GPS makes a good replacement Speedometer

I agree. I bought a Zumo 550 for my motorcycle. Motorcycle's are notorious for inaccurate speedometers. Now I know how fast I'm really going. You could probably also use a track to prove you weren't speeding if you ever had to defend yourself in court for a speeding ticket. Provided, of course, you weren't actually speeding.

You fitted an extra, unnecessary part to a Lotus?!! - Colin Chapman will turn in his grave )

Something else to try (not applicable to the Lotus!):-

If your car has Cruise Control and a Trip Computer...

Put the car on Cruise at say 50mph (by the speedo.) Reset the Average Speed on the Trip Computer.
Since you are travelling at a steady speed, your Current Speed and your Average Speed are one and the same.

The chances are the speed now indicated by the Trip Computer will disagree with the speedo., but will agree with the GPS.

I was very surprised at how accurate the GPS was on the speed. It matched my speedometer to the tee. I know that my speedometer is correct because I have had it calibrated and I shoot myself several times with Radar every now and then to make sure that it is correct.

Well these days, of course, there's no intrinsic reason for them to be inaccurate, given that they're just counting pulses. In the UK, at least, they're allowed to be optimistic by 10%, but not allowed to be pessimistic.
I read a "Reader's bikes test", in a UK magazine - one guy had adapted a bicycle computer and it was within 0.1 mph of the laser rig (at 180.1mph!).

Out of interest, how have you done that? - re-marked the dial, or are you saying it was correct in the first place?

Naw, it was correct to begin with. I just ensured that it was by getting it tested. I have always called it being "calibrated" since that is what the courts use as well. It may be an incorrect terminology though.

I use my Nuvi as my primary speedometer since my winter tires are larger than stock.

Wow, good idea! didn't think about that one. There are a lot of 'oversized rimmed out' cars that need to get a gps, of course, they probably spend all their money on rims and stereo equipment...lol.

I had the Super Seven weighed and it's 330 kilograms on the from wheels and 340 on the back so if anything the addition of the GPS will make the balance perfect!

My next trick will be to mount it somehow that doesn't block the entire window! I have one sticker but there isn't enough horizontal dash (Skuttle box) to fix it. The other option is to stick it on the vertical part of the dash but then it will be mounted far over in the passenger compartment. There isn't enough room between the window and the steering wheel so that is out! I could fit it on the transmission tunnel but it might not get a good lock on the satellites there.

The other question will be what happens when it rains. Is there any commercial rain cover available that works with the touch screen. It should also let the voice commands through although that might never work with the wind noise and engine sounds. How do the motorcycle guys work it? There should be a mod to allow attaching an ear piece.

Bob

I generally have found the speedometer on my Nuvi 660to be accurate; however, the current high speed reads 221 MPH. While I have been known to exceed the speed limit on occasion, my VW Passat could only reach 221 MPH if it was dropped out of an airplane. I find it so amusing that I haven't reset the high speed in months.

221????? What does resetting it have to do with it being 221? Sounds like you have a problem. No matter if you reset it or not, the high speed shouldn't be 221 unless you actually went 221. I am not sure I understand you

Sorry if my post wasn't clear. In addition to giving the current vehicle speed the Nuvi 660 also gives other statistics, one of which is the maximum speed at which the vehicle was driven. The reading on my unit says that my max speed was 221 MHP. Clearly I did not go 221 MPH but that is what the unit says. I find it amusing and thought others might also.

i found my max speed was 800+MPH on my garmin ique m5. I think it's due to me shutting the gps off (in San Jose, CA), then move to a different location and turn it on again. It think (at the instance I turn it on) that I was at my original point (San Jose, CA)(b4 it acquire satelites). Then a few seconds later, I might be 50 miles away from the original location (San Francisco, CA).

But most GPSs only acquire satelite signals once each second, then take sometime to process the data and display it. So what you see is old data (1 or more seconds old). I guess you can still use GPS as speedometer if you set your car in cruise control so you speed is maintain (so gps data is not "old" data).

Yeah, I noticed that the speed was about 1-2 seconds behind the actual speed. But if you want to get a really good average over 2 or 3 seconds, it works fine.

My max speed is 531 mph, cause I was in an airplane.

I've seen some pretty crazy speeds/locations on my old Garmin eTrex Legend.

Once I was in a shuttle bus going between the terminal and parking lot at the DCA airport, and it showed my location as about a mile from the coast (in the ocean) traveling at ~300mph. It fixed itself within a few seconds, but multi-pathing and reflection of the satellite signals can cause some really bizarre readings.

This is not unusual, I and other people have had the same thing happen. I have had it happen with two different Garmins. The speed I am traveling at the time is always accurate, but occasionally the max speed will be some ridicules speed like 150 MPH. I have no idea why and I doubt if Garmin would have a clue either. The only guess I could make would be it has something to do with it losing and acquiring new satellites.

If your car has Cruise Control and a Trip Computer...

Put the car on Cruise at say 50mph (by the speedo.) Reset the Average Speed on the Trip Computer.
Since you are travelling at a steady speed, your Current Speed and your Average Speed are one and the same.

The chances are the speed now indicated by the Trip Computer will disagree with the speedo., but will agree with the GPS.

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Doesn't the trip computer get it's information from the speedometer? If so wouldn't they be the same? My Impala has a GPS in the trunk that the OnStar uses, but I don't think it passes any info to the trip computer.

I've used my Nuvi 350 to check my speedometers. My Honda CRV reads about 2 mph slow in that important 70 to 76 mph range. So I set my cruise with the Nuvi and drive past traffic checkers all day.

Both of my cars are 2 MPH slower then a speed checker install permanently by the city near my house. The GPS give the same speed as the speed checker. I am assuming the speed checker and the GPS are correct.

that what i found also

Yeap, I have found that the GPS is more accurate. This was done by shooting myself (well someone else was) with radar.

Most of the rental cars I get (for work) are always a bit faster (it says that it is going 60, but you are really going 58). The only exception was the last one which was a Dodge 300 and it was a bit slow.

Actually you can buy XYZ brand GPS, and if you happen to be driving a Lotus, riding a bicycle or even taking a hike, you can rest assured that you will know how fast you are traveling!

The way the System works is by calculating time.
A GPS receiver needs to receive transmissions from at least three or more GPS satellites to calculate its position effectively. GPS receivers usually measure the time delays between the transmission and reception of each of the satellites GPS radio signal. This technique of calculating distance using three separate transmissions is called trilateration.

I tend to agree with everyone else here. Drove a brand new motor coach last week. I set the cruise at 70mph, the GPS said 68, also passed a radar device along the interstate and it also said 68mph. I suspect there is an intentional discrepancy programed into speedos.