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post #1 of 10 (permalink) Old 08-11-2012, 04:48 AM Thread Starter
 
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Speedo/Odo Question

After my wheel & tire upgrade I had the dealer reprogram the computer for the new size but I've since figured out that my speedo reads +/- 4 KM/H higher than my actual speed.

I'm having trouble figuring out if this will make my odometer run faster or slower than it should; it should run faster, correct?
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post #2 of 10 (permalink) Old 08-11-2012, 06:31 AM
 
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I see a similar effect when I run my winter tires, which are slightly smaller than the stock summer tires.

I used one of those online calculators to crunch the numbers, and with the smaller tires, when the speedo reads 100 km/hr, actual speed is 96 km/hr, but I've never actually compared the odometer itself to check how it's measuring the distance comparatively.

Depends on where the odometer gets its information I suppose, whether it's dependent on a signal from the speedo, or independent.

In the old days, you had a speedo cable that fed both, but with computers and such these days I have no idea how that information makes it way onto the instrument cluster.

Isn't that why the dealer re-programmed the computer......to eliminate that problem? Sounds like they missed a button, and besides that, how did you figure out the speedo was 4 km/hr off ?

Bottom line guess to the question is, without knowing if your running +4 or -4 ....speedo reading +4 km/hr, possibly more on the odometer, speedo reading -4 km/hr, possibly less on the odometer.

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post #3 of 10 (permalink) Old 08-11-2012, 07:06 AM
 
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Most programers will allow you to set your spedo. to your tire size. The tires on Henry are 1 1/2" taller on the 20" rims that threw out my speedo by 5k at 100kph. the programer ballanced that.

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post #4 of 10 (permalink) Old 08-11-2012, 11:14 AM
 
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Are you going by a gps? They are usually 3-4km/hr out. Not sure why though.

I think it's a time delay of the satellites and gps unit processing the data.


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post #5 of 10 (permalink) Old 08-11-2012, 12:40 PM
 
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http://www.miata.net/garage/tirecalc.html



Really? Its the Internet take it easy.
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post #6 of 10 (permalink) Old 08-11-2012, 01:03 PM
 
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Another good one is

http://www.tacomaworld.com/forum/tirecalc.php


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post #7 of 10 (permalink) Old 08-11-2012, 04:51 PM
 
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Quote:
Originally Posted by SVT4MT View Post
Bottom line guess to the question is, without knowing if your running +4 or -4 ....speedo reading +4 km/hr, possibly more on the odometer, speedo reading -4 km/hr, possibly less on the odometer.
I think that's the right answer.

The only thing we don't know is at what speed he's reading +4 Km high? If he's doing 100 KPH, then that's only a 4% error, but if he's going 50 and his speedometer reads 54, then that's an 8% error, and that would be pretty far off. Personally, I could live with a 4% error. The speedometer isn't a precision instrument anyway.

My Mazda reads a few KPH high at 100, and that's with the stock wheels and tires. That's compared with 2 completely different GPS units. For accuracy with GPS, you need to maintain a steady speed for a while, and the faster the better. 100 KPH seems pretty good for this. I've checked this against measured 5 KM stretches on the highway, and I'd say the GPS can easily get you within 1-2 Kph accuracy. Depends on your GPS, too, I guess.

I suspect the reason for the unavoidable speedo error may be that the Ford programming software for the PCM doesn't let the technician enter in a precise number for the tire diameter. It may just have a drop-down list of various sizes and he has to pick the closest one. Sort of like buying a pair of shoes -- your feet may be size 10.378245, but they only sell size 10 or 10 1/2.

I'm not sure about the PCM programming thing, since I've never seen it, so it's just a guess on my part. But it sounds pretty convincing, eh?

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post #8 of 10 (permalink) Old 08-12-2012, 07:11 PM
 
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If your speedo is reading +4 km/h i.e. 104 km/h and your actual road speed is 100 km/h, then you are rolling your odometer faster which means when you've travelled 100 km actual your odometer/tripometer will read that you travelled 104 km. This means that after 100,000 km, your odometer will say 104,000 km which means YOU missed out on 4000 km of fun that you'll never be able to get back unless you exceed warp 10 while sling shotting around Ford HQ...another time! -4 km/h = 96 km/h while doing 100 km/h which means slower odometer and more time to enjoy your truck! However, be aware that while exceeding the speed limit, your actually going 4 km/h faster, and at 146 km/h, it can make a huge difference when the OPP finally catch you!

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post #9 of 10 (permalink) Old 08-19-2012, 07:56 AM Thread Starter
 
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Thanks for all of the info guys!

I'm sure b4000 hit the nail on the head with the available tire size thing, because although the tires are 31" if you go to the BFG website the actual diameter is an odd number.

I've used 2 GPS units, as well as several other vehicles for comparison - I did several checks at varying speeds and it pretty much remained a 4 km/h differential.

I was looking at programmers to resolve it, but the cost for one small change is something I can't justify. I lose a little bit of life on the odo, but the speedo works in my favour for keeping the popo away.

Thanks again guys.
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post #10 of 10 (permalink) Old 08-19-2012, 03:09 PM
 
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You have to figure any odometer is not a precision instrument. It just provides a pretty close estimate. The diameter of the tires can change depending on pressure/temperature, too. Who knows, that could be good for +/- 0.5% right there?

And for what you need the information for mostly, ie. "When do I change my oil?", etc., I doubt even a thousand km error makes any difference. And has anyone ever gotten a ticket for doing 104 in a 100 zone?

On my old RWD truck with cheap all-seasons, when it was really icy or snowy, I used to joke that my 2 km drive to work would put 150 km on my odometer. Never seemed to hurt anything.

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