my gears?
01-26-2011, 08:34 AM
#38
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LOL 84wagoneer beat me to it. Correct me, if I'm wrong, but doesn't the vehicle speed sensor pick up wheel speed? Gearing would then have no bearing on the speedometer being off, but it would change engine speeds (rpm). Like, when you have your car in neutral (tranny disconnected) or disengage the clutch, you still get a speedo readout, as long as the car is moving. The only thing that provides a speedometer signal in that case are the rolling wheels. I bet, you could even lift the front of the car and spin a wheel and get a speedometer readout. I've done it on other cars.
Keep in mind that there is multiple factors for your final drive ratio. And changing anything will effect your spedo or mileage.
01-26-2011, 09:14 AM
#39
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i'm gonna make this REALLY easy for all of you, and some may hate me after putting in the math work, but....
http://www.1010tires.com/TireSizeCal...?action=submit
http://www.1010tires.com/TireSizeCal...?action=submit
01-26-2011, 11:20 AM
#40
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LOL 84wagoneer beat me to it. Correct me, if I'm wrong, but doesn't the vehicle speed sensor pick up wheel speed? Gearing would then have no bearing on the speedometer being off, but it would change engine speeds (rpm). Like, when you have your car in neutral (tranny disconnected) or disengage the clutch, you still get a speedo readout, as long as the car is moving. The only thing that provides a speedometer signal in that case are the rolling wheels. I bet, you could even lift the front of the car and spin a wheel and get a speedometer readout. I've done it on other cars.
01-26-2011, 11:24 AM
#41
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Year: 1986
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AGREED!!!!
The speedo cable is connected to gearing on your transmission, the gearing spins as fast as your DRIVESHAFT spins, which is why with larger tires, some people actually replace the gear on the transmission with a different size to get a more accurate reading. Also why changing your diff alters your total final drive to balance out as well.
Keep in mind that there is multiple factors for your final drive ratio. And changing anything will effect your spedo or mileage.
Keep in mind that there is multiple factors for your final drive ratio. And changing anything will effect your spedo or mileage.
06-18-2024, 05:22 PM
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