1998 F150 Speedometer error

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David M - 11 May 2005 11:08 GMT

So, I have the 98 F150 4.6 parked for 6 weeks whilst I go overseas.
When I come back, I noticed the speedometer is reading about 10%
high (ie. when I am going 70mph by the speedo, I am actually travelling
about 63 mph according to timing the mile markers).

Would disconnecting the battery cause the computer to lose the speedo
calibration? The engine runs fine, no MIL. Any other thoughts?
Bad VSS? The tires on the vehicle are slightly larger than stock so
I would expect the speedo to read a little low.

TIA

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David M (dmacchiarolo)
http://home.triad.rr.com/redsled
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David M - 13 May 2005 10:05 GMT

> So, I have the 98 F150 4.6 parked for 6 weeks whilst I go overseas.
> When I come back, I noticed the speedometer is reading about 10%
[quoted text clipped - 7 lines]
>
> TIA

Well, I took it to a speedo shop, they put it on their dyno, and,
it was dead on. I would not have thought that the stretch of highway
that I chose to do my timing apparently has mile markers that are
not placed accurately. Weird. I tried it on a different highway
and, yes, apparently the mile markers on that stretch of
highway were different.

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David M (dmacchiarolo)
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Matt Macchiarolo - 19 May 2005 19:12 GMT

Bro,

I recently installed a Tru Speed speedometer calibrator when I upsized my
tires on my Superduty, and I can't find any mile markers near me that are
accurate. I-96 between Novi MI and Brighton are especially off.

I think it's set a little low still, and my GPS died several months ago, so
until I replace the GPS I'll be off a little...

>> So, I have the 98 F150 4.6 parked for 6 weeks whilst I go overseas.
>> When I come back, I noticed the speedometer is reading about 10%
[quoted text clipped - 14 lines]
> and, yes, apparently the mile markers on that stretch of
> highway were different.

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David M - 20 May 2005 00:26 GMT

> Bro,
>
> I recently installed a Tru Speed speedometer calibrator when I upsized my
> tires on my Superduty, and I can't find any mile markers near me that are
> accurate. I-96 between Novi MI and Brighton are especially off.

I searched for a little while and could not find any accuracy
specs for mileposts.

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David M (dmacchiarolo)
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Matt Macchiarolo - 20 May 2005 02:18 GMT

Given the work ethic of many state DOT workers, or their lowest-bidder
construction contractors, do you think that two damns are given about the
accuracy of mileposts?

I'm sure there are hard-working, diligent highway construction workers out
there. Just too damn few of them.

And for the record I always slow down in construction zones. :-)

>> Bro,
>>
[quoted text clipped - 4 lines]
> I searched for a little while and could not find any accuracy
> specs for mileposts.

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The OTHER Kevin in San Diego - 20 May 2005 02:06 GMT

>Bro,
>
[quoted text clipped - 4 lines]
>I think it's set a little low still, and my GPS died several months ago, so
>until I replace the GPS I'll be off a little...

Some quick math (described in the TruSpeed docs) will get you pretty
darn close.

I used my GPS to get mine dialed in. It's dead on accurate when the
tires are new, but reads high as they wear and the diameter decreases.

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Matt Macchiarolo - 20 May 2005 02:12 GMT

Yep, that's what I did, and my miles per tank came back to about where they
were before the tire replacement...between the tire replacement and the Tru
Speed install, my miles per tank decreased by almost 50 miles...I was going
the same distance, but the odometer were clocking more slowly, so after a
tank of gas it appeared like I didn't go as far...which is about 300-320
miles unloaded in mixed but primarily freeway miles.

When I get a new GPS I will dial it in exactly. My Jeep is still off by
about 10% since the large tires and final ratio change, even with the
recommended speedo gear replacement...I'll have to tweak that one of these
days with a different speedo gear, which is a lot easier to change out than
installing a Tru Speed but harder to adjust. :-)

>>Bro,
>>
[quoted text clipped - 12 lines]
> tires are new, but reads high as they wear and the diameter decreases.
> :)

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The OTHER Kevin in San Diego - 20 May 2005 04:39 GMT

>Yep, that's what I did, and my miles per tank came back to about where they
>were before the tire replacement...between the tire replacement and the Tru
>Speed install, my miles per tank decreased by almost 50 miles...I was going
>the same distance, but the odometer were clocking more slowly, so after a
>tank of gas it appeared like I didn't go as far...which is about 300-320
>miles unloaded in mixed but primarily freeway miles.

I also lost about 50 miles/tank, but I'm pretty sure it's due to the
big tires and increased rolling resistance. (And probably the tweaked
ECM I run too.. ehhe)

>When I get a new GPS I will dial it in exactly. My Jeep is still off by
>about 10% since the large tires and final ratio change, even with the
>recommended speedo gear replacement...I'll have to tweak that one of these
>days with a different speedo gear, which is a lot easier to change out than
>installing a Tru Speed but harder to adjust. :-)

Had to do that on the 4Runner when I bolted the 33's on it. PITA to
say the least...

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