Garmin Gdr E350

[Danny Hosford]

Assumingthe tires on your vehicle are the ones mounted from the factory and inflated to the proper air pressure it has been discussed in previous threads and the majority here believes the GPS is the most accurate.

There used to be somewhat of a reason for a car having an inaccurate speedometer because they were driven by a gear in the transmission moving a cable to the speedometer and the speedometer being electro-mehanically driven by the spinning cable. There were multiple nylon gears but you were limited to a finite set of adjustability of the display. When the gearing and tires fit one of the nylon gears, the speedometer was accurate. Sometime they were off a little.

But these days the speedometer is drivn by electronics and the input is just a revolution sensor. So the connection is totally software and there is no reason the programming cannot be changed to make the speedometer accurate. But some manufactureres, BMW is one, will not let dealers "fix" the software. They want the speedometer to read faster than the car is going. My son calls this sort of thinking "nannying". I don't know if Toyota lets the dealer "fix" this by connecting a computer to the OBD II port and re-programming or not.

There used to be somewhat of a reason for a car having an inaccurate speedometer because they were driven by a gear in the transmission moving a cable to the speedometer and the speedometer being electro-mehanically driven by the spinning cable. There were multiple nylon gears but you were limited to a finite set of adjustability of the display. When the gearing and tires fit one of the nylon gears, the speedometer was accurate. Sometime they were off a little. ........That reminds me that I bought a brand new 1968 Mercury Cougar XR-7 and got a speeding ticket on the way home from the dealer because the factory had put in a wrong gear. The judge accepted a letter from the dealer admitting that the gear was wrong and caused the excess speed at which I was clocked, and dropped the charge.

Has anyone else noticed the the mph display on the garmin is different between the map screen and the trip computer display. It seems to be about 1 mph different. Not a big difference but I would think they should be the same. And being as there is a difference, which one is right?

I put new tires on my wife's Avalon and noticed that before the GPS and display speed were the same. Now the GPS is 3 MPH faster than what I see on the speedometer. New tires are lower proflie and wider but same rim size. I would conclude the GPS is accurate and tires sizing can cause speed errors.

When I purchased my last car, I got a substantial difference between reported speed on the speedometer verses the gps. I tried two gps units, one of which is waas enabled, and got identical readings between the two. Thanks to a google search, I found out that model of car (BMW E39 series) consistently reports a higher than true mph. The reading I was getting was consistent with what other owners of the same model report. It is a factory setting that cannot be corrected by a dealer. I rely on the gps reading.

I put new tires on my wife's Avalon and noticed that before the GPS and display speed were the same. Now the GPS is 3 MPH faster than what I see on the speedometer. New tires are lower proflie and wider but same rim size. I would conclude the GPS is accurate and tires sizing can cause speed errors. The speedometer indicated speed (at any given actual speed) will go up as the outer diameter of the tire decreases. Thus the speedometer indicated speed will even go up a bit as the tire tread wears off.

"Position drift" while the GPS receiver is static can cause temporary small speed readings of 1 or 2 MPH. This is caused by position calculation error from small variations in signal strength and accuracy from the satellites. When moving, these small errors get averaged into the displayed speed and usually get cancelled out, having little or no effect on the displayed speed accuracy. I have tested the displayed speed on both of my nuvi's on many occasions using mile markers and a stop watch. The nuvi's are always spot on at highway speed (within the 1 MPH precision that the nuvi shows), while the speedometer readings on my 3 cars always read 2 to 3 MPH fast. I have also checked the nuvis aginst my OBD connected Ultragauge and it also matches. I find it interesting that the car's speedometer is off by about 2 MPH, but the OBD is reporting the correct speed.

and see a difference but only about 1 to 2 MPH with the Garmin giving the higher MPH. I also compared two Garmin units and they both gave the same speed. Not sure how much the diameter of the car tires make on the MPH on the odometer.

I just returned from a road trip in my Toyota Avalon. While traveling the speed displayed on my GPS was always about 3 mph faster than the speed on my car. Which one is the most accurate? this issue has been discussed extensively. See node 6695:

Here's a link where you can see the effect of different tire sizes. The circumference is the key parameter when you're figuring out the impact on speed. The OEM tire size should give you a baseline that you can compare the effect of different sizes to.

So I would have thought that your GPS should now be reading less than the speedometer, since the two were reading the same before you put on lower profile tires.There's a bit of a caveat with that. Tire sizes are expressed as aspect ratios. So, a 205/70 will be 70% high as the tire is wide, i.e. 205 millimeters.

I can put on a 215/65 and it will be a marginal difference of -1.1% in diameter. Go wider yet, and the difference can actually go to the positive side. The wheel offset determines how wide you can go.

I noticed this a year ago when I purchased a Magellan Roadmate 1470, the Magellan speed was always 3 mph faster than my car speedometer, then I got a Garmin 2350LMT, the Garmin speed matched the car speedometer exactly, so one day I put both GPS's in the car and the GPS speeds where different.

I can put on a 215/65 and it will be a marginal difference of -1.1% in diameter. Go wider yet, and the difference can actually go to the positive side. The wheel offset determines how wide you can go.I stand corrected. I was thinking "lower profile lower tire height" and ignored the statement that the new tire is wider.

I have noticed the same thing. In my RV I have a Scan Gauge II, The mph indicator is always a mile or two under what the speedometer indicates, my GPS is usually one mile above what the Scan Gauge indicates.

By now you've come to the (correct) conclusion the GPS is more accurate than your vehicle speedometer. BUT, there's a way to fix that if it bothers you. There's a product called YellowBox that takes the reading from the sending unit (usually in the transmission) then corrects the number before sending the info to the speedometer.

I have one on my motorcycle (that was 10% off from the factory) and it's very simple to use. Just install, figure out the percentage your speed is off (comparing your GPS to the speedo) then set the dip switches on the YellowBox accordingly. The device is almost a must for those who've gone to much larger tires/rims on a truck.

I set my cruise control at 9 miles over the speed limit on the freeway and coast hwy.

Drive by the cops with the radar guns and never have been stoped.

I'll have to check my car speedo with the GPS.

Maybe I could be doing 3 more miles an hour

I think I'll just stick with my current 9 over, whatever it really is seems to work.

That is not quite true. I notice if I stop at a red light the GPS speed number they are still going down,while I have already come to a full stop. Yes it is close but not accurate.The GPS calculates your speed over a measured distance determined by GPS location over a rolling time scale. Therefore the speed displayed will lag a bit during acceleration and deceleration. But when driving at a relatively constant speed the GPS will be more accurate than the car's speedometer.

I drive a 2006 Ford E350 van and do "expedite deliveries". I have a garmin streetpilot c330. I know, I know, and yes it has grey whiskers and is very wrinkled. I have found it to be very adequate for what I do. I agree that I trust the gps speed over a short distance more so than the speedometer. The c330 has a feature that shows you your fastest speed, since you did a speed reset. Occasionally when I check fastest speed, It has read as high as 169 mph. Please go back and re-read my vehicle type. This is not a problem for me, I just get a kick out of seeing those speeds.

The one on the police's radar... If you ever get a RADAR (LIDAR) ticket, be sure to check the calibration certificate. Many law enforcement agencies now a day don't have much budget to do it regularly.

Personally, I got away 3 tickets in one city over a period of time.I will keep this in mind, hopefully won't be pulled over anytime soon...Agreed with the first quote. I doubt if Garmin gives any calibration certs with their products.

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