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Catch me if you can (MPG)


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Originally Posted by Rogan68 Before I got the Edge Juice w/Attitude, I noticed that my larger tires vs speedo wasn't ...


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Old 02-09-2012, 03:49 PM   #31 (permalink)
 
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Originally Posted by Rogan68 View Post
Before I got the Edge Juice w/Attitude, I noticed that my larger tires vs speedo wasn't linear.. sub 30-35mph, the speedo was on the money with the gps. as I went faster, the speedo started to lag.
The speedometer is off by a percentage when increasing tire size. A 31-35 will cause the speedometer to be off about %10 which is why it is more noticeable at higher speeds.
I'm just guessing here but to figure out the odometer difference you would use to divide a mile by circumference in ft. And get how many rev. your tire does and divide it by what it did
(31"*pi)/12= 8.11583333 ft/rev
5280/8.11583333= 650.580142 rev/mile
(35"*pi)/12= 9.16 ft/rev
9.162978577 = 576.231839 rev/mile
That's about 12.902% or for ever 1 mile on your odometer, you went 1.12902 miles

Orrr you could just divide new tire size by the old and you can get the difference.....Lol


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Last edited by Stangshcky12; 02-09-2012 at 03:57 PM.
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Old 02-09-2012, 04:25 PM   #32 (permalink)
 
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My stock tire is 245/75-16. I've got 235/85-16s on it now. 4.1% offset.

245/75-16:
30.5" dia.
7.98 ft/rev
662 rev/mi

235/85-16:
31.7" dia
8.29 ft/rev
636 rev/mi


662 / 636 = 1.0408, or 4.1% (rounding)

60mph + 4.1% = 62.46, or 62.5mph.


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Old 02-09-2012, 09:22 PM   #33 (permalink)
 
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So how do you explain that I'm currently running a stock sized tire 265/75 R16 and my speedmeter is off by 0.5 MPH and total trip difference for a tank of fuel can vary 20-30 miles? GPS and ScanGauge II are right on but the odometer is off???

As for the cetane grid I got it from a ASTM document... I had a connection with a guy that allowed me to grab it... But this is all I got left of that document... I lost the document in one of my hard drive crashes.


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Old 02-09-2012, 09:55 PM   #34 (permalink)
 
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I might have to go to the library this weekend and see what I can find there as far as ASTM standards on this.

How many miles a tank are you getting? The speedo/odometer gears in your truck might be for 245/75/16 (everywhere list that as the stock size, although I'm not doubting that 265/75/16 may have been an option) so if your using 25-30 gallons a tank at 19mpg and your odometer is off 3.73% your looking at 17.7 to 21.261 gallons a tank.
This is of course all nominal numbers and we know tires are not always ideal size and average mpg is ever changing.

As far as the speedometer, speedometers suck lol I dont really have much of an idea there. Ive driven countless vehicles from my cousins car dealerships and the speedometer was typically 3-5 mph faster then what the GPS read (which of course can be up to 1 mph off (older ones) newer ones seem to be .3-.4 mph off since they use several signals, not just four like old ones)


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Old 02-09-2012, 10:05 PM   #35 (permalink)
 
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Thank you... That's what I want someone else to voice right there... So if your looking for true number you got a use a device calibrated for your application. Hence my setup of the SGII and GPS.

I know know that MPG's is just averaged numbers using the best information you can provide for the math. So if the information is skewed then so is the results.

As for the tires that would be a 2.3 MPH change so I'm right on the tires just the stock speedo/odo is calibrated poorly.


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