aztoyman

1990 2wd, SR5, 22RE and 5 speed manual trans. I bought it recently and have been fixing it up. Runs and drives great. Looks to be an original un molested old truck but, I really don't know the entire history.

The speedometer is off 6 MPH according to two different GPS readings. It reads lower than actual speed. If the speedo shows 50 MPH, GPS is showing I'm going 56 MPH.

I have the factory size tires on it. 205/75R14. The speedometer "seems" to be working quietly and smoothly. The rear diff doesn't "look" like it's been into or swapped but, I haven't verified the ratio. It doesn't seem gutless or anything like my 4WD with big tires before a gear change.

I've had speedometers jump around, squeak and flat out stop working but, I never had any read that far off on a stock truck with stock size tires. I've had speedometer cables fail or get dry and cause problems but this one seems smooth.

Unless someone has an idea, all I can think of is to try to figure out the gear ratio to see if it got changed at some point. I don't even know what the 2wd trucks came with from the factory.

Any ideas?

millball

These speedos work by magnetic inductance that is resisted by a hairspring attached to the speedo pointer. There is no direct mechanical coupling to the speedo as there is for the odometer.

Any small change in friction in the system, (Wear, dirt, physical manipulation of the hairspring, ect, ect,) is sufficient to cause the speedo to read incorrectly.

It is not a nuclear grade instrument.

I would ask if the odometer shows a comparable percentage of error, a different error, or no error.???

aztoyman

Thanks for the fast reply. IDK if there is an error with the odometer. I guess I better check. Is there a way to clean the speedo or service it? I've had a lot of Toyota trucks but, never had this issue.

The truck was parked and not run since 2011 so maybe just needs to run for a while or clean it?

I did find that the factory installed diff should be a 7.5" with 3.58 gear set based on the tag on the door jamb. I'll have to check my odometer as suggested to see if it's correct.

millball

If there is little, or no error in the distance recorded by the odometer, the speedo drive gearing is correct and the fault must lie in the speedo itself.

aztoyman

Thanks millball, I will check the odometer and go from there. I have a feeling it's the speedo but I need to verify.

I will be sure to post up whatever I find because dead end threads suck. It may be a few days.

JoeS

If it is 6 mph off at all speeds, then it's a needle out of position on the shaft issue. Pull the needle and reset it at an est'd 6 mph position lower. It has to come off as it was put on the shaft after the speedo face plate was secured to the assembly. It should be just a press on fit. I'm sure there is a YouTube video on how to pull the needle as part of a speedo repair/rebuild.

aztoyman

Thanks for that suggestion Joe. I think it sits at zero but now I'm going to look closer. Being as it reads a consistent and smooth quiet 6 mph low, it might be my best and easiest option. Even though I doubt the rear end was swapped, I'm going to see what the odometer accuracy is. Just for peace of mind.

JoeS

My 1986 came new with 5 mph off. The needle set on the 5 mph mark. It's the last mark, but it's only half the distance of normal 10 mph marker distance. I couldn't get the needle off. I didn't have a way to apply equal pressure on both sides of the needle base as they do with 2 plastic levers to pop off watch hands. I just tested mine to see if I could manually rotate it down and it went with ease. Kept the clear cover off to test it with my GPS and those police radar speed trailers. Adjusted it manually until it was as close as i could get it. Now the needle rests below that last half mark about the same half mark distance lower. There is no mark there, but doesn't matter. It's been reading about right at all speeds for the last 36 years.

aztoyman

Just a little update. I pulled the dash cluster and removed the speedometer to clean it up. It had some really crusty grease around the bearing/bushing where the cable attaches and on the inside where it spins to actuate the needle. @millball, I see how it works like you said. I spun the input with a drill and the non connected part with the needle started moving up in speed.

I used plastic safe contact cleaner and alcohol and got all the gunk out. I put a drop of lube on the bushing, installed it back in the dash without the clear cover and took it for a ride. The needle was at zero by the way. I compared it to my GPS at many speeds locked into cruise. It was still off 6 mph BUT..........I had only been checking up to 50 MPH. Today I went freeway speeds on the frontage road and the faster I went, the more it was off.

It's easy to adjust the needle so I moved it to read about 5-6 mph at a stop and tested again. It was correct up to 40 mph then started being off the faster I went. No mile markers on the frontage where I was so I can't confirm the odometer. I set the zero back after this.

The grease on the cable end is really nasty and rusty looking so I'm going to pull the inner cable out of the housing, flush and blow out the housing and replace the inner cable. It's only $7 and I used the same cable kit to fix my son's 91 pickup years ago when his speedometer needle was bouncing up and down. Hopefully that's all it is. I'll update this thread when I get it done. The cable is supposed to be delivered Friday.

aztoyman

I took the inner speedo cable out today and it's nasty dirty but in good condition. I cleaned the inner cable and flushed out the outer housing and blew it out with compressed air until clean. I cleaned the speedo one more time, lubed the cable and reassembled everything. Took it out for some test runs and no change.

According to my Android phone GPS and my wife's iPhone I'm going 6 mph faster than the speedometer shows. BUT........

I used an online calculator and did some more testing. I punched in my 3.58 ratio rear diff, tires are 205/75R14 listed as 26" diameter, set the cruise at 2000 RPM and at 2500 RPM in 4th gear (1:1 ratio)

At 2000 RPM in 4th gear I should be going 43.23 MPH as per the calculator. My speedo showed just under 45 MPH

At 2500 RPM in 4th gear I should be going 54.04 MPH as per calculator. My speedo showed just under 55MPH

So I decided to verify my tach is reading accurately. When I got home I compared the tach to the one on my timing light and they matched.

So I think I'm chasing my tail. I'm calling it good. I haven't driven in such a low truck in years and it feels like I'm hauling ass. I got this truck to fix up and sell and I don't want to have the buyer getting speeding tickets if they like to drive 5 mph over the speed limit.

Robert m

iTrader: (2)

My '94s stock tire size is 195/75-14. I think 205s were an option but, If you are running 225/75, I'd say that's your problem.

aztoyman

Oops, Robert you got me. It's a typo and I just edited my last post. My tires are 205/75 R14 from the factory as per the label in the door jamb. I still calculated using the diameter listed for the actual Hankook tires I put on.

Here's the calculator I used. https://www.strangeengineering.net/g...io-calculator/

JoeS

Mine came with 195SR-14s (~75 aspect). Yea, they did list 205/70-14 as optional. I think that was on the turbo verison with the mag wheels. I switched to them on my steel wheels after my OEM Bridgestones wore out. Looked better and had better quality tire choices. I didn't notice any speedo error between the 2 sizes. Seems the 195's turn ~791 revs per mile while the 205's turn ~798 revs per mile. Close enough. My speedo needle was just not installed/adjusted right from factory.

Tire data comparison https://tiresize.com/calculator/

My speedo after needle correction. It has about the same distance from the needle to 10 mph as between 10 mph to 20 mph. Just looks lower. Yes, I am just about ready for another 5,000 mile oil change.

aztoyman

Last update. I bought another gauge cluster cheap because the tach was bad. I took the speedo out of it to swap with mine. And then I saw the "snout" where the cable attached was different. Everything else looked the same so I took them apart and built one out of two. I used the magnet side from my speedo since it had the proper cable attachment. I also kept my odometer. I swapped on the needle /pointer side of the newer unit. Basically three sections screwed together. The needle/pointer comes off fairly easy. Just snug, no tool needed.

I couldn't see anything wrong with my speedo. It looked exactly like the other. But it definitely felt different when I moved the needle. The bad one was loose feeling and bounced when it fell back to zero. The other felt like it was "damped". It was smooth and settled back to zero like it hit a cushion.

Anyway, my pieced together speedometer is spot on with my GPS up to about 45 mph and is just a little off at highway speeds.