Checking speedometer accuracy.


Old 08-24-16, 08:04 PM
Norm201's Avatar
Thread Starter
Join Date: Sep 2013
Location: United States
Posts: 9,199
Received 256 Votes on 230 Posts
Checking speedometer accuracy.

Is using a GPS and/or cell phone app good enough to check the accuracy on a speedometer? I always thought my van was about 3 to 5 mph reading over the actual speed. And seemed to check with an old GPS I used. However, I now started using a smart phone app and it reads almost dead on (although there is a time delay of a 1/2 second or so).

On my car I have a HUD display attached to the OBD2 connection and it corresponds exactly to the speedo needle on the dash. It also checks out with the smart phone app (DigiHUd Speedometer).
Sponsored Links
Old 08-25-16, 04:24 AM
chandler's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,965
Received 6 Votes on 6 Posts
I find my GPS to be pretty accurate. I have never used Maps to gauge speed, but I guess with any satellite oriented receiver, there is a lag time. I know this from watching altitude markings. Here in the mountains it will read ascending altitude even after I top a gap and begin downhill.
Old 08-25-16, 05:08 AM
Forum Topic Moderator
Join Date: Mar 2005
Location: USA - N.E.Tn
Posts: 47,708
Received 331 Votes on 294 Posts
Before GPS I always checked the odometer with the mile markers on the highway. Might not be as accurate and requires some math but always gave me a good idea of how far off the speedo was after a gear or tire size change.
Old 08-25-16, 05:40 AM
the_tow_guy's Avatar
Group Moderator
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,988
Received 60 Votes on 49 Posts
Wife's Camry indicates about 1-2 mph below what the gps does when we have it on. Also most times when we go by a portable radar speed sign, they also show the same error, so I'm assuming the car is actually off. Kind of surprising really.

Old 08-25-16, 05:56 AM
pugsl's Avatar
Forum Topic Moderator
Join Date: Mar 2008
Location: United States
Posts: 9,029
Received 74 Votes on 67 Posts
I get about a 2 mp difference between GPS and speedo.
Old 08-25-16, 06:28 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 26,221
Received 721 Votes on 667 Posts
Mine is +/- 2 mph. If my cruise is set on 70, gps speed varies from 68 to 72.
Old 08-25-16, 06:40 AM
Join Date: Jun 2010
Location: Canada (near The Motor City)
Posts: 686
Received 10 Votes on 7 Posts
There are many different tires out there to fit your vehicle from numerous manufacturers in various quality levels and design (all-season, performance. M/S etc)

You would have to check the circumference of all those tires and verify that they are all identical to expect a speedometer to be accurate in all cases.

And don't forget tire pressure - a couple of pounds either way will affect speedometer readings.

So many variables you can't expect speedometer readings to be "right on"
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title: