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Old 01-22-2009, 01:33 PM   #1
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Tire Size compared to MPG

I have a 99 4runner that came stock with the 265/70/16. I now have 285/75/16 and I've been trying to calculate my MPG but I know that my speedo is slightly skewed. I was wondering if someone could point me to a prior post regarding this issue or if someone could just help me figure it out.

Here's what I've come up with so far

265/70/16= 30.6

285/75/16= 32.8

A difference of 2.2 in diameter.

That's a 7% difference can I assume that my speedo is essentially off by 7%. So if I travel 200 miles I actually traveled 200*1.07= 214???

Not sure if I'm totally off but I would appreciate it if someone could clear this up for me.
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Old 01-22-2009, 01:40 PM   #2
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Yup, that's pretty much how you do it. But, You are of course assuming your speedo is correct with 265x70r16's on it, which I'm not sure about. My 4Runner's speedo is off by about 4% with 285x70r17's, which means the speedo probably read a bit fast with 265x70r16's on it.
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Old 01-22-2009, 01:56 PM   #3
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The speedometer and odometer take their signal from different sensors. The speedo gets it from the ABS system. The odo gets it from the output shaft of the transmission.

I did a test with 265/75/16 and GPS. My speedo was dead-on with the larger tires. However, I determined that the odometer was off by 3.5 percent, whereas previously it was dead-on with the stock tires.

With 285/75 tires, both the speedo and odo will be off. Your assumption of 7 percent on the odometer is reasonable, but could be checked with a GPS unit.

Hope this helps.

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Old 01-22-2009, 02:02 PM   #4
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mastacox I didn't even think about it being really that much off while I had everything stock


Quote:
Originally Posted by MadCityRich View Post
The speedometer and odometer take their signal from different sensors. The speedo gets it from the ABS system. The odo gets it from the output shaft of the transmission.

I did this test with 265/75/16. My speedo was dead-on with the larger tires. However, I determined that the odometer was off by 3.5 percent, whereas previously it was dead-on with the stock tires.

With 285/75 tires, both the speedo and odo will be off. Your assumption of 7 percent on the odometer is reasonable, but could be checked with a GPS unit.

Hope this helps.

MadCityRich
I'm trying to get a hold of somebody with a GPS unit to confirm my question for sure. But I was trying to do some calculations and see how close I come.
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Old 01-22-2009, 06:40 PM   #5
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http://www.discounttire.com/dtcs/infoTireMath.dos
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Old 01-22-2009, 07:49 PM   #6
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What I did was drive on the hwy and look for the mile marker signs, and then see what your trip meter does in between mile marker signs. If you check this a few times in different places you should see how far off your trip meter is and you can figure your mpg out based off of that error.
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Old 01-22-2009, 08:39 PM   #7
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On non abs trucks it takes it from the tcase, and you can change this on abs rigs so it takes it from the tcase.
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Old 01-23-2009, 06:26 AM   #8
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Quote:
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Thanks for the link! It looks like my math was right after all
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Old 01-25-2009, 10:28 AM   #9
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I've used internet mapping sites (google, yahoo, etc.) to track mileage across an entire tank of gas tracked against my odometer, and arrived at a very consistent differential which I then calculated as a percentage. From that point forward, I was able to consistently calculate the percentage into mileage on the odometer whenever I'd calculate my gas mileage.

Just as with some other methods here, there is a built in assumption here that mapping sites are accurate. Whatever works for you is best, I suppose.
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Old 01-25-2009, 04:53 PM   #10
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According to the police that pulled me over my speedo is off about 3mph higher at over 70mph but the slower you go the more back to accurate it is even with the 265/75s I have. Under 50mph there really isn't a difference.
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Old 01-27-2009, 08:30 AM   #11
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With my 285/75/16's my speedo is off by about 4-5 kilometers per hour (2.5-3.1 miles), I verified this yesterday with my GPS. BTW my milage is horrible, I get 180-200 miles per tank!!, I have no idea how others are getting 300+ miles per tank???, I think i'll swap in a V8 for better fuel milage
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Old 01-27-2009, 08:38 AM   #12
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Hmm, well since I dont have ABS then that means my speedo and odo come from the T-case? Interesting. I dont have a GPS but it seems that now with a true 35" tire and 5.29 gears my ratios are close to stock. Anyone know if that is true? Or how would one calculate the ratio between stock gears and tires and what i run now?
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Old 01-27-2009, 09:10 AM   #13
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Quote:
Originally Posted by BIC performance View Post
With my 285/75/16's my speedo is off by about 4-5 kilometers per hour (2.5-3.1 miles), I verified this yesterday with my GPS. BTW my milage is horrible, I get 180-200 miles per tank!!, I have no idea how others are getting 300+ miles per tank???, I think i'll swap in a V8 for better fuel milage
during harsh cold consecutive days and cold starts I get anywhere from 180-200 miles out of a tank and this is before any calculations for tire size. But on a decent warm day I can usually squeeze about 230-240 from a tank. well see what happens in summer. I've said it before as well I have no idea how people are claiming 300+ miles on a v6?

Also I would love love to do a lexus V8 swap!
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Old 01-27-2009, 09:15 AM   #14
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Quote:
Originally Posted by BIC performance View Post
With my 285/75/16's my speedo is off by about 4-5 kilometers per hour (2.5-3.1 miles), I verified this yesterday with my GPS. BTW my milage is horrible, I get 180-200 miles per tank!!, I have no idea how others are getting 300+ miles per tank???, I think i'll swap in a V8 for better fuel milage
Where you drive makes a large difference as well. I noticed my city mpg drop considerably upon moving to Vancouver from a city with less frequent stop and go driving. I had a chance to hit the highway the other day and was relieved to see my mpg return to normal.
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Old 01-27-2009, 09:17 AM   #15
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I dont see how that is possible either. Heck, I have a 2.7L with 5speed and best I ever got before the low fuel light came on was 220 miles and that was on a 700 mile trip. Of course the 35's dont help much.


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during harsh cold consecutive days and cold starts I get anywhere from 180-200 miles out of a tank and this is before any calculations for tire size. But on a decent warm day I can usually squeeze about 230-240 from a tank. well see what happens in summer. I've said it before as well I have no idea how people are claiming 300+ miles on a v6?

Also I would love love to do a lexus V8 swap!
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Old 01-27-2009, 09:17 AM
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