Speedometer and Odometer error corrected easily

For those whom I've shared my problem with, I had a speedometer that was reading up to FIVE miles per hour faster than I was really going. When I was physically going 65 my speedometer was telling me I was going 70. That was on OEM size tires and all stock gears. AND the odometer was adding an extra mile every 28 miles.

Here's what I did (and I got help on this forum for this solution): I changed the driven gear in the speed sensor housing. Factory OEM is an N-29 gear (with 29 teeth). I replaced that with an N-30 (which fits like a glove). That change reduced the error from 3.45% to less than 1%. That reduction brought my speedometer down significantly too. But not all the way.

The slope of the speedometer (for those of you who understand charts) is still off. But by repositioning the speedometer needle about 1 MPH BELOW the bottom of the Zero mark, things are pretty darn close.

Now when the speedometer tells me I'm going 25 I'm actually going 26. Not a big problem at those low speeds. At 50 it's spot on. And at 75 I'm actually going 74. Since that's the side where you get the bigger tickets, I can live with a mile per hour over. I estimate that not until 85 I'm two miles under the read speed. So the faster you go the more likely you are to be going slower than you think.

If you have any questions or need help - I have a little experience on this now. Feel free to drop me a line. However, don't expect a quick answer, it may take me a week or two before I find your message.

'av'a g'day my Toyota mates.

'')

 

Interesting. Can you post a few photos? Not sure where the speed sensor housing is.

 

I fixed the error in my speedometer/odometer by going from a 265/70/16 to a 265/75/16...worked perfectly.

I had about the same margin of error as you did...common to all second gen trucks. Verified by gps

 

I'd be glad to post pics if someone can tell me how to.

 

Thanks. Figured it out. Check the original post for pictures.

And Patrick: A bit dramatic don't you think? Kind of insulting if you ask me. But I can think of a thing or two to say back, but I don't want to sink to a new level.

As for tires, my tires (245 75R16) are brand new. Was also told that putting a 265 tire would likely rub the well. I don't know if anyone who has done that has experienced this, but based (at that time) on what I was told I opted for the factory OEM tire. What WAS on the truck when I bought it was 255 70R16, about a half inch lower (bigger speed error - SEVEN MPH at 65).

 

Do you have any gear ideas for 285/75/16 ?
I guess even the % closed would be better than what it is.

 

Help me out here. There are two gears and both have to be the same number of teeth to work properly. How did you change to a 30 when the other one that it runs against is a 29? Isn't that like putting a 30 chain on a 29 sprocket? Yes you can make it go, but you wear out both the sprocket and the chain.

 

I had to think about that and you are correct as long as the diameter of the new gear meshes properly.

 

To understand it better, imagine a bicycle: A 10 speed in this case. When you switch gears you get different ratio's. By changing to a 30 from a 29 the change is so small that the mesh of the teeth is negligible. Besides, they offer the N30 with 265 75R16 anyway. The single tooth makes very very little difference. Besides, if you keep the N29 gear and the N30 wears out (it's the plastic one) you can always switch back.

But from what I've heard, there are no issues with this. The SIZE of the teeth may be the same but the diameter is a mere tooth longer.

Let me do some math and I'll give you a better answer.

OK, here's some numbers: The N29 has a diameter of 1.029 inches. That times Pi equals 3.2327 inch circumference. That means (roughly) there is one tooth per every 0.111 inch peak to peak. Since I'm not going to pull the gear out of the truck to measure it I'll just assume (based on the numbers) that it is 1.0645 inch diameter. That's a mere 0.035 inch diameter difference. In terms of radius it's only18 thousandths of an inch. Rather small. It's a small change, and if there is a .010 tolerance then you can easily accommodate 020 difference in gear diameter (or 010 radius). Simple GD&T. (Geometric Dimensions & Tolerancing)

But here's the important thing: If you put a larger or smaller diameter gear with the same size and angle of the tooth you CHANGE the ratio BUT the gears still mesh. Believe me, I thought of this when I first heard of the idea of using an oversized gear. But so far it's working nicely. Perhaps in 80,000 miles I'll be thinking differently about this. Maybe in 50,000 miles. MAYBE in 500 miles. I'll let you know. But so far, no problems. I've already got 300 plus miles on it.

And if it makes you feel better I'll keep a 12 mm wrench handy and keep the old N29 gear in the truck JUST IN CASE.

 

Thanks for the reply. Yes as long as the diameter changes the teeth can be added or taken away and it will mesh properly. In this case the difference is so little that it works without having to be able to adjust the driver gear.

 

More information is needed. What ring & pinion are you running?

Do you know how to calculate tire size? If so - great. But just in case you don't, the 285 is the number of centimeters across the tread. The 75 is the sidewall PERCENTAGE OF THE TREAD. This case, 75% of 285, or 213.75 cm tall. Double that because there is sidewall above the rim and below the rim. That's important because you also have to add the rim into the equation too.

If you have a 16 inch rim, converting that to centimeters, that is 16 x 25.4, which equals 406 cm. THAT plus the 213.75 x 2 (431.5) will give you tire height. This case, 431.5 + 406 = 837.5 cm tall (or 32.97 inches tall or diameter)

Next you need to calculate distance traveled for every rotation. 32.97 x Pi (3.1416) = 103.586 inches per revolution. There are 5280 feet in a mile. Multiply that by 12 to get the number of inches. (63360 inches in a mile).

OK, divide that by the distance the tire travels. This case, 103.586. (63360 / 103.586 = 611.6657 revolutions per mile. Keep that number handy, you'll need it because THAT number times the ring & pinion gear will give you engine revs per mile. If this tire were on my truck the engine RPM would have to be 2507.83 revs per mile (assume you're going 60 miles per hour, you should see 2508 engine RPM) THAT is where to start. What gear do you have in the truck now and how far off (by percentage) it is will clue you in on what change you need to make to the gear.

Unfortunately that doesn't look like you're going to get close enough changing from an N29 to an N30. The next size gear is an N32 and it won't fit in my truck.

But logic dictates the truck will be going faster than what is shown. So likely you will be going to a smaller gear. Perhaps an N28 or even an N27. But at that point I don't know if you can get that gear OR IF EVEN it would work. It might not mesh with the teeth on the output shaft. In that case you might have to do more disassembly and investigation. And I'm sorry, that is where I can't help.

So a lot depends on what gears you're running in the rear end. Do you know how to check the gear ratio? It's not very hard. Just have to make some marks on a piece of tape and wrap it around the drive shaft, then count the number of turns.

Let me know if you need assistance with that.

 

Correct me if I'm wrong. Adding teeth would increase your center distance(related to diametric pitch of the gear) of the gear set.. you might be doing damage. Did it go together ok?

 

That assembly is designed to handle any number of different drive gear sizes.
Different models come with different size tires and rear end gearing. They come from the factory with different gears.
The replacement gear is simply from a different model truck.
I'm sure my 5-lug did not come with an N29... before doing this mod, I would want to check my existing gear before ordering a new one.

This mod has been commonplace for 40 years with no ill effects. In 1980 when our speedometers were limited to 85mph (in 10mph increments, starting at 5 and with a big red "55" in the middle) a lot of Corvette owners would replace their gear with one that was much smaller, and allowed you to read MPH directly from the KPH scale. Simple gear change turned the pathetic 85mph speedo into a 140mph speedo.

 

I've noticed on stock tire size my speedo reads 2 MPH faster (GPS calculated) at 70 MPH (actually doing 68 MPH). I can live with that. At least this way I am always under the posted speed limit. Sure, over the life of the truck it will read a few more miles than actual but not too much to be concerned about and I'm too lazy to do the calculations tonight.

 

Mine is a little worse... 66 or so on 70 indicated.

 

Yeah... I agree that is a little worse. Guess you could do the OP's fix or run slightly bigger tires.

 

Debating which way to go.
I've been considering a rim/tire swap to something a little larger and wider on a 17" rim.

 

When you do post pictures. I've always liked the clean look of your truck.

 

Where would someone get the N-30 gear part? Dealership? Is there a partnumber?