Calibrating Speedometer

[PorkchopSandwiches]

My speedometer reads 10% faster than I'm actually traveling. (I.e. I have to go 66 according to the speedometer to be going 60 on the freeway) I'm pretty sure my tire size is factory spec. I tested speed with GPS, radar sign, and mile markers and they all agree that my speedometer is fast. Is there a way to recalibrate? I have the automatic 4spd trans btw

[Zues Marine]

the way i did it;

 

pop off the plastic cluster cover on the gauge, pull out the speedo needle about .25" (this 'unlocks' the needle, pull a bit more and you it'll pop all the way out)

 

hook everything but the plastic cover back up so you have access to the needles while driving

 

set cruise at an easy, gps-verified speed, like 60mph..

 

align needle to 60mph, press to lock (or reinsert) needle.

 

go home. reassemble. done.

[PorkchopSandwiches]

I'm not sure that would get it to be accurate though since it's off by a percentage not a set number - so if its off by 7 mph at 70 and 3 mph at 30, adjusting it back 7mph to be accurate at 70 would make it read 4mph too slow at 30. If that makes any sense lol If they're digital speedometers I thought maybe they could be reset or reprogrammed

[Zues Marine]

i guess that makes sense but the way i did it above worked fine off my little Garmin...for me anyway

 

what size are your tires?

[PorkchopSandwiches]

i guess that makes sense but the way i did it above worked fine off my little Garmin...for me anyway

 

what size are your tires?

 

I'm pretty sure they're 205 55 R16 (judging by a zoomed in cell phone picture of my car )

[johnegg]

there is no way to calibrate the speedo, other that the above.

 

so there are 3 possibilities:

1/ it is stock / factory and has been wrong since it was made.

2/ the tires are 10% larger than the stock tires.

3/ the speedo has been replaced with the wrong one.

 

what is the top speed on the speedo, 120 or 140?

[David E]

I had similar problem on my 99 Outback. Speedo was reading 5% higher than I was traveling (tire diameter too small). Double checked with GPS on speed and distance traveled. Fixed when I upgraded from 15 to 16" wheels by using 5% larger diameter tires. I made a chart - attached - when I was figuring out what I wanted. Ended up with 225/60-16. Only rub on turns combined with up a ramp.

TireWheelSizes.pdf

[PorkchopSandwiches]

there is no way to calibrate the speedo, other that the above.

 

so there are 3 possibilities:

1/ it is stock / factory and has been wrong since it was made.

2/ the tires are 10% larger than the stock tires.

3/ the speedo has been replaced with the wrong one.

 

what is the top speed on the speedo, 120 or 140?

 

I went and checked to be sure about the tires. The door jamb sticker says 205/55/R16 89H and the tires on the car are 205/55/R16 89T so the size is identical.

 

The speedometer goes up to 120. It's in a 98 Legacy GT. Is that the wrong one??

 

Here's a pic for reference

 

http://i.imgur.com/8SO2mgK.jpg?1

[David E]

Did the math:

205/55-16 = 24.88" diameter

* 10% = 27.37 diameter needed

For 16" wheel:

265/55 (27.48")- these are some pretty wide tires to stay at the low aspect ratio

245/60 (27.57")

225/65 (27.52")

Note to get my speedometer to read correctly needed the ~27.5" from 225/65-16

[johnegg]

Note to get my speedometer to read correctly needed the ~27.5" 225/65/16

that is 1.2 inches larger than stock outback tires.

i doubt, but do not know, if you can run those on a GT without rubbing.

but i know you would have a better chance of running them if you swapped in outback struts and springs.

 

by my math, that size tire would in fact increase you actual speed by 10.6%. so your speed would go from 70 to 70.4 mph.

 

however, if those tires are too large, you could in crease your speed from 70 to 74.0 mph by running stock outback tires 205/70/15, (205/65/16 = 74.54mph and are only .2 inches larger than outback.)

 

either of these will erase more than half of the error, but not all. but i think both will require taller struts.

[PorkchopSandwiches]

Althought I appreciate the math and thought that went into calculating oversized tires, I don't really want to go that route. The tires on my car are practically brand new and I don't want to lift it or put beefy tires on a GT. I called a local large subaru dealer to see if they'd had that problem before and they said they'd never heard of it. When I'm home for the summer I'll hook it up to the Subaru computer and see if there's anything that can be adjusted or changed on there. If not, I'll probably just leave it and get used to what speed should be what.

[johnegg]

there is no way to calibrate the speedo, other that the above.

 

so there are 3 possibilities:

1/ it is stock / factory and has been wrong since it was made.

2/ the tires are 10% larger than the stock tires.

3/ the speedo has been replaced with the wrong one.

 

what is the top speed on the speedo, 120 or 140?

 

there is a 4th possibility, maybe.

 

the trans has been replaced with an outback trans.

this would account for some , but not all, of the error.

[Zues Marine]

low tire pressure?

 

less air smaller diameter..

 

??

[spooln30]

There should be no reason for it to be off other then the trans or sensor was replaced with a outback version. I run 205/70/15 outback wheels and winter tires and my speedo would say 60 when my GPS said 62 so it wasn't off by much.

[johnegg]

i don't remember the year, but i think subaru had a court case about this. an owner sued because not only was the speedo not within accepted industry standards, but his odo was reporting his total miles incorrectly.

 

example: at 100k miles of actual driving the odo read 110k miles. this effectively depreciates, undervalues, the car.

 

btw: the speed sensor has nothing to do with it.

the speedo units are all the same. this saves suabru money.

but they change / alter how they work for the lego L, GT, Outbacks by adjusting the tire size, the speedo driver gears in the front diff, and the face plate on the speedo it self.

 

riddle me this batman,

the GT and outback speedos go to 120 mph.

the lego L and brighton go to 140 mph. (probably impreza too)

huh?

 

all part of subaru's ''let's make this speedo work on that car'' program.

[BarManBean]

Did the math:

205/55-16 = 24.88" diameter

* 10% = 27.37 diameter needed

For 16" wheel:

265/55 (27.48")- these are some pretty wide tires to stay at the low aspect ratio

245/60 (27.57")

225/65 (27.52")

Note to get my speedometer to read correctly needed the ~27.5" from 225/65-16

 

^^Pretty sure this math is not as useful as possible. You need to use outside tire circumference (rolling circumference) instead of diameter.

[johnegg]

^^Pretty sure this math is useless. You need to use outside tire circumference (rolling circumference) instead of diameter.

 

the math is not intended as an absolute. but as a percentage estimator.

 

as you know, different the same size tires by different manufacturers will measure a different circumference.

 

and if you ever are shopping for larger tires to match a speedo, i would not go for the max, 100% match. because if your new tires happens to be larger than you thought / want, you speedo will read too fast.

[PorkchopSandwiches]

low tire pressure?

 

less air smaller diameter..

 

??

 

Tire pressures are all where they should be.

 

There should be no reason for it to be off other then the trans or sensor was replaced with a outback version. I run 205/70/15 outback wheels and winter tires and my speedo would say 60 when my GPS said 62 so it wasn't off by much.

 

Doing the math on 205 55 R16 vs 205 70 R15 (Legacy GT vs Outback) shows that the speedometer would only read 5% faster with an Outback speed sensor - roughly what you were getting there. I think it's gotta be something other than outback parts if I'm seeing double that difference.

[johnegg]

again, the speed sensor has nothing to do with it.

they are all the same.

 

but what the hell, buy a used speedo or cluster for a GT and try it.

an outback cluster will also work but the wire connector on the back is wrong. so getting one from a pick and pull with the wire pigtail would be ideal.

 

it may be possible so swap the back plate, but i have never tried it or heard of it being done.

[GEORGEROV]

Most manufacturers have a percentage error built in to the speedo , usually about 5% fast , so that they can't be blamed for you breaking the speed limit when the gauge says you were not . Also worn tyres can alter effective diameter of wheel by say 15mm's and this can add another 2% error .