can I fix my speedo and tranny shift and odometer? - Ford Powerstroke Diesel Forum

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post #1 of 9 Old 01-28-2013, 11:44 AM Thread Starter
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can I fix my speedo and tranny shift and odometer?

Tire size increase?

I was at the ford dealer, and they plugged their computer into the little port and there was an option for tires. he put in 275/65/20 which is the same size as my 35's and the speedometer is still off. Makes me think that the odometer is off too as well as shifting and everything else. can I fix all this?

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post #2 of 9 Old 01-28-2013, 05:14 PM
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275/65-20 should be about 34". That's probably the issue.

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post #3 of 9 Old 01-29-2013, 02:22 AM
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Quote:
Originally Posted by cjackermann View Post
Tire size increase?

I was at the ford dealer, and they plugged their computer into the little port and there was an option for tires. he put in 275/65/20 which is the same size as my 35's and the speedometer is still off. Makes me think that the odometer is off too as well as shifting and everything else. can I fix all this?
275/25.4= 10.83 x1.3= 14.08+20 = 34.08"

34.08x3.14=107"
35.00x3.14=109.9"

Your speedo should only be off about 2.5% off. What are you seeing?

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post #4 of 9 Old 01-29-2013, 02:26 AM
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Your odometer should be off 2.5% too. I don't know why you are worried about tranny shifts. Millions of people run a much larger difference in tire sizes without a problem. When you go up 4-5 inches or more in tire diameter, is when some people start re-gearing there trucks. I went from 32.8 to 34.5" tires and never had any issues...
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post #5 of 9 Old 01-29-2013, 04:44 AM Thread Starter
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according to my GPS my speedometer says 65 and I am doing 70. when it says 70 I am doing 76

Just thought it had a negative effect on everything about the truck if it was not correct. Are you saying that I can just leave it like that?

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post #6 of 9 Old 01-29-2013, 09:03 PM
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Originally Posted by cjackermann View Post
according to my GPS my speedometer says 65 and I am doing 70. when it says 70 I am doing 76

Just thought it had a negative effect on everything about the truck if it was not correct. Are you saying that I can just leave it like that?
Thats 7.5% off... wonder why it's that far off? I would not be worried about it at all.

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post #7 of 9 Old 01-30-2013, 03:17 AM
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according to my GPS my speedometer says 65 and I am doing 70. when it says 70 I am doing 76
?
You just need to have it programmed for a bigger tire size, or whatever is closest to the actual measurement of your installed tires. This may involve some trial and error to get any accuracy. It is a known condition that the rated tire size (as stamped on the sidewall) and the actual measurements of the tire are not the same. You can go to the Toyo or Nitto websites for examples of what I am talking about (i.e. a Toyo 35x12.50-20 ATII is not the same size as a 35x12.50-20 MT. The ATII is smaller.)

Good hunting!

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post #8 of 9 Old 01-31-2013, 06:35 PM
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Quote:
Originally Posted by cjackermann View Post
according to my GPS my speedometer says 65 and I am doing 70. when it says 70 I am doing 76

Just thought it had a negative effect on everything about the truck if it was not correct. Are you saying that I can just leave it like that?
Whatever was done to your speedo, it wasn't done right. Your speedo is reading as if NOTHING was done.
70mph/76mph = .921 for a 7.9% rate of change or error rate
31.5 OEM tire/34.08 current tire = .924 for a 7.5% rate of change or error rate

The numbers don't lie, but the dealership sure did.

I don't know about your 2003, but mine shifts the same whether it's on 31.5s or 35s. When I say the same, I mean at the same INDICATED speed. 45 mph indicated, light pedal pressure, converter locks. However, on my 315s I have a GPS speed of 49 when it locks.

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Last edited by Karls03; 01-31-2013 at 06:42 PM.
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post #9 of 9 Old 02-01-2013, 10:53 AM
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I did some more math for you (tire/gear/rpm/speed/336) and assuming that you have a 3.73 gear, here's what you should see on your tachometer at 70 mph INDICATED.

Uncorrected speedo: 1979 let's call it 2000 rpm for argument's sake.
Corrected speedo : 1829 this would be just above the small line below "2" on your tach.

Because your GPS shows more than your speedo, I'm thinking that the first one is what you are seeing. SCT told me a few months ago that while the 2005 and up was reprogrammable through their unit (OBDII port interface), but the 2004 and below speedos read through the ABS and are therefore harder to recalibrate.
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