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Calculating speedometer error

How does tire size effect your speedometer?

Calculating speedometer error

Postby EKinMN [OP] » Feb 04 2012, 4:00pm

I saw StarsnBars thread about a Speedometer recalabration Tool, and it got me thinking...

So lets say that a guy does not recalibrate his truck after a tire size change. If I go from a 265/65/18 (OEM) to a 285/65/18, my tire diameter goes from 31.5" to 32.6". Is there a way to easily calculate speedometer error? Or, how about the new effective gear ratio (I currently have a 3.42 axle)? I am in the same boat as StarsnBars, in only wanting to be able to change the speedo when I upgrade. I don't need a tuner. But, if the difference is not substantial, I will just live with the speedo error.

Thinking out loud, here. The difference in diameter is 1.1". 1.1" is about a 3.49% increase in total diameter. Is it fair to say that at 60MPH, my tires are turning 3.49% slower, and thus my speedo should read 57.9MPH? If that is accurate, I can probably just live with the slight change without a recalibration.

Is it the same for the axle ratio? At a 3.49% change in tire size, is my new effective gear ratio 3.30? Again, I could probably live with that, since these trucks seem to run pretty well on wither the 3.08 or 3.42 axle.

Also, as FYI, the tires I am comparing have a significant weight difference. 44lbs vs. 56lbs. So I already can imagine what this would do to MPG. Not that that is a factor in these calculations, but I do know what I am getting myself into. ;)

What am I missing?
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Re: Calculating speedometer error

Postby scottja » Feb 04 2012, 4:18pm

Hang a GPS on the windshield and comparethe speed it say's your going to the speedometer and see what you got. I went from a 245/70/17 up to 265/70/17 on my truck about a 2 inch difference in height. with the 245 the gps was accurate with the speedo with the 265 the gps showed 2 mph more than the speedo.
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Re: Calculating speedometer error

Postby EKinMN [OP] » Feb 04 2012, 4:21pm

Good call! I should have thought of that! :oops: Well, I guess that means that my math is at least directionally accurate.

But -- I am just wondering if there is a way to calculate it before I make the purchase of the larger tires.....
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Re: Calculating speedometer error

Postby O'Brien » Feb 04 2012, 7:16pm

Check out this site. You can compare tire sizes from original and select brands as well.

http://www.1010tires.com/tiresizecalculator.asp
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Re: Calculating speedometer error

Postby EKinMN [OP] » Feb 04 2012, 7:34pm

That was very helpful! Looks like my math was only off a little bit. It's about a 3% change, which honestly, I can live with. I am not buying a programmer just to adjust a couple percentage points.
Sort of unrelated, I've noticed that my current tire size is a "P" metric tire. Most of the 285's are an LT. Just one more thing to consider as I shop for tires. I still have lots of time. Won't need them until next winter...
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Re: Calculating speedometer error

Postby StarsnBars » Feb 05 2012, 3:21am

Good post Eric. I went from a 31.5 stock tire to a 32.5 tire to a 34.5 tire. 72 lbs of rolling rubber per tire on the 34.5's. :) Yikes. Not looking back though. They are way sweet. :D . When I had the 32.5's before these I really couldnt feel much difference. But the new 34.5's have made a noticeable difference in tranny, mileage, and speedometer. Could probably live with it to, but I haul a pretty hefty camping trailer and want to get shift points somewhat close again. Really leaning towards the Diablo. Alittle extra but it sounds like the best bet.
Nice link O'Brien. This forum rocks. I would of never found that..
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Re: Calculating speedometer error

Postby SMOKE » Feb 05 2012, 3:37am

Hey guys I think the main thing is getting your transmission shifting proper;y to prevent excessive wear and heat build up. Its not good to be ina higher gear at a low speed. The minor diff. you have probably isnt a big deal but Stars you really should find a way to re-cal things. I just went through all this when I did my gear swap. I was off by 13mph and the truck was in 6th at 40. If this doesn't sound right you guys can throw stones ... lol
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Re: Calculating speedometer error

Postby EKinMN [OP] » Feb 05 2012, 3:05pm

StarsnBars,
I am happy to hear that you could not feel much difference in the 32.5's. I am leaning that way. I just want to fill that wheel well just a bit more (and use a leveling kit, of course). I want to do that without significantly altering the performance of the truck. Sounds like I can get away with that using a 285 tire.

Thanks gang.
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Re: Calculating speedometer error

Postby starspangled6.0 » Feb 05 2012, 3:10pm

Of course, a CAI or tuner would help negate any performance losses ;)...
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Re: Calculating speedometer error

Postby SMOKE » Feb 05 2012, 5:15pm

EK, I'm running the 285/70/17 with a 5.3 and 3.08's and we couldn't tell a.difference even after recalibration. No evident power loss or diff. in mpg.
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