GPS speed/mileage vs. dashboard

[lhrocker]

I just picked up a 2002 Legacy GT and I love the car. I noticed, thought, that the speed/mileage on the cars speed/odometer doesn't match the GPS. Compared to the GPS, the car's speed is slower, but the miles are higher.

 

I had a 97 Legacy where the speed was spot on with the GPS, but the mileage was a little less.

 

Is this normal, as I normally use the GPS to track my mileage for expenses?

[johnceggleston]

how far off is it?

 

what size tires are on the car? this makes a difference.

 

what is the stock size tire for the car?

 

almost all speedos are a little off, usually showing faster than you are actually going.

[lhrocker]

I'm pretty sure the stock tires are 205/55/16. I will check when I get back to the car. My older legacy had 14's, but once I swapped to 16's, the speedometer matched up.

[SuBrat84]

I would definitely check your tire sizes on this one.. From what I understand typically they calibrate the speedometer to be reading 8-12% faster than what you're actually going. (10% is a good way to calculate it. E.G. 60mph indicated is more likely 54mph actual. DO NOT USE THIS AS AN EXCUSE TO SPEED!) Supposedly this doesn't affect the odometers. Also, GPS don't accurately account for changes in elevation. Are you testing on relatively flat roads?

[johnceggleston]

while you are at it check the trans code number on the ID tag under the hood and see if it matches up with the trans code number on the bell housing at the starter. the speedo gears are matched to the stock tire size. if an outback trans was installed the speedo gears would be different since outbacks have a different tire size.

 

how far off is your speedo???

[lhrocker]

I would definitely check your tire sizes on this one.. From what I understand typically they calibrate the speedometer to be reading 8-12% faster than what you're actually going. (10% is a good way to calculate it. E.G. 60mph indicated is more likely 54mph actual. DO NOT USE THIS AS AN EXCUSE TO SPEED!) Supposedly this doesn't affect the odometers. Also, GPS don't accurately account for changes in elevation. Are you testing on relatively flat roads?

 

I checked the tires and they're a set of Continental 205/55/16's. The speedometer reads about 2mph faster than the GPS. My old Legacy had a set of Hancook 205/55/16's and they spedometer was the same as the GPS. I've had the same Garmin GPS since I owned my 97 Legacy bought 3 years ago.

[johnceggleston]

IMO, 2 mph is close enough, especially since you are driving slower than the spedo says. 2 mph at 60 is ~3.3% at 70 it's ~2.8%.

 

i have aged out of driving on the edge, speed wise. i set my cruise control 7 mph over the limit on the hiway and forget it. this allows for a 2mhp variation in the cruise going up hills. any error in the speedo is in my favor and i'm always, hopefully, still less than 9 over the limit, which around here is considered acceptable. gone are the days of driving 10 -20 over.

[unibrook]

My 2001 Forester speedo shows 2 mph faster than my actual speed.

[johnceggleston]

My 2001 Forester speedo shows 2 mph faster than my actual speed.

 

his too.

[rverdoold]

My impreza speedo is about 9km (6 m/h) to fast compared to two GPS instruments. The odo is also more, about 2 km per 100 km. The tires i have are 205-55-15 and are far from standard which are 175-70-13 i think (1.6 AWD).

However there is a tricky thing with GPS speedo. A GPS can only indicate accurate speed in a straight line with continious identical reception. So if you have a curved road it will most likely indicate less distance in the same time results in lower speed as well.

Next tires i will get will be 205-60-15 or even 195-65-15 if they fit with the standard suspension.

 

On our off-road camper it is hooking up to pc and filling in the tire size and it is spot on.

I have no idea if it is possible to change gearbox VSS pulses linked to the tire size.

[lhrocker]

Well I'm comparing apples to apples with the GPS. The speedo matched my old car running the same routes to work. With the new car, the average speedo thru the same route was shown fast on the dash than the GPS. No real issues - I was just curious, and it would be nice if the cars speedo matched the GPS, since there's no way to get the GPS to match the dash.

 

 

My impreza speedo is about 9km (6 m/h) to fast compared to two GPS instruments. The odo is also more, about 2 km per 100 km. The tires i have are 205-55-15 and are far from standard which are 175-70-13 i think (1.6 AWD).

However there is a tricky thing with GPS speedo. A GPS can only indicate accurate speed in a straight line with continious identical reception. So if you have a curved road it will most likely indicate less distance in the same time results in lower speed as well.

Next tires i will get will be 205-60-15 or even 195-65-15 if they fit with the standard suspension.

 

On our off-road camper it is hooking up to pc and filling in the tire size and it is spot on.

I have no idea if it is possible to change gearbox VSS pulses linked to the tire size.

[rverdoold]

Of course would be nice to have them matched, also curious how to. I know on our off-road camper we can read the pulses from the VSS and then simply calculated with the tire diameter the revolutions per km and there you go. It should be possible.

[johnceggleston]

My impreza speedo is about 9km (6 m/h) to fast compared to two GPS instruments. The odo is also more, about 2 km per 100 km. The tires i have are 205-55-15 and are far from standard which are 175-70-13 i think (1.6 AWD).

However there is a tricky thing with GPS speedo. A GPS can only indicate accurate speed in a straight line with continious identical reception. So if you have a curved road it will most likely indicate less distance in the same time results in lower speed as well.

Next tires i will get will be 205-60-15 or even 195-65-15 if they fit with the standard suspension.

 

On our off-road camper it is hooking up to pc and filling in the tire size and it is spot on.

I have no idea if it is possible to change gearbox VSS pulses linked to the tire size.

 

stock impreza tire size, 93 - 99, in the us is 185-70-14, which is very very very close to the 97 legacy GT tire size of 205-50-16.

 

if you google miata tire size calculator you will be able to plug in your size and the stock size and get the difference.

 

short of swapping your speedo drive gears in the trans , or the whole trans, there is no way to adjust your speedo. the only other option is to swap your speedo, but you are limited to stock subaru tire sizes in a very limited year range.

 

the 3 stock tire sizes for the late 90s legacys are 185-70-14, 195-60-15, and 205-50-16. they are all very close in diameter and circumference so they are interchangeable with out too much error on the speedo, (the 16 is the smallest by 3/8 inch in circumference which is about 1/2 of one percent. this equalls 0.3 mph @ 60mph). in the us in the 90s, imprezas used the same size as legacys, usually, i think.

 

if your odo is off more than a few percentage points, your tire size is off. the odo counts wheel rotations to make up a mile, smaller or larger tires will change the actual distance in a given number of rotations.

Edited August 3, 2010 by johnceggleston

[rverdoold]

stock impreza tire size, 93 - 99, in the us is 185-70-14, which is very very very close to the 97 legacy GT tire size of 205-50-16.

 

if you google miata tire size calculator you will be able to plug in your size and the stock size and get the difference.

 

Miata calculator is a brilliant tool. want a bit higher tire as well so most likely will go for the 195-65-15 just will have to see if they fit the steering as well.

[lhrocker]

So if I wanted to get a tire size that would "slow down" my speedometer, would I get a slighty larger tire or a smaller tire? I'm running stock now.

[johnceggleston]

So if I wanted to get a tire size that would "slow down" my speedometer, would I get a slighty larger tire or a smaller tire? I'm running stock now.

 

a larger tire will make you go farther / faster than the car / speedo thinks you are.

 

i would check out a tire size calculator and keep trying larger tires until you find the one that matches your speedo error, about 3.3%.

[lhrocker]

Thanx John