1997 Speedometer reading fast, computer scanner correct.

On my 97 the speedometer reads 70 when you are actually going 65. I figured maybe somebody had put in a wrong drive gear.

Today I hooked up a scan tool and the computer is getting the correct vehicle speed compared to a gps. They both use a common drive right? This threw me for a loop.

So maybe the speedometer itself in the cluster is out of calibration? Any other ideas?

This is an all stock 3.4L 4x4 with factory 31x10.5 tires.

 

The tires are undersized for the stock speedo gear thus giving you a higher readout on the gauge (also inflating your odometer). I have the same issue. From around 20 mph and higher is when my speedometer starts getting noticeably higher than actual speed.

32" tires or equivalent is the perfect size to match the stock speedo.

 

ayup, my stock was off...put on 265/75/16, speedo is now correct.
Don't feel bad, most bikes are off 10 or more at 70.

 

there are two sensor systems for speed, apparently your scan picked up the tranny
1) transmission to ecu (your scan) (fixed speed based on known gear ratios and driveshaft speed)
2) combo meter (tire size affects this, uses wheel speed sensors)

 

hmmm, where are the combo meter sensors located?

 

front 2 wheel speed sensors, for abs... if it has abs

 

ah, so no secondary speed sensors w/ no ABS. I didn't think I had any wires down there that I didn't know what they went to.

 

i have the same issue. I just drive faster knowing my speedo is like 6% off.

 

I found the same thing with my 04. Going to 235/85r16s made the spedo spot on.

With 33s I use a Dakota Digital to dial it back to perfection. Worth the $80 bucks IMO.

 

Yep, smaller tires gives you an inflated speed. Bigger tires gets you speeding tickets

If will also affect your odometer and any mpg calculations.

 

even though my speedo is now correct, I notice my odo. isn't.

 

The ECU would have to take into account tire size also, it's effective gearing. And us last millennia guys just have the 1 reading.

 

I don't think I can go this route as my 97 looks to have a cable driven speedometer. I guess I will see what gear is in it now and get a different one.

It's odd how far off it is with the correct stock size tire.

 

yeah your are right I am completely off, not thinking

 

Odd yes, yet still normal. Maybe they INTEND for us to go up one size tire.

 

In my experience most vehicles, from the manufacturer, the spedo reads a touch high. I always assumed it was a liability thing. If you were driving along at what you think is the speed limit and got a ticket because your brand new car's spedo was reading low... doesn't bode well for anyone.

For example every Ford truck I owned, the spedo on the dash read about 3 mph faster than the ECU. As for my Taco, there is a 2 MPH difference and the ECU it fairly accurate.