tobey457

I just put new wheels and tires on my car (4cyl base) and compared to my GPS, my speedometer is slightly off. At 30mph I'm off around 1mph, and at 70 - 80 I'm off 2 - 3mph. So when my speedo is reading 70mph my GPS says I'm going 68.

I switched to an 18x8 with 235/40/18 tires, which per most of the calculators was within spec. So now I'm wondering (since I never checked it against the GPS when it was stock) if the speedo was off anyway, or if it was the wheels and tires... or maybe it's the GPS...

Not a big deal, just curious if you guys are noticing anything similar.

EL_PIC

This is common.
Check your odometer also.
A few years ago there was a class action suit with Acura Honda that
mileage would rack up too fast thus voiding your warranty sooner
and overstating your MPG.
What will these car companies think of next ??

ed_423

hmm..an alarming issue people should take note of when changing wheels...

tobey457

Well not necessarily, it may have nothing to do with the new wheels / tires. The car may just be off from the factory, or it may be the GPS.

My guess is that the car speedo is off by 1mph from the factory, and the tires / wheels put me off by another 1mph...

At any rate being off by 2mph is probably within tolerence at 70mph, just wondering what others are seeing.

KillerG

unless you got that shit top gear uses to do speed runs then your GPS is probably off by a teensy bit

THAT BEING SAID

changing your wheel size/diameter affects this too. a tire size calculator, such as the one the miata guys use, will help a ton.

SO

it is a combination of both minute factors.

ressling

It's the GPS...

GPS is a triangulation of satellites, it's not an exact science. Your speedometer on the other hand, is mechanically integrated into your car.

To draw a similar comparison, it's like wi-fi vs. 3G/4G. Wi-fi will be more "precise" of a connection, because it's running off a network that's home based via DSL or Cable (coaxial), whereas 3G/4G uses satellites.

Puppetmaster

As posted, it isn't an exact science. Every measurement has some variance - the speedo, the GPS, and add to that the differences from the new diameter, however small.

HondoGermany

In Germany or Europe the speedo must show more speed than you drive. The car manufacturers must set the speedo faster than you drive. I Think its for that you dont drive too fast, if 100mph is allowed and your speedo shows 100mph than you have a little reserve, so you dont drive too fast. Different wheel size is a factor on your speedo too. Like Killer posted, some tire calculator shows how much the difference on bigger or smaller wheels is

tobey457

I realize there are too many variables to really determine what the correct speed is for my car. Too many factors to narrow it down at the moment, so really just wondering if anyone else has noticed (on their stock TSX) if their GPS and their speedo are close in MPH, and if not how far off are they?

Mr. NC

Only way to solve this issue....remove the speedo entirely, who needs to know how fast we're goin these days?

BaoZZeR

Maybe it's off because of air resistance?

DJMcGee

Could be a bit to tires and a bit to GPS. Again, calculate the difference in rolling diameter for the new tires compared to stock. For that tire size your speedo will show 61 when actually going 60.

Keep in mind, manufacturers have to stay within a tolerance, which the germans have used to their advantages over the years which at 60 mph is +/- 3 mph.

And, that is the speedo, the odo needs to remain accurate for warranty purposes.

HeavyDuty

With OE wheels/tires, mine was off about the same as what you indicated above. ~2mph at 60.

Honda is famous for their tachs reading higher too, by 1-200 rpm at redline.

tobey457

Cool, I appreciate the info.

Anyone else seeing similar results on stock wheels / tires?