Danny.T

Average, when I'm doing 55 MPH, my GPS shows 51 MPH - The differences vary depending on what the speed I'm doing, like 40-80 mph then the differences on GPS range 3-6 miles/h slower than my Speedometer. Anyway can I adjust it? I did remove the Dashboard Displays to replace the bulbs a couple of times.

My car: 99 Accord 4-DR V6 EX

Your help is much appreciate

Danny

TheMuffinMan

iTrader: (1)

Most cars have inaccurate speedometers.......over time they just get more inaccurate as components age.

If you replaced the wheels and/or tires with non-factory sizing it will throw the speedometer off as well. If you're incredibly concerned over the difference you can buy something similar to the Modifry Yellow Jacket ( http://www.modifry.com/products/yj/yj_faq.htm ) but would need something more universal.

Targa250R

Completely normal. No inexpensive way to adjust it accurately. Just ignore it.

Every factory Honda speedo I have seen reads approximately 7% high.

redbull-1

Agreed for the most part. My current 2009 Accord seems accurate; for now. It pretty much matches my Garmin GPS speed readings.

My previous 2 Hondas always read higher than actual, based on comparing to radar sign's speed readings.

Targa250R

The class action lawsuit settlement in December '07 for high odometer mileage accrual may have prompted them to address that issue

redbull-1

That actually was in the back of my mind.

Balwin

Completely normal. Most cars read a few percentage over what you are really going. The gps has the true speed.. notice if you go by one of those radars that tell you your speed, the gps is always spot on.

Danny.T

Very appreciate for your inputs...

Danny

"A"

You can change the aspect ratio of your tires or combination of wheels/tires to give you more accurate speedo reading.

My 90 Mazda 323 has OEM 155/60 -13 tires, I changed them to 175/70-14 Miata alloy wheels/tires, better traction, more accurate speedo reading.. not sure if it effects odometer reading though.

andyz

GPs can be wrong too. There is something known as Geometric Dilution of Position that occurs when the angle between the satellites you are using becomes too wide. When the angle is wide, extremely small errors due to atmospheric conditions can be magnified.

Some GPS use the known position of ground based stations to correct the errors. If you find the right menu in your GPS, try to monitor the EPE or Estimated Position Error carefully.

You will probably notice that it changes as you move, reflecting different configuration of satellites. Like if it says 500 ft, then your position could be plus of minus 500 ft of your displayed coordinates. This definitely could throw off speed readings.

Real accurate speed measurements probably should be taken with a radar gun or radio telemetry , but certainly not from 50 miles up.

IMHO.

TheMuffinMan

iTrader: (1)

I'll take a GPS reading over a 19 year old speedometer

*edit*

Not saying you are incorrect, but when comparing 10+ year old electronics to the latest technology, the GPS is probably going to be more accurate providing you are in a clear signal area.

gandalf

A few months back there was a post in Popular Mechanics and it mentioned that the Speedo was calibrated to the size tires that originally came on the car. Mine are P195R6015 I used my GPS in my 42 mile commute the other day and it was Right On. They said that it was calibrated to the Rpm's of the tire/size, there is the possibility that it might affect the ODO too

G

chikin pickle

The speedometer is driven by a gear, that is driven by the final driven gear in your differential. Electronic speedometers have a hall effect sensor to measure the angular velocity of the shaft, older mechanical ones use a rotating cable and a magnetic field.

Our electronic speedometers use the angular velocity of the shaft, with the known gear ratios, and outer diameter of the factory sized tires/rims (wheel) in order to determine the rotational velocity of the wheels, and then they can calculate the instantaneous speed of the car.

The odometer uses the instantaneous speed in order to calculate distance traveled at any given speed for a period of time. If your speedo is off, your odometer will be inaccurate by the same margin of error.

If you change your final driven gear ratio (more of a rear wheel drive car type of thing higher final driven gear ratio the more torque multiplication), or wheel size you will cause the speedometer to become inaccurate. Also, as new tires wear the speedometer will become inaccurate as the tire diameter slowly shrinks.

If your speedometer if off, the cheapest way to bring it closer to accurate is to increase/decrease the tire size accordingly when it's time for new rubber. My speedometer was off by around 4-5 miles an hour with 195/60/r15 falkens. When I bought new tires I bought 195/65/r15

(for those of you unfamiliar with tire sizing the first number is tread width in mm, the second number is sidewall height [as a percentage of treadwidth] and the final number is the rim size, the R means it's a radial tire by the way, not rim)

and now my speedometer is spot on accurate.

Targa250R

Not in a FWD or 4WD Honda. The speedo/odo drive gear is either pressed onto or machined into the differential housing inside the transmission case, therefore the VSS only measures the RPM of the diff. Since the diff RPM is obviously equal to the RPM of the driven CV axle(s) and attached wheel hub(s), the only two ways to affect the ratio of the pulse measured by the VSS to the actual ground speed of the vehicle are to 1) change the tire size, or 2) change the speedo gearing.

Changing the final drive ratio will not affect the readings of any of the instruments.