Calibrating the Speedometer

[BigWheel]

I searched for this and found nothing so here goes. With talk of changing tires to a larger size and needing more options there are often a lot of questions about having an accurate speedo. Has anyone tried to calibrate their speedo for a new size tire? I talked to a fellow Mustang nut today who will be recalibrating his speedo due to new wheels. He asked me why I couldn't do the same on a Festy. Thoughts?

[FB71]

I searched for this and found nothing so here goes. With talk of changing tires to a larger size and needing more options there are often a lot of questions about having an accurate speedo. Has anyone tried to calibrate their speedo for a new size tire? I talked to a fellow Mustang nut today who will be recalibrating his speedo due to new wheels. He asked me why I couldn't do the same on a Festy. Thoughts?

depending on the year Mustang, he's either changing the driven gear on the trans end of the speedo cable, or using a pulse modifier on the trans speed sensor (electronic speedo). Or, if he has a 2005 or newer Stang, he can change the software in the PCM to correct the speedo. The Festiva has a plain old mechanical speedo, but there are no different tooth-count driven gears available for it. That's why we can't correct our speedos.

[BigWheel]

depending on the year Mustang, he's either changing the driven gear on the trans end of the speedo cable, or using a pulse modifier on the trans speed sensor (electronic speedo). Or, if he has a 2005 or newer Stang, he can change the software in the PCM to correct the speedo. The Festiva has a plain old mechanical speedo, but there are no different tooth-count driven gears available for it. That's why we can't correct our speedos.

I figured as much but had to ask. There are so many cars out there with parts that are exchangeable you would think we could find speedo gears to work. Makes sense though and thanks.

[ohsoslow]

all i did was remove my speedo needle and turn it till it was correct

[muscle_Car1]

^^^
Well its off by a percentage not specific measurement so that has an error factor also

[BigWheel]

all i did was remove my speedo needle and turn it till it was correct

Huh??? Sounds like a Flintstone moment :-)

[getnpsi]

I just used an online tire calculator and know to compute mileage that way.
I'm running huge tires on my aspire. I'm off 8.9%. when it says I am going 65 I'm speeding by quite a bit. I usually dont drive the car that fast, eats the mpg's.

I thought some speedo shops are able to mount some sort of inline gear reduction unit. Probably upwards of 100 bucks installed. ( just do what i do in other words)

[ohsoslow]

Huh??? Sounds like a Flintstone moment :-)

ie at 40 mph the speedo was showing 43 so i marked it and turned it back a hair. i kept doing this till it was correct.

its kinda nice having your godfather as a state trooper....also clocked my golf cart at 43mph

[packrat427]

I upgraded form 12's to 175/70/13 and according to my GPS which is pretty reliable , it says my speedo is on the money against the GPS.

I always thought it was off and I adjusted numbers for mileage , but I was wasting my time.

Jim (FB71), what is that little lever for on the back of the cluster that in front of where the cable attaches to the back , I always thought it was to adjust for minor differences in speedo calibrations.

[FB71]

Doug, you're correct. But it only compensates for a 1%-2% difference.

[skeeters_keeper]

all i did was remove my speedo needle and turn it till it was correct

That will only work for one speed. i.e., you can make it just right @ 40mph, but you will still be off by a little @ 65, and a little in the other direction @ 15.

with 175/70/13's my GPS said I'm off by about 6 mph. I messed with the adjustment on the back of the cluster a little, and its worse. Now I tend to drive off the tach a little - I know @ 3k in 5th I'm going a little under 80, but my speedo is reading around 70. I just use the rpm to mph calculator I wrote a little while back to know aprox how fast I'm going, and checking it against the GPS, its almost dead on.


Alternatively, I have considered modifying one of those "cluster background" by scaling the cluster readout. With my speedo off by almost 10mph @ 75mph, I should be able to have a speedo that reads correctly up to around 95 - 100 mph. The only reason I haven't is that it seems I change tire sizes every few months, so every few months I would need to swap out cluster backgrounds if I wanted to keep the readout accurate.

[ohsoslow]

i set it for 50 mph (i rarely drive it anywhere)

it was 2 mph off at 10 but who cares about 10 mph