On my 94 Disco I, My buddy and I went out photographing bridges. Went about 500 miles in one day. We took his laptop with external GPS receiver, so we could find the obscure bridges on back county roads. The GPS reads miles per hour when we are moving. The Disco speedometer was exactly 5 miles per hour fast, compared to the GPS readings. In other words when the Disco speedometer said we were traveling 70mph, the GPS indicated we were traveling 65. When the Disco speedometer said we were traveling 50, the GPS said 45. Now one might think this would be the case with incorrect tire size, but I am running factory tire size 235/70R 16. Are disco speedometers just optimistic? I tend to believe the GPS as being more accurate.

 

Over time the pointer (or whatever you want to call it) will get off a bit. I looked at an old expedition about a year ago and all the pointers where off on the speed readout, and the gas. I know mine is accurate because there is a police speed check thing on my drive home and mine is always accurate with that when I drive by. Is there a way to fix it? i can say I know, other than just doing the math! Mike might have an exact anwser, but I though I would let you know not all are optimistic like that.

Hope you got some good pics!

 

Its not the tires most read faster than you are going.
If you look the miles you travel will be in line with the gps unless you have larger tires.
With 245/75/16 tires mine is one mile per hour slow. 70mph is 69mphs and my trip meter is 4 % slow so 100 miles traveled is really 104 miles.

 

My '97 does the exact same thing, my OD is dead on, I always thought that the OD ran off the speedo, but I guess not.
It is fun though when you have a passenger, go 85 on the speedo and watch them freak. Ok, ok, it doesnt take much to entertain me.

 

Sounds like its possibly a common DI occurrence??

 

I have the exact same thing with my '03. Speedo's ~ 5mph fast with stock 255/55/18s

 

my 01 disco reads 4 mph SLOW when i go past the little radar trap things they set up on post. this is consistent for 10 mph and 50 mph. and im running stock tires...

 

hmmm maybe I need to check mine again, I thought it was right on, but then again that was at 25 MPH. I wonder if mine is off when I exceed 30-40 MPH. This is an interesting topic, i'll let you guys know what I find out. I was hoping Mike would chime in with something, it will be interesting to see what he says.

(I am running 255/60/18)

 

I have a hand held GPS that I use for backpacking and have found almost every car and truck I have been in does the same thing, and brand of car does not matter.

 

I'm running stock 235/70/16s on my '94 D1 and I too have the same occurrence. The speedometer reads 5mph fast at all speeds and the odometer is correct; I ran a long trip with GPS and the distance was exact. This is much different than the effect from oversize tires like on my truck, where the error grows as you drive faster (5mph at 30, 10 at 60), so it must just be a needle issue.