speedometer off?

[sb001]

Hi all- so I went out of town on a family trip this past holiday weekend in my newly purchased 1998 LX sedan. While driving on the interstate with cruise set at 73mph, I started noticing lots of folks flying past me- even a couple of semis. I didn't really pay much attention to it at first-- just figured a lot of folks were to eager to get to the lake or whatever-- but the more it kept happening the more suspicious I got. So on the way back home this evening I decided to calculate the actual MPH I was driving, using the trip odometer and the clock. I set the cruise at 77mph, and then reset the trip odometer to 0 right as my clock changed to 7pm. Then I calculated the actual mph I was driving at intervals of 5, 10, 15, and 20 minutes (all easily divided by 60, since 60 minutes= 1 hour.)
For example 60 minutes (1 hour) divided by 5 minutes = 12. So, right as the clock changed to 7:05pm, I logged the trip odometer mileage and multiplied by 12. If the speedo was accurate, the trip odometer should have read around 6.42 miles (since 6.42miles x 12 = 77.04mph.) However the trip odometer only read 6.1 miles, and 6.1miles x 12 = 73.2mph. I did this calculation again at 10 minutes (x6), 15 minutes (x4), and 20 minutes (x3.) Every time it came out to somewhere between 73.2mph and 73.6mph. So it looks like my speedometer is off by an average of about 3.4 mph (at least at 77mph.)
The most obvious explanation would be wrong sized tires or rims but they are stock 195 65 15. Is it possible for the speedo needle to just get that far "off" over time? Thank u all

 

[kn0x47]

yes the speedo is off. it's more off the faster you go. so it the techometer too. I've verified it both with gps and my scangauge II. Speedo reads 84mph when you're actually going 80

 

[sb001]

yes the speedo is off. it's more off the faster you go. so it the techometer too. I've verified it both with gps and my scangauge II. Speedo reads 84mph when you're actually going 80

That would certainly correlate with my findings-
no wonder these cars last so long- no one is wearing them out by going as fast as they think they are
Surprised no recall or bulletin

EDIT: Interesting:

http://www.caranddriver.com/features/speedometer-scandal

In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.

 

[cmgreensr]

Actually, this was researched before by folks concerned that their vehicles would show higher mileage due to the honda speedometer reading high. It was found that even though the speedometer reads high in all of these, the odometer was spot on.

It's been a long time since I've read up on this because it was a concern to me when I first got my car but honestly, it's probably just helped to keep me out of trouble on occasion.

 

[RedRyder]

Interesting. My speedo always reads 3mph slower than what I’m going, but that’s because of the plus sized rim/tire setup. Whenever my radar detector goes off it shows my speed, and I always compare that to the speedo, & every time it’s a 3mph difference. Same with apps I’ve used on my phone too. I’m pretty sure if I put the stocks back on it would be pretty much exact, but I’d have to try it to confirm.

 

[xci.ed6]

mines off, ~10%, gps or signs tell me, I just drive ~10% over

 

[CDsDontBurn]

Same issue here. Every time I drive by a sign with a radar gun, it shows me as 5MPH over regardless of the speed I'm going.

 

[flowhard]

Mine actually tells me I'm going faster than i really am going. Only by 2 or 3 mph though.

 

[sb001]

Mine actually tells me I'm going faster than i really am going. Only by 2 or 3 mph though.

That is what I am saying mine does as well. When my speedo shows 77, I am actually doing 73. I guess the poster above who says his doing that has kept him out of trouble, has a point...
Now if my speedo was showing SLOWER than what I was actually driving, I'd have a real problem with it.

 

[xci.ed6]

Mine actually tells me I'm going faster than i really am going. Only by 2 or 3 mph though.

tire size? I'm goin 215/45-18,~4% faster speedo

-edit-ditto redryder