Speedometer/odometer correction for oversized tires

I have an early 97 taco that has the old school cable driven speedometer gear in the transfer case, the truck has no ABS and I believe all of the modern electronic speedometer correction devices are too new for my old taco. I recently got new tires on my truck I went from 225/75R15 to 265/75R16 so my question is what are my options here, I have searched a little bit already and can't find a for sure source for what I need. I really like my odometer and speedometer to be accurate as can be so me just calculating how fast I am really going is not a long term option. Obviously I need to change the gear on the speedo cable but to what?

 

Not sure on what size gear you need to correct the speedo but another option is buy a scan guage or ultra guage and calibrate it to use as your speedo? They are so usefull for so many things anyway i cant imagine you'd regret the purchase..
I will never not have one of those things anymore!

 

Increase gear ratio to equal the tire size increase? you increased your tire size by X%, increase the size of the adjusting gear by the same %

 

they're a little off to begin with but going 2" in your case would over-correct the speedometer (31" -> 32" is dead on)

maybe see if the T100 speedometer is cable and get that gear?

 

Ok, so I just strait copy and pasted this from CelicaSupra.com but I'm pretty sure this is what I need to do even though it is not your typical plug in new part here and voila it has been fixed. This method will obviously take a little more figuring out for the proper end result.


The end to speedometer gear questions
First of all, these are all mathmatical equations for your own use, I will not be giving responses for you.

Second, I am an amatuer vehical modifier and will not take any responsibility for money spent on improper parts.

The following is all of the mathmatics needed to figure out what speedometer drive and driven gears you would need in your 5-speed transmisson for a proper working speedometer using tire size and rear gear ratio.

Start out by determining your current tires diameter, calculate the tires you are running or plan to use, not the stock size. The diameter can be found from manufacturers online or you can use this formula. For this example I will use mytires of choice, 275/40/17

Tire width (275) multiplied by the sidewall as a percentage (40% or .4), divide by 25.4 to convert to inches, and then double this number and add the wheel diamater (17)
---so 275x.4=110, 110/25.4=4.33, 4.33x2=8.66, 8.66+17= 25.66. So a 275/40/17 tires has a diamater of 25.66 inches.

Now you will need to determine your tires revelotions per mile. This is found by dividing 20168 by the tires diameter.

--- 20168/25.66 = 785.97 revs per mile.

Next determine your number of teeth on the driven gear. You have two options here with Toyota gears, the drive gear in the transmission was offered with 10 or 11 teeth, you can get driven gears to match either and the best bet is to find out what is in your transmission currently. Pull the driven gear out and using a flashlight you sould be able to see a 10 X xx-xx or an 11 X xx-xx stamped on the side of the drive gear. Most MKII W58's have 10 tooth drive gears

So, this formula is drive gear (10 or 11) multiplied by rear gear (3.73 for me) multiplied by tire revs per mile (785.97), then divide this number by 1001 to get your driven gear tooth count.

---10x3.73x785.97 = 29316.681/1001 = 29.29, this means a driven gear with 29 teeth made for a 10 tooth drive gear = 29:10
-or-
---11x3.73x785.97 = 32248.349/1001 = 32.21, this means a driven gear with 32 teeth made for a 11 tooth drive gear = 32:10

Finally, dividing the driven gear number by the drive gear number gives you a final number. Doing this will allow you to find the correct setup in a 10 or 11 tooth drive gear.

---29/10 = 2.9 or 32/11 = 2.9

The following is a compiled list of Toyota speedometer gears, these are found on many vehicles besides Supra's, most 80's and 90's trucks, suv's and vans had these parts in some form also. I have included the final number after the ratio, to make it easier to find your proper setup.

These are gears from supra's and cressida's with part numbers
33403-29015 30 tooth, 30x11 2.7
33403-19245 31 tooth, 31x11 2.8
33403-19255 32 tooth, 32x11 2.9
33403-29115 33 tooth, 33x11 3.0 3.73's
33403-29145 31 tooth, 31x10 3.1
33403-29125 32 tooth, 32x10 3.2 3.90's
33403-29135 33 tooth, 33x10 3.3 4.10's
33403-39075 34 tooth, 34x10 3.4
33403-19235 35 tooth, 35x10 3.5 4.30's

These are gears from truck's and suv's with part numbers and model codes, I have removed any duplicates to the supra list above.
33403-39345 31 tooth, 31x11 2.8 RN131
33403-39425 32 tooth, 32x11 2.9 RN101/RN110/RN13
33403-39415 33 tooth, 33x11 3.0 VZN10*/VZN110/VZN13*
33403-29315 32 tooth, 32x10 3.2 RN101/RN110/RN13

When ordering any driven gear, you will need to make sure that you have a matching drive gear. They are matched to each other and like was posted above, the drive gears are stamped with what driven gears it can be used with.

This is what I have found so far in my hours of searching a lot of Toyota forums. I hope this has helped some people, and like I said in the begining, I am not responsible for time or money wasted on parts. The best bet here is to speak with your Toyota parts guy of choice when ordering and explain to him what your looking to do.