On line map milage compared to actual miles

So does anyone have some actual data to compare. GPS miles vs. odo miles over a longer span 100+ miles. I am curious.

CA Traveler wrote:
On a highway with MMs simply use them to check your mileage. And if you can drive at a constant speed for 5+ miles have the copilot use a phone etc to capture the elapsed time.

If if you are on a reasonably flat road a GPS should reasonably accurate at least I believe my Garmins are accurate.

I've found that the accuracy of my Garmins miles and arrival time to be accurate. The arrival time estimate is update on a per minute basis based on changing speed or route. Route changes also change the miles.

Unless it is a designated speedometer check station the MM signs aren't that accurate.

On a highway with MMs simply use them to check your mileage. And if you can drive at a constant speed for 5+ miles have the copilot use a phone etc to capture the elapsed time.

If if you are on a reasonably flat road a GPS should reasonably accurate at least I believe my Garmins are accurate.

I've found that the accuracy of my Garmins miles and arrival time to be accurate. The arrival time estimate is update on a per minute basis based on changing speed or route. Route changes also change the miles.

AH64ID wrote:

GoPackGo wrote:
If you have a GPS that reads out MPH, set your vehicle speed at 60 mph as shown on the speedometer and compare to what the GPS says. The GPS should be accurate. If I remember correctly, My GPS says I'm actually going 63 at an indicated 65 on my speedometer.

I have yet to see a vehicle where the speedo and odo match, meaning if you adjust/learn your error based on the speedo the odo may be wrong.

I personally prefer to reset the 2nd trip meter and gps while stopped then clock a difference over a long easy drive. My old tires were within 0.3%, and the new ones look to be within 0.5% but I need another drove to verify them. My speedo reads close, but it doesn't read the same as the gps speed.

I've observed this difference with 4 different vehicles now using the same Tom-Tom and here's my thoughts? Again, this goes back to the hills vs flat theory.

The GPS in the Tom Tom is getting the speed from the sky, looking down. Granted, it might be getting the signal from 3 to 7 satellites, but it's still from "above" looking down. From straight above, a vehicle traveling up or down a hill will appear to be moving at a different speed, than the actual ground speed.

Like I said, I've seen this discrepancy in every vehicle I've used my Tom-Tom in.

My current truck is interesting. It has an actual needle type speedometer. It also has a digital speedometer. And I use the Tom Tom. Funny, that the three are never exactly the same, only very rarely are they exact, and I've noticed that is only on very flat ground. So, I come to accept this as the norm now.

When planning a trip, Tom-Tom shows the miles and time. I use this as a guide, not a fast rule. Traffic delays, detours, rest stops all add time. The secret in planning is to determine a comfortable driving distance, say 200 miles a day. And if you drive that in 4 hours, or 10 hours, well ... if 10 hours ... you must have stopped and smelled the roses! That in itself is a good thing!

My speedo, because of a change of tire size's, shows 65 when I'm actually going 68 according to GPS. Got to carefull incase I run by a cranky cop. LOL

GoPackGo wrote:
Ok, I admit to assuming the speedo was linked to the odo. But if that's not the case, then where does the odo get it's info ?

It is linked to the speedo, but there always seems to be a little needle error even on new rigs with digitally controlled gauges.

Ok, I admit to assuming the speedo was linked to the odo. But if that's not the case, then where does the odo get it's info ?

GoPackGo wrote:
If you have a GPS that reads out MPH, set your vehicle speed at 60 mph as shown on the speedometer and compare to what the GPS says. The GPS should be accurate. If I remember correctly, My GPS says I'm actually going 63 at an indicated 65 on my speedometer.

I have yet to see a vehicle where the speedo and odo match, meaning if you adjust/learn your error based on the speedo the odo may be wrong.

I personally prefer to reset the 2nd trip meter and gps while stopped then clock a difference over a long easy drive. My old tires were within 0.3%, and the new ones look to be within 0.5% but I need another drove to verify them. My speedo reads close, but it doesn't read the same as the gps speed.

If you have a GPS that reads out MPH, set your vehicle speed at 60 mph as shown on the speedometer and compare to what the GPS says. The GPS should be accurate. If I remember correctly, My GPS says I'm actually going 63 at an indicated 65 on my speedometer.

path1 wrote:

AH64ID wrote:
How much of an error are you talking about?

Being nobody else has the problem so it must be me. I'm just going to watch it more closely.

Can't believe how long of drive it turned out to be. We went from Yuma to Bakersfield, Google maps says 382 miles in 6 hours 6 min.

That's somewhat long for us as prefer 200-300 miles a day, but didn't want to overnight anywhere so we decided to drive it in one shot. LA traffic was a big delay (starring at cars in front of us). And grapevine was messed up because of something, so we went thru Lancaster and Mojave down thru Tehachapi to Bakersfield. Stopped a couple of times to eat and fuel up. All said and done the 6 hours and 6 min drive turned into little over 12 hours.

We keep a log (somewhere) of fuel and mileage, but I'm going to start keeping track of our mileage at beginning of the day to get a better feel of what is best for us. (Step one is finding the thing)

I'm glad people chimed in and got me straight on this. I thought maybe the on-line maps showed "air miles" and not actual miles on the ground travel.

So what was the difference in miles? Time especially in rush hour or heavy traffic areas is not a good gauge. One traffic jam can easily add an hour.