inaccurate speedometer

[artm]

Mine reads 5mph on the high side - at 50-85, more or less - unless my gps is wrong. Stock truck, had it for about a year and just noticed it.

Anyone with the same???

[Mike Hippert]

My GPS and speedo are about 2 mph different no matter what the speed, the speedo matches those radar things that tell you how fast you are going. Have you put new tires on it that are about an inch taller then the stock tires? An inch would give you about 3 mph off and then the GPS to Speedo thing is another 2 mph, so that gets you to 5 mph. Just a thought.

[artm]

Stock wheels and tires. I guess the next time I'm on the dynamometer I'll see what's up.

[jimmy salmon]

mine is 10mph out at any speed checked with gps and radar thing so i just deduct 10mph

[rover4x4]

mine is off by 5.

[KeithD]

Mine seemed a bit optimistic with the 205/80 tires it came with....on a GPS it is now about right - on 285/75s!

[smenzel]

First thing I did when I bought my GPS was check it against all of my vehicles. Turns out all of my speedos read at least five MPH on the high side as of 50 MPH.

I'm not sure if I really believe this but someone I spoke with said that speedos are designed this way. Perhaps it's to make people think they're driving faster than they really are. :)

[BarryO]

I'm not sure if I really believe this but someone I spoke with said that speedos are designed this way. Perhaps it's to make people think they're driving faster than they really are.

I've heard this, too. Back in the days before Hall-effect speed sensors and digital electronics, it was very difficult to make accurate speedos. In other words, stock units had a pretty loose tolerance on them. So they were designed to read on the high side, to ensure that just about every one of them read above the true speed: in order to avoid the situation where some read slower than actual, and thus might generate unintended tickets.

I bet the average speedo is alot more accurate now, such that this much slop ain't needed, but tradition and habit keeps it the way it is.

[smenzel]

Barry, your reasoning makes more sense but I sometimes wonder if there isn't some weight to the goverment conspiracy theory. :)

[ttegt]

my '95 d-90 runs 5 mph slower than my gps shows as well, does this mean that the odometer is showing more miles than the truck has been driven? :rolleyes

[Mike Hippert]

Replace the speedo output gear in the T-case to correct.