I've been searching the forum and I haven't found anything yet, but have any of you guys noticed a difference in your mph on your speedometer and your gps (I have an after market gps/tomtom brand). I am getting about a 5 mph difference. Last April I got a ticket for doing 46 in a 35, no real biggie except when I saw the police officer with his lights I thought that I was doing 51. I thought nothing of the difference then, but now that my gps is saying the same thing I'm starting to wonder if this is something that I should have looked at. Thanks for any help!

 

Are you riding on stock tires? If so, If I were you I'd take it to the dealer and have them verify the PCM program is correct for your tire size.

 

Mine is 1-2 mph off according to my Garmin.

 

MS Streets and Trips... showing about 2-4mph off... (w/a PCM recali here for a smidgen of a taller tire)

 

Exact same as my Garmin Nuvi.

 

I noticed the same thing a few weeks ago when I got my first speeding ticket in my new F-150. I had the same tires (Pirelli) & 20" rims from the factory and I was clocked by radar at a speed 3 mph above what I thought I was traveling. After looking at the trooper's radar to verify the speed that he stated, I got back on the road and turned on my hand held gps and sure enough I was 3 mph above my truck's speedometer. Since I have a Superchip Programmer, I just adjusted the tire size until the gps & speedometer were reading the same.

 

My Garmin StreetPilot shows my speedometer about 2-3 MPH fast. It is dead on with my car. My truck is pure stock.

 

Same here my Garmin Streetpilot shows the speedo on my 04 is reading 3 mph fast.

 

I just checked my speedo vs. my Pioneer NAV on a quick trip to the store and mine is spot on...Honestly, I was a little surprised. I expected +/- 3mph. I ride on the stock 20" scorpions. Might have something to do with the fact that my NAV gets input from the VSS wire.

 

Mine is reversed. I have 26" rims on my F150 and my GPS shows 3mph faster than speedo at around 70mph.

 

2mph off. if im goin 40 on speedo it reads 42 on my gps. i put on 18" fx4 rims on my xl so i figured that was it.

 

2 to 3 mph off here....i checked with my garmin and if you have a lease i would say it's a valid point to make to your dealer about the amount of miles you are allowed.

 

Guys,
I have 07 Supercrew Lariat ,its bone stock 18s and the BFGs. Now I checked mine with my Garmin Quuest and then with my 2610 street pilot. I was spot on with both units. Ihad to go to the larger unit as eyes dont do so well on the motorbike as they used to....The quest was too small....THe 2610 does jus fine. try checking with different speeds, some vary more than others. My bike is notoriously wrong....at 100 its like 89, 55 its 51 so is that to my advantage? I think so . To the fellow who got a ticked get it recalibrated then go to DA mabey that will help you some...depends on how fast you were going...Good luck.....

Rod

 

I'd bet the variances in speed btwn the various GPS units and what your car/truck is saying are mostly due to how the GPS calculates your speed. 1st, a GPS must 'find' at least 3x satellites and triangulate your longitude/latitude position. 2nd, it will 'sample' the satellites it has found for changes in your longitude/latitude. Based on these deltas, it does some basic math, attempts to average things, and display a speed. Different GPS units will sample at different rates. Also, higher-end GPS units allow you to adjust the sample rate. Thus if you had multiple GPS units in the same car/truck, you'd also probably get different speeds.

A GPS is also sensitive (depending on the unit) to any motion. If your car/truck/motorcycle has alot of motion or bouncing around, your GPS will incorporate this movement into the speed (realize that your GPS is calculating the speed of the GPS unit itself). If you pick up your GPS quickly from a standing position, you'll see the GPS thinks it just sped at ~60MPH (which it did).

(also, for what it's worth a GPS can only calculate your 'speed over ground'. On a boat or in an airplane they can not account for water current or jetstreams). In these applications, the more interesting calculation is VMG (Velocity Made Good) towards the destination.)

 

My speedo and Garmin Nav are nuts on, in the mph dept. One size up on the tires.