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  #1  
Old 03-23-2005, 12:38 PM
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Join Date: Feb 2005
Location: North Carolina
Vehicle: 1995 Ford F150
Posts: 154
235/75R-15 vs. 265/75R-15 ??

Hey Folks -

Per the manual, I am supposed to be running 235/75R-15's but my '95 has 265/75R-15's.

What is the size difference between these tires? What is the meaning of the first set if numbers? I think the 75R is a width number, and the 15 refers to the wheel size, correct?

Will these tires make any significant difference in the speed as displayed on the speedo? What about the odometer - will the miles traveled be any more/less than what is shown?

Thanks for the info!
John
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1995 F150 XLT Reg Cab Short Bed
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302 V8...with a 4R70W and 3:08 rear end
224,556 miles/Avg MPG=15.3
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  #2  
Old 03-23-2005, 01:16 PM
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Join Date: Sep 2004
Location: Watertown, New York
Vehicle: 2004 ford f-150 4x4
Posts: 87
Sizes

The 235 / 75 15 is 235mm wide or about 9.25 inches across the tread. while the 265 is 265mm wide or about 10.43 inches wide.

The next number refers to the percent of height to width both are at 75 percent. so the first tire would have a total diameter of

235 / 75 15 is about 28.88 diameter

235 x .75 = 176.25 (Two side walls in the dia. so)

176.25 x 2 = 352.5 (then convert to inches) 352.5 / 25.4 = 13.88

then add the rim size 13.88 + 15 = 28.88

while the 265 / 75 15 is about 30.64 inches diameter.

The speedometer will read slower than you are going and the truck will also show less milage than is actually on the truck. You will also have less power than you would with the smaller tires. Putting on a bigger diameter tire has the same effect as running a taller gear ratio.

Hope that this helps.
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  #3  
Old 03-23-2005, 01:34 PM
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Join Date: Feb 2005
Location: North Carolina
Vehicle: 1995 Ford F150
Posts: 154
Yes, this does help. Thanks.

Now let me ask this: I have 30.64" and should be running about 28.88", or roughly 6% larger tires. In relation to miles traveled and speed (and even MPG I guess) can this 6% be correctly applied? In other words, if the odometer says 100 miles traveled, is it really 106 miles. And if I am going 65 MPH, is it really like 68.9 MPH?

And if my OD drops in and out frequently at 65 MPH, if I had the correct tires, then this might correct this problem, if smaller tires = more power?

Thanks for the info.
John
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  #4  
Old 03-23-2005, 04:52 PM
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Join Date: Sep 2004
Location: Watertown, New York
Vehicle: 2004 ford f-150 4x4
Posts: 87
Tires

Yes, Smaller tires will give more power, act like a lower gear ratio.

I have a 4:10 gear ratio in my ford, so I actually want bigger tires to get it closer to a 3:73.

In your case it is not a fixed number, the difference in speed variation will increase with speed. Example if you are reading 4mph wrong at 40, you might be 6 off at 60 you would have to do the math based on the circumferance of the wheels.

The smaller tire has a circumferance of 90.729 inches

The larger tires circumferance is 96.258

Your smaller tire will rotate 698.34 times per mile traveled

The larger tire will rotate 658.23 times per mile.

The truck will run a lower RPM at the same speed with the bigger tires, and can be part of the cause for the shifting especially when coupled with the extra power needed to turn the larger tires.

BLAH BLAH BLAH. There is a lot more, you can also just use the difference between the two sizes to actually determine how far off your speedometer is off at five mile per hour intervals, that will be simple algerbra.

Hope that this helps
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  #5  
Old 03-27-2005, 08:26 PM
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Join Date: Feb 2005
Location: North Carolina
Vehicle: 1995 Ford F150
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A follow-up...

Using my Garmin eTrex GPS-r, I did a comparison of miles traveled as measured by the odometer vs. actual miles traveled as measured by the GPS. Now, this is a small sample, as I only travelled 10 miles by the odometer reading, but here are the results:

Miles traveled odometer.....10
Miles traveled GPS-r............10.5

So on this small sample the odometer is about 5% off from being perfectly accurate. This also translated into the MPH reading, although this was hard to read. But it looks like the speedo is about 2.5 MPH lower than actual speed.

I guess when I change the tires out (they are at about 80% treadware remaining) I will be switching back to the stock 235/75-15's.

Any thoughts from the crew here?
John
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Old 03-27-2005, 08:26 PM


 
 
 
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