Can we change the tire size in the computer?

I have a 1999 SR5 4WD Extended Cab, 5 speed manual.
Is there a method to change the computer to reflect the tire size on my truck?
I use AutoEnginuity on my Ford F250 to add the correct tire size in the computer. Keeps the speedo showing the correct mph and the odometer is correct.

AE said that their software won't change the tire size on a Toyota.

 

Welcome to TacomaWorld
No, but you can change the gear in the speed sensor sending unit on transfer case.

 

Thanks for the welcome.

Is there a source or reference for the gearing change? I don't see a sticky thread on it.

 

https://adventuretaco.com/replacing-the-speedo-gear/

But honestly, most people just live with it.

 

1st gens use a mechanical cable to the speedo, so no, there's no adjustment in the ECU.

As said, you can change the gear in the sending unit in the transfercase. The problem is that you're very limited on the possible gear ratios. Adventuretaco He went from a 30 to a 33 tooth gear, which still wasn't enough for the right correction.

There's another link in adventuretacos link that describes more about how the speedo/odo work.
https://adventuretaco.com/speedometers-odometers-and-gas-mileage-all-lies/

Basically, the speedo/odo is fed by the sender that's coming from the T-case, which means the speed/distance is affected by tire size, t-case, diff, and speedo gears.

Personally, the "best" way to correct your speedo is with the diff gears. There are several on-line gear calculators out there (I like this one: http://www.grimmjeeper.com/gears.html). So as long as you focus on changing your new diff ratios to closely match your original engine RPMs with stock tires (at different speeds), that will get you close. Diff gears are more expensive for sure, but it's not just about your speedo readings, it's about engine power/torque etc...

i.e. say you rev'd at 2300 in 5th gear at 75mph. Enter your new tire size in the calculator, then play around with the gear ratios to find a close match. Again, you'll only get close since there are only a few ring and pinion sizes that are physically possible (4.10, 4.30., 4.56, 4.88, etc...).

On newer vehicles, it's all driven by computer code, so it's all infinitely adjustable, but on your 23 year old pickup truck, it's driven by mechanical gears, which mean you have physical limitations to how accurate your speedo can get. That's really why there's such a large variance that's legally allowed for speedo readings.

I actually prefer the speedo to be off. That way I can feel like I'm doing 80, but I'm really not, lol.

 

This is a jumble of partially correct and entirely incorrect information...

Anywho, after 1997 all tacomas use electronic speedo sensors which you can absolutely change the output of easily without buying speedo gears and guessing you'll get close. https://www.lutzauto.com/product-page/toyota-92-manual-transmission-3-wire-speed-sensor-calibrator

95-97 only have the option of changing the speedo gear as they do have mechanical speedometers (which in the gauge cluster then convert to a digital signal to feed the ECU and TCU, but that's a much harder signal to intercept)

 

That's pretty cool, never heard of that before.

I was going off what Adventuretaco posted (using a 2000 with the mechanical speedo gear), and what I experienced with my 97 5-lug. When I changed the dash cluster, I'm fairly certain it came from a newer (early 2000's) truck, and plugged right in to my '97. Speedo, tach, odo all worked perfectly with the mechanical input. Granted that was a long time ago...

 

This is awesome. When I get home from work, I'm going to look under my truck. Hopefully my 04 w/ auto trans has a 3 wire connector.

 

It almost for sure will, my friends 02 Tacoma does, my 93 land cruiser even does haha. Toyota hasn't changed this connection in forever

 

@Speedytech7 Thanks for the tip.

 

I went from a ~31" to ~32" and the speedo is now accurate, just the mileage recording is not due to how it was programmed. good enough for me, I still keep mileage records but never look at them anymore

 

Nice. I'm going to look into this as well, since even with the 33-tooth gear, I'm still ~7-9% off with my speedo. Not that it really matters, but I might as well burn money, right? Is this new? In the past, I've known of the Dakota Digital, but never of this.

Yeah, you can do both/either. Really, it comes down to how much you want to spend - the gear is like $15-30, and the electronic stuff is all in the $100 range.

I should note that, in some of the comments on the page you linked, I did say that if I were to do it again, I'd go the DD route :

...after I've done a mediocre job of answering your questions... I'll add this as well - the speedo gear fix worked well for me, but if I were to do it again, I'd probably consider doing it differently. I'd likely purchase a Dakota Digital SGI-100BT Universal Speedometer and Tachometer Interface electronic adapter. I know you've looked at some of these and they are pricey - but for me, the ability to (a) not needing to figure out the current speedo gear size - as well as all the other important bits of information necessary to determine what new gear to purchase, (b) not needing to worry about changing speedo gears in the future and (c) the ability to adjust for different tire sizes in the future - I think those things could be worth the extra price of the electronic converter.​

Another option, besides the one Zane posted, is the Dakota Digital. Does basically the same thing by modifying the signal. Slightly cheaper than Zane's link, but also a bit less user-friendly interface for adjustments.

 

Yeah, the speedo gear and the programmer both still don't address the final drive ratio, which as I said IMO is the most important aspect when going to larger tires. Only way to regain the power/torque after larger tires is to re-gear the diffs, which is, of course, the most expensive. Anything after that is just figuring out how accurate you want the speedo to be. ~$100 really isn't all that much, and knowing that's available, that seems like far and away the best route to go for sure.

 

Oh, yeah, for power, of course the only way to get anything is to do the gearing. I guess I never really thought about it that way - I always viewed the power/gearing as something that would get you a certain performance profile, and as the speedo as something that just gives you a readout of speed. (And then of course, also of gas mileage, ODO, etc.) Your obviously right that they are connected/related, which I suppose is what the speedo gear is supposed to do - it's supposed to "translate" from whatever gears you have, to what the computer expects for a "mile of distance."

Thanks for making me think that through.

 

Oh I definitely learned something too. I had no idea about the electronics being around. I guess I SHOULD have known, since we live in an age where there's always "an app for that" lol. I also just tend to think of first gens as analog dinosaurs, lol.

 

I would think that even with the signal modifiers, you're still playing a guessing game. Using GPS to calibrate your speed is completely inaccurate (unless you have military-grade or special RTK gear in the 10mm accuracy range). Consumer-grade GPS equipment is 10m (32.8 ft) accuracy at best, and when you're moving.. it's not even close to that, so it's relying on an algorithm to "smooth out the pings" giving an average over time.