I drove to northern Kentucky and back this past week. I used my Garmin Nuvi both ways to track my time, average speed, ect. I didn't really need it to get there, been going back and forth for about 15 years now. Only time I needed it was to find my kid's apartment, mostly because their street kind of sneaks up on you.

Anyway, I noticed my speedometer reads about 1.5-2 mph slow at 70 and is off by a good 3 mph at 85. I'd actually noticed it in the past but now I know it is consistant over a long drive. I also found that when my odometer shows 100 miles traveled, the nav says 104.3, again that was consistant over the entire trip. On the way back, my trip computer B showed 915.6 miles traveled and my Garmin showed 955.5.

I have the stock tire size, 225/50/17s but the Falkens do seem a little taller than the Michelin and Yokohama tires I've had in the past. However, last year, I thought it was odd that typically from my brother's house (I spend the night there before leaving on my trips because he is at the eastern edge of the Dallas area) to my ex-wife's house is 910 miles but last year, my trip odometer showed 877.

Anyone else notice this with aftermarket nav units? No big deal, I guess but using the nav's mileage gave me a 26 mph average (average speed was 71.3 on the way home), using my odometer it was around 25 mpg.

 

Mine is off by 2 Mph, thats going buy the speed my GPS shows me.

 

I have a Mio and speed is normally matched up pretty well, sometimes is jumps 1mph but i assume thats normal

 

My speedo is off because of my tires, but im going about 67 when the speedo reads 70. Even a tiny difference in tire diameter can make a difference at highway speeds. That three mph might have kept me outta some tickets though

 

Doesn't the navi give you linear speed whereas speedometer gives you tire speed?

 

isnt it the same thing as long as your tires are maintaining traction and not spinning freely which should never happen at freeway speeds unless you and your car were about to be in some big trouble.

 

mine is 1 mph faster at 60mph and 1mph slower at 80 mph

 

my 680 nuvi is off by 2-3 mph. I've been on several trips but never thought about watching the difference. Next time I go to Miami or Tampa I'll make note. I'm also riding on 20's and thought that was the difference in the speedo readings.

 

at anything below 65 or so, it's dead on with my garmin. At 80, it's 1 mph slow.