View Full Version : Gear cutting for a speedometer

11-07-2012, 03:03 AM
Anyone ever made a replacement gear for a speedometer to compensate for different tire size? I have an outback that the speedo is off by about 3% and can't find a source for different gears to change the calibration. Just wondering if this is even a practical project (learning experience) or a total waste of time.

Opinions anyone?



The Artful Bodger
11-07-2012, 03:43 AM
Anyone ever made a replacement gear for a speedometer to compensate for different tire size? I have an outback that the speedo is off by about 3% and can't find a source for different gears to change the calibration. Just wondering if this is even a practical project (learning experience) or a total waste of time.

Opinions anyone?


David, I would first confirm if the speedometer is driven by a cable, which seems unlikely.

Modern speedos are usually(?) electronic and somewhere there will be a divisor programmed into memory that can be changed, you just need someone who know how to do it. (Outback, I assume thats Subaru?)

11-07-2012, 06:28 AM
Industry standard used to be 5%, and some production cars were more than that. The Code of Federal Regulations CFR 49 allows some slack due to different sized tires, tire wear, inflation pressure variations, the deterioration of springs and permanent magnets in old-type speedometers, and (possibly) the range of practical ratios you can cut to mate with a fixed number of teeth on a steel output shaft, which was normal before removable gears came into vogue.

J Tiers
11-07-2012, 09:25 AM
Have not done it, but I need to.

I have a car with a non-working odometer, although the speedometer works. It's a known problem (Volvo 240DL) but the gears do not seem to be available.

To the point about the new vehicles, the Chevy truck has it all electronic, but the wagon is old enough to vote and drink.

Before you kindly explain that I should buy a new car, I should point out that the wagon will accept a 10 foot length of board, pipe, or whatever INSIDE with the doors closed, which is considerably more than my truck, or most other pickups, will do.

As for the CFR and 5% tolerances, that may not play out too well when the cops nab you for 1 mph over the limit.....which is 2% at 50 mph..... When they get a bug in their ###, they get fussy and actually start to ticket for that small a "violation". generally in "speed trap" towns.

11-07-2012, 12:17 PM
Artful: yep, it's cable driven. The 2000 (generation 2) subaru outbacks are electronic. Much simpler to change. Mines a 98, but it gets close to twice the mpg of my Dakota so I find myself driving it a lot more.

TRX: I can deal with it if I have to, it's just an annoyance more than anything.

J Tiers: mines not old enough to drink but getting there. And she hauls almost as much as my truck just doesn't pull a trailer. Speed traps are my main concern, been pulled for 1 mph over before. The joys of driving in VA with TN plates.

Guess I need to do more research.

Thanks Gents

11-07-2012, 12:23 PM
I've never seen a stock speedo setup that didn't over-read by a good margin in order to protect drivers from prosecution.

Tyres vary quite a lot in rolling circumference, even when nominally the same designation.

GPS is an easy way to get an accurate speed reading that isn't going to be affected by changing wheel/tyre combinations.

11-07-2012, 01:02 PM
I put a new set of higher profile load-rated tires on my truck and my speed was off (low reading) by nearly 9%! I could have got another set of gears for the gearbox end that will drop this to 5%, but that's almost as bad. At one point I found an adjustable gizmo that supposedly worked in-line with the existing cable, but I never got around to it. My tires are nearly worn out now (14 years!) and with all the rubber loss its a lot better, but when I change them (a few months, maybe) it will all start again. I'll probably just use the tach again.

Heck, I even thought of painting 5 scales on the tach - one for each gear :)

11-07-2012, 01:58 PM
My Suzuki has a little lever behind the speedo (mechanical) to adjust displayed speed...

11-07-2012, 03:09 PM
If you are unable to find a suitable gear, you can use a GPS for speed in a pinch.

The Artful Bodger
11-07-2012, 04:31 PM
In really, really old, (i.e. old enough to have children who can vote) vehicles it was sometimes possible to reach in behind, pull out a speedo light and poke your finger in to touch the rotating aluminium cup/disk attached to the needle. It was just a matter then of carefully moving the cup with the little finger until the needle hit the stop then a little more to 'adjust' the needle. Be careful not to damage anything and take rings and watch off (you dont want to get anything metallic across the terminals of an amp meter!).

However, I very much doubt your Outback would be like that.

11-07-2012, 04:48 PM
There was a speedo calculator app on my phone along time ago that helped figure out speedo changes for different reasons like gear change,tire change ,so maybe see if any phone apps could help also.

11-07-2012, 07:09 PM
Calibrating your speedo using a GPS makes sense as the accuracy is very good.

But ..................................

%-age error seems to assume the error is linear (same %-age ove the whole range) but it may not be. It may vary at various speeds - ie it is non-linear. It may be a sum of linear and non-linear.

Plotting it out on graph paper will soon show where/what the errors are - better than a calculator.

Using the graph, extract "speed actual" (from the GPS) and speed reading (from the speedometer) and construct a table from it.

Its a simple but very exact method.

Use/remember the table readings on your speedometer.

Gary Paine
11-07-2012, 07:56 PM
In my earlier engineering years making instrumentation for cars, all the speedos were mechanical, and some tachs too. We converted with model changes through the 80s to electric, but the mechanicals all used the same mechanism. It consisted of a rotating magnet acting on a drag cup attached to the spindle that was spring loaded against a peg for 0. The mechanism was calibrated by starting with a higher than necessary magnetic charge and then knocking down the magnetism with a bucking field a bit at a time until the pointer was within the tolerance band. The pointer was a press fit onto the spindle. If this sounds like your mechanism, you can probably pull the instrument out and get it to the point that you can carefully prevent the cup from rotating and then twist the pointer a bit. Because it is an analog instrument, realize that it will be pretty linear and you must take that into account. A 4 mile per hour error at 60 would likely only be a 2 mile per hour error at 30. Also, this is simply setting accuracy at one point. The gain term has not changed by this adjustment.

michigan doug
11-08-2012, 06:51 AM
You could just drive 5 under the posted speed limit.

Would probably improve your fuel economy a touch too.

Nothing wrong with fixing it the right way though.