Speedometer is inaccurate? How to recalibrate?

[Sizzlinninja]

My speedometer is extremely inaccurate. For example, with the speedometer reads 75mph, I'm truely going 55mph. And when the speedometer reads 105mph, I'm actually just going 80. How do I go about fixing this? Thanks.

(1988 Toyota Supra non-turbo)

 

[Maple191]

are you wheels and tires the original size.

 

[AbsoluteSpeed]

That is too big a difference to be the tire size.

Has the speed been off the entire time you have owned the car?
Sounds to me like someone swapped the differential but not the transmission speedo gear.

Your car would have come with a 4.30 diff (assuming it is LHD) check the ID tag and see if it is different.

 

[Backlash2032]

When I went from a 4.30 to a 3.91 it was only like a 5 mph change IIRC

Sent from my terrible HTC Rezound using Tapatalk. Don't buy HTC...

 

[Orion ZyGarian]

I wonder if someone swapped in the wrong sized speedo gear for the transmission

 

[AbsoluteSpeed]

When I went from a 4.30 to a 3.91 it was only like a 5 mph change IIRC

You’re right. I just messed around with a speed calculator and the worst it will change ìs around 10mph.

 

[Dan_Gyoba]

There is a tension adjustment on the speedo that can be changed. If your odometer isn't bad (I don't think that they're ever 100%) just the speed, then you can make a change there.

The speedo needle is pushed up when the cable turns, which slips, and pushes against a spring. Make the spring tighter, and it reads lower. Make the spring looser, and it reads higher.

When I swapped out my differential, I had a 3.909:1 in where it wanted a 4.300:1. If my speedo read 100km/h I was going about 110. I never adjusted it then, I just got used to never having my speedo read more than the posted speed, which kept me out of ticket range.

 

[titanjgg76]

Did you ever figure out what was causing this? Mine is doing the EXACT same thing!

 

[suprahflyg]

mines doing the same thing but it depends on how i accelerate. did you figure it out?

 

[Dan_Gyoba]

The speedo in my '90 was doing some odd stuff when it was really cold, accompanied by a buzzing noise. It would spin up a more or less random amount. It only did so while cold.

As I said though, if you remove the speedo from the instrument cluster, there's a lever which adjusts the tension on the spring. If you increase the tension, the speedo reads lower. Decrease the tension and it ready higher. There's actually quite an adjustment range. Kind of a pain since you can only adjust while it's out of the car. I imagine that places where this is done have a calibrated spinning cable so that they can adjust it.

 

[titanjgg76]

No...I swapped my transmission for a newer one and it still does the same thing. Next is a differential rebuild or replace with LSD (Cusco maybe). The RPM and Speedo reading is accurate as far as they relate to each other and how fast I SHOULD be going. However, GPS still says I'm going about 20 mph slower than what speedo reads.

 

[Dan_Gyoba]

You've got to get correctly sized tires, or deal with it.

If the engine RPM/gear/speedo agree, then that's it.

The odometer will remain out, but you can adjust the speedo reading, which will help you avoid tickets.

If you figure out the difference in your tire size, there are also offroad 4X4 places that might be able to get you a speedo gear adjuster, since oversize tires on trucks is somewhat common. Maybe you can find an adjuster for a Toyta R series transmission.