Speedometer Accuracy
 

Has anyone else tested their speedometers? With my phone GPS and those police signs on the side of the road my car registers 63mph when the speedometer says 65mph.

This is probably good for me because I'm going to a 225/45 tire which will be slightly taller than the 215/45.

I find most vehicles are like this, my Dodges are always 4-5mph slower, my Celica and Sentra (was) is 2-3mph slower. My wife's old MX-6 was 2 over due to a larger tire size.

Also most speedo's get more off the faster you go, doing 125 in my Celica recorded 119 on my various GPS's.

Yeah, Generally they're off by a percentage, not a fixed amount. I'm figuring mine is off by 3%.

Subarus tend to be universally 3mph slower at 60mph.

I just clock my car against a GPS speedometer whenever I get new tires and mentally calibrate the mechanical speedometer.

That's consistent with what I've seen on mine when running with the GPS, and I believe is pretty standard.

I've found it all over the place on the cars I've owned, both up and down. My Tundra was one of the only cars I've had that it was dead on.

according to GPS on my phone, car shows 65mph whereas the phone varies between 60-62mph depending on how it feels. Road side speedo signs clock me 60-61 when Im doing 65mph, so thats cool.

Now if mine was that big I'd probably worry cause of the extra miles being tacked on to your odo can get quite large.

Odometers and speedometers are no longer mechanically linked like the old days. Odometer checks should be performed separately from speedometer checks.

On my Subaru, the speedo will read high, but the ECU will read correctly if you are logging. As an aside, the tachometer also gets a bit ahead of itself under acceleration. Both are most likely done to protect you. The ECU generally knows what is going on, and the odometer may interpret differently or get a different signal than the speedometer.

Now that's just weird.

Unfortunately I won't have any longer trips to test the odo until after I put on the new tires.

Federal regulations mandate accuracy of speedometers. It's generally safer for the manufacturer to error on the high side, rather than have customers getting angry about tickets because it reads low.

You can do an odometer check in a relatively short distance using the posted mile markers on an Interstate or many US Highways.

Learn something new every day.

I took a detour on the way home and a 14 mile trip according the the mile markers on I-64 registered exactly 14.0 miles on the odo.

Most speedos are set up to read faster because there is a good chance people will install bigger tires...

It's a common thing for pickup/4x4 owners who increase tire size and suddenly realize 75 is really 80.

Finally decided to post about this...

Anyone else have a problem where their speedos are off (on the fr-s/brz). It doesn't matter what speed I'm going, but the analog speedo will read slightly faster than the digital speedo. E.g. if the digital speedo says I'm goin 35mph, the analog will read ~37/38mph. 60mph : 62/63mph, etc.....

Not sure which one to believe. I've just been using the digital one. Everything on the car is stock.


But officer, I was going both 80 And 83mph!