Speedometer reads too high

check the gear ratio will be pretty easy if you just jack the truck up so the rear axle is suspended. Then mark with chalk on the drive shaft and a tire. Have some one spin the tire and you can count the ammount of times it takes the drive shaft to rotate per one rotation of the tire...won't be perfect..but it'll give us a better idea..


that said..my 96 came with 31x10.5r15s..think it was an option for the SR5 though..can't remember..but my RPMs for 65 where about 2500ish and i have 4.10 diffs.

 

B02A is the diff code...
That's a 3.58 with a 7.5" ring.

With a 3.55, you'd be running 2500 at 65, so if you're hitting 2330 at 55, that would put you at about 2750 at 65, which would be about right for a rear end in the 3.91 ballpark.
2750 to 2500 is only about a 10% difference.... the tire change subtracted about 10%, so there's no error there.
Back to square-1.

Manually check the ratio as BD describes. If the number seems to be way off, either double or halve your count. Since we know you're somewhere in the 3-5 range, if you only count 2.x, spin the tire once more, if you're at 7, then divide by 2.

If you only jack up one tire, you need to run two revolutions, and that's a better way to go since jacking up both you would need to have two people to spin both tires.

It was an optional size.


For reference:

 

You guy are over thinking this. The B02A ratio of 3.58:1 would really sap performance of a the 2.7L with 31's. A regear is a reasonable solution especially if the truck has enough power now to push the big wheels.

Toyota put 4.56s into trucks that came stock with 31s and the 4 banger.

As for % errors, on 1st gens, a 5% under reported error was typical, where 65 indicated on the speedo would be closerr 62 on the GPS. We have seen this over and over again on trucks that came stock with 31s and 4.10s.

If the truck originally had 3.58s and now has 4.56s, this could easily explain the speedo error now being seen.

 

According to the Electrical Wiring Manual, on the 4 cyl. engine, the speed from the speed sensor in the speedometer is one of the inputs fed into the control until to set ignition timing. So now I'm wondering if this is negatively affecting engine performance.

 

As to the gear ratio, I was away for awhile and am just now getting back to this. I'll try to find out using the method you've suggested. Thanks

 

Jacking up one tire, 7 turns of the drive shaft give about 3 1/8 turns of the tire. 1 turn of the tire gives about 2 1/4 turns of the drive shaft
That's a ratio of about 2.25. What does that tell us?

 

If I'm understanding you right, taking the 2.22-2.25 ratio gives a gearing in the 4.56 range. Digiratus, you said this would explain the speedometer error, right?

On the ignition timing issue, I'm going to take a reading with my scanner at highway speeds and see what that tells me.

 

The speedo in my '09 does the same thing, but the difference isn't constant. When speedo reads 20, actual speed (read from OBS scanner, GPS and radar scanners on the side of the road) is 15-17. 30mph is accurate. 75mph indicated is 65 mph actual.

I'll try to use the light grease on the back of the speedo and see how that works.

 

On the speedometer repair, take a look up this thread a bit. The lubricant has to be very light!

 

Up the threat a bit, I concluded that my ratio was in the 4.56 range. Was my calculation right?

Since the speed sensor in the speedometer console is one of the feeds into the control module for the ignition timing, I'm wondering whether my speed error is affecting performance. I have a scanner that connects to my iPhone, and at 55mph (GPS reading) my RPMs are about 2300 and the timing advance is about 25. Does this seem reasonable?

 

Don't trust the roadside radar units. They are not calibrated regularly.
We have some out on I-15 near Barstow that, at least a couple of years ago, were reading 10mph high.