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235/75/15 tires and gear ratio/speedo change?

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Old 07-21-2012, 10:56 PM   #1
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235/75/15 tires and gear ratio/speedo change?

If you replace the OEM tires with 235/75/15 tires, how do the gear ratio differences effect the performance of an automatic transmission? How about the speedometer? I see in the tool it shows:



Are these changes accurate in your experience?
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Old 07-21-2012, 11:32 PM   #2
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Yes and no.

The changes are not exact... even two tires of the same size from the same manufacturer but of different models will be different, but the app gives a rough idea of what to expect.

Now...

Your stock speedo is likely optimistic by between 6% and 10%.
Your stock odo is likely optimistic by between 2% and 3%.

The larger tire will slow the indicated speed and compensate for the error.
Of course, you'll want to verify the final result with a GPS.
From looking at the chart, I'd guesstimate that your final result will be very close to actual... maybe just a hair lower than actual (overcompensation). Of course, the odometer will be 3 or 4% lower than actual.


However, the impact on the final drive ratio is very real, so the engine will be under more of a load during acceleration and hill climbing. If you have a V6 it's not a big deal, but you'll notice the power loss on the 2.7
Also, MPG will show two impacts:
1 - Due to the higher engine load, you will have an actual drop in MPG unless your driving is primarily rural highways at 50-60mph.
2 - Due to the "slower" odometer, your calculated MPG will drop by (at least) the same percentage that the odometer changes.

Example:
You currently travel 300 miles and use 12 gallons, you calculate 25mpg.
With the larger tires, you travel the same distance and burn the same 12 gallons (no ACTUAL change in economy), but due to the 6% difference in tire size, your odometer now reads only 282 miles.... you now calculate 23.5mpg

This ignores the real MPG loss from the final drive ratio change, so your actual number may be only 22mpg.
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Old 07-22-2012, 03:58 AM   #3
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With the 30's I run....

Speedo wise - It throws it off by about 3 MPH at the worst and thats at higher (highway) speeds. Actually have had the chance to check with a GPS.

Odo wise - It's thrown off. my ratio is about 1:1.1. 10 miles on the odo is really about 11 mi. 100 miles on the odo is really about 110. I ran 225/75/15 for awhile and the ratio then was about 1:1.05. My truck says I have about 59K on the Odo. I'm over 60K in reality more than likely.

MPG wise - It really depends on how hard you want to throttle it. Even when I've been "heavy" on the gas, I can still regularly get more than 20 MPGs, maybe 21 or so. It's still better than what the sticker said, so I'm cool. Been taking it easy lately and have gotten the Ultragauge to go up a notch or two in the Avg MPG section. I can still easily clear 350 miles to a tank of gas right around the time the gas light comes on.

The Slug - The bitch is a slug. She stills rolls nice when she gets going and even though she's sluggish, there IS a hint of pep still underneath all of it. Just have to learn how to drive.

I would change nothing about what I've done, nothing. The truck looks like a real fucking (little, but real) truck with some real tires on it instead of that 215/70/15 bullshit.
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Old 07-22-2012, 12:09 PM   #4
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Thanks for the information guys, good stuff. We'll see what I'll do....
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Old 07-22-2012, 04:18 PM   #5
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I say go for it!!! I upgraded to the grabbers 215/70/15 to 235/75/15 and like Chris said it looks like a real truck now with the more aggressive tires. Toyota wanted to claim a low mpg do they stuck those wagon wheels on but we got over 2.5' of snow last winter and I'm not worried about bogging down now. I went from a 8 cyl to my new 4 cyl so the way I look at it I'm in the green now matter how I figure it out. Plus I got a manual so I can determine when it feels right for the truck to shift. Go for the bigger tires
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Old 07-22-2012, 04:21 PM   #6
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Plus these tires are technically 28.9" and I've read here that you don't "really" have to regear until 30"-31"
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Old 07-24-2012, 01:15 PM   #7
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If you want your exact gas mileage or odometer reading when running 235/75/15 on base truck multiply the trip miles by .0702 and add that number to the total miles. It throws it off by approximately 7.2% so it says you traveled less distance than you really did. I do not regret going up to 235s, it looks a million times better and you will get used to the power loss.
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