jrunager

I find that when I'm cruising on the interstate that when I'm doing 70mph I'm actually doing about 67mph according to my GPS. I'm using stock sized tires 185/65/14. I'm now considering using 195/60/15 inch tires now that I have 15 inch rims. Has anyone done this? If so how has it affected your speedometer accuracy?

Thanks,
Jason

fragmare

The same thing happens when I use my GPS and not just in my Honda. The speedo in every car I've ever used my GPS in always starts to register higher than the speed the GPS says I'm traveling once I hit about 40 or so. Honestly, I'm starting to think its the GPS rather than the various cars I've used it in.

Former User

It's a well known Civic problem. Both of my Civics behave this^ way.

redm3turbo

Using GPS, a device is able to calculate a lot of information about a moving object. Using even basic time and location data, a GPS unit can quickly calculate the relative speed of the object, based on how much distance it covered in a given time.

GPS devices are positional speedometers, based on how far the device has moved since the last measurement. The algorithm also uses the doppler shift in the pseudo range signals from the satellites. It should also be noted that the speed reading is normalized, and is not an instant speed.

Speeds are updated at short intervals to maintain accuracy at all times. It uses frequent calculations to determine the vehicle’s speed. For example, using a standard movement per time calculation, if you have covered 80 feet in one second, the GPS device works out and converts that to MPH, which in this case is 55MPH.

GPS speed calculations are more accurate than a vehicle’s speedometer since it is not affected by the same inaccuracies, including the vehicle’s wheel size or drive ratios. It is dependent however on GPS satellite signal quality but with the use of moving average calculations any errors are minimal.

Vehicle speedometers also require calibration to maintain perfect accuracy as general wear and tear, changing wheel sizes, and the manufacturers own ‘erring on the side of caution’ and setting the speedometer to read higher than the actual speed, all contribute to inaccuracies.

Generally gps manufacturers claim a 0.1MPH tolerance with speed readings, however will go as far to say a 0.5MPH inaccuracy is possible, to make allowances for any satellite signal variations beyond their control.

fragmare

Nice info, guys! So I guess my Civic speedo really is off slightly at higher speeds.

Former User

My Civic speedometers read approximately 3 mph faster than normal at nearly all speeds.

jrunager

My 2003 Cobra I sold this past summer when compared to my GPS speed was spot on perfect at any speed even at triple digits. Yes I did compensate for the delayed signal of the GPS.

diatibe24

Yup, mine is off by about 3-4 mph.

cheggie

Yeah, according to a check with GPS on a long stretch at steady speed, mine reads 4-5 mph fast. I wonder if this is also affecting the odometer?

Is there any reasonable way to correct this? I have replaced Ford driven speedometer gears before, does Honda offer different gears for the speed sensor?

fragmare

I believe there's something in the FAQs about recalibrating the speedo. Probably not a cure all, but it might fix SOME people's problems.

Axatax

1) This is "normal" behavior for a Honda speedometer.

2) You can remove the needle and replace it while clocking the vehicle to a GPS.

Obviously, breaking down the cluster, removing the needle, and doing this while driving (esp. this last point) require the utmost care.

3) There is a device that can be placed inline with the VSS that will allow you to dial-in a correction to the speedometer. I have seen this mentioned on European forms where a UK spec'd car is brought into W. Europe and vice-versa. Don't know anything else about this.

Checkmarks

Jason

Changing your tires to the next larger size 195's will correct your speedometer. But you will manipulate your fuel economy's. Do a test for the forum. Check your gas mileage for say 500 miles correcting for the inaccuracy your spedo will report today. Check the mileage against your gps not your car odometer. Change the tires to the 195's and run the same patch of ground. The issue is you change the rpm the engine runs when manipulating the size of the tires. This can work ether to benefit you or cost you more fuel to travel the same distance. It will change the load on the motor. Bigger tires more load lower rpm but possible out of your 'power' band. There are causes why Honda uses 185's

EyePhoto

Every car/truck I have ever driven and checked speed with GPS shows the speedo reading high. My 2000 EX shows 108 when I'm really going 100Km/h. However, the odometer seems pretty close. hanging the gearing at the cable with an adjustment gearbox will fix the speed indication but mess up the odometer.

Former User

I should have mentioned that this^ is true for my Civics too.

jrunager

I'm guessing a taller tire will help my mileage on longer stretches and probably hurt accelleration from a stop. I recall my brother's 87 Mustang LX having 2.73 gears which are really tall gears and this allowed him to drive 60mph at about 1500rpm (about 28mpg haha). I think the 195/60/15 is only 3% taller than the 195/55/15 so I don't think I'll really notice much difference especially after I get a torquey B20 swapped in.