Have kind of a strange issue, just looking to see if anyone has any bright ideas that I might not be aware of. My 88 4runner speedometer was always fairly far off since I bought it a few years ago, but it was fast not slow, reading probably 75 when doing 65 with 32" tires on it. I figured that someone had changed out the speedometer gear on the transfer case when running larger tires in a previous life it had. That's ok, I can live with it.

After installing 35" tires my speedometer is pretty close now, maybe a bit slow but much closer. When it shows 65, I'm probably closer to 68-70. Great I thought, its plenty close enough for my daily needs and I don't really speed at all so no need to fuss with it.

I have been roughly tracking my MPG, its a bit low, around 15mpg. I chalk it up to the larger tires for a while.. but its 3.4 swapped and my pickup gets better mileage with a super charger on it. Yesterday I decide to do a long test drive with a fresh tank of gas and reset my trip odometer. My trip odometer mileage was 76 miles, but the google maps trip distance traveled was 92 miles. This is like 21% off! How can it be this far off when my speedometer is within 5-7%? These are all mechanically driven off the speedocable. Is it possible that someone fiddled with gearing in the gauge cluster somehow? Curious what you guys think, I had always assumed that the speedo and odometers worked together on these older mechanical driven ones.

Maybe my trip odometer is just broken, it doesn't like to reset very well, gotta pressure the button a lot for it to fully reset. Ill have to check its milage compared to the normal odo mileage next.

 

The trip odometer and the main odometer should Always record the same mileage, They are directly mechanicly coupled.

It is certainly normal that a rig running larger than stock tires would record less miles than the machine has actually travelled.

Once you observe how many miles you have actually travelled when the odometer has recorded 100 miles, it is easy to calculate the actual miles you have travelled any other time.

The speed indicating portion of your speedometer is magnetic induction coupled, not actually mechanicly connected, and its accuracy is
entirely dependent on the adjustment and strength of a hairspring that tends to return the needle to zero.

So,,,, it is not uncommon to see a speedo and odometer combo that each have errors in different directions from true.

 

Thanks millball for the information. Do you know if the magnetic speedometer / hairspring is user adjustable?

 

The spring adjustment/calibration is the work of a speedometer shop that can apply different specific cable speeds to the speedo head so as to observe and make the tension appropriate.

Not User Adjustable.

 

I took my speedo completely apart. It wasn't too bad. I initially reassembled and put a few drops of very light oil on a few spots. Turned out to be a mistake(created too much resistance for the needle) and had to disassemble to remove the oil.

 

I just use a speedometer app on my phone when I drive my 4 runner on 35s with stock gearing. It uses GPS to calculate speed. Much More accurate.