Increasing Tire Size and effect on electronics

[scavoj]

I have a couple of questions regarding the change from the stock 265 70R16 to 75R16s on an 04 DC Tundra. Is there a way to correct the spedo reading (and therefore the ODO)? Also, does anyone know if there would be any effect on the VSC and TRAC system?

[Emmette]

I ran into the speedo problem when I bought my used Tundra with 75s instead of the normal 70s. When I was stopped for speeding, the officer had me going 5 mph more than my speedo read. I had my wife pace me in our Toyota van and confirmed that my speedo readout was 5 mph too low. I checked with the dealer, and they told me Toyota has a margin of error of up to 7mph, so they could not (or would not) recalibrate the speedo.

[maisano]

http://www.miata.net/garage/tirecalc.html


i got this site from okctundra this has good info on it.

[scavoj]

Originally Posted by maisano

http://www.miata.net/garage/tirecalc.html

i got this site from okctundra this has good info on it.

Yea, this is the best tire size calcualtor I've seen. I found it probably about a year ago. It's been in my favorites since then. Thanks.

Originally Posted by Emmette

... I checked with the dealer, and they told me Toyota has a margin of error of up to 7mph, so they could not (or would not) recalibrate the speedo.

7mph !!! That seems pretty high. Thanks for the reply.

Does anyone know if the increased tire size will affect the VSC and Active-TRAC systems?

I'm going to call Toyota...but I'm probably going to get the same response I always get from them..."We don't recommend making any modifications to our vehicles"

[toyotafreak]

Joe, this is in keeping with the thread title, but not with your question. The TS archives are friggin huge, which is a great thing, but I haven't made the time to get through all of it.

I've spent a significant amount of time bouncing indicated speed/odometry against GPS, and what I've found is that the OEM 265/65-17 Dunlops at recommended pressure yield an error of 4.7%. This is not based on a small or single sample, and is very repeatable. (Btw, if you reset the AVG MPH every tank, subtract 0.9 MPG for truth - anything beyond one tank the accuracy drops even further). So what am I getting at?...in terms of how the speedo is calibrated from the factory, their choice of tires is 4.5% too large.

I guess my crackpot head has had this wrong for a long time. The 1.047 figure I mentioned is the number I have to multiply the odometer reading by to derive true mileage. My problem is that I've always thought the OEM tires' diameter was too small, when in reality they're too big. 4.5% too big. A tire 4.5% too big requires a 1.047 odometer increase to show true miles traveled. A tire 4.5% too big also requires you to add 4.7% to your indicated MPH to find how fast you're really traveling. Not rocket science, but big for me cuz I've had the wrong idea the whole time, AND the wheel wells are empty enough, AND the highway RPMs are too high for decent mileage (not towing's concern either - that's what OD OFF is for).

Toyota original equipment:

Tire_________Diam____Circ________Error__________No tes_________
245/70-16----29.50----92.69----1.1% too big----SR5 Standard 2WD
265/70-16----30.61----96.15----4.6% too big----SR5 Standard 4WD
265/65-17----30.56----96.02----4.5% too big----OEM Upgrade 2WD & 4WD
275/45-20----29.74----93.44----1.9% too big----TRD wheelset

Next, I solved for zero error, which becomes 29.2" diameter and 91.72" circumference. Here's some other fictional sizes that zero out the speedo error (I specified aspect ratio and wheel size and let Excel determine tread width):

Tire_________Diam____Circ_____Error
238/65-17----29.20----91.72----0.0%
258/60-17----29.19----91.71----0.0%
281/55-17----29.18----91.67----0.0%
260/45-20----29.20----91.73----0.0%
292/40-20----29.19----91.71----0.0%



And finally, where real tire sizes fit:

Tire_________Diam_____Circ_____Error___________Not es
245/70-16----29.50----92.69----1.1% too big----SR5 Standard 2WD
265/70-16----30.61----96.15----4.6% too big----SR5 Standard 4WD
265/60-17----29.52----92.74----1.1% too big
265/65-17----30.56----96.02----4.5% too big----Upgraded 2WD & 4WD
275/55-17----28.91----90.82----1.0% too small
265/45-20----29.39----92.33----0.7% too big
275/45-20----29.74----93.44----1.9% too big----TRD wheelset
285/40-20----28.98----91.03----0.7% too small

The whole problem with this now is that highway rpm and gas mileage would have to rise, although braking and acceleration would probably improve (maybe TRD's onto something here, as theirs splits the difference between speedo cal and the OEM 17's - probably results in a sportier feel).

So what now? I saw a 275/55-20 on a Sequoia at lunch and it looked real healthy. Fender looked pretty full. Wonder if he knows his speedo's off 8.5%. Can we pay to have a technican reprogram the truck's CPU ("need to drop the indications 8.5%".) In that case, we'd get accurate speedo/ODO indications while effectively changing our final drive ratio to a more highway-happy number.

Is it safe to assume that larger tires than stock result in better highway mileage, better tire wear, worse city mileage and worse braking?

[Thwarter]

I had the same question now that I'm getting ready to replace my stock BFG Rugged Trails (265/70-16) with Michelin LTX M/S (LT265/75-16). While I didn't really like the BFG's (too slippery/squeally), I have about 38,500 miles on them so I can't complain about durability.

I think the real reason Toyota (or any other mfg for that matter) doesn't want people to modify the speedometer reading for different size tires is that some people may change it just to lower the mileage being counted on the odometer, regardless of what tire size they have.

Like others have said - when you get larger tires, you are in fact reducing the mileage that your odometer/speedometer registers. According to the specs for the above tires, the revolutions per mile goes down from 680 to 652. Technically this could raise the vehicle re-sale value a bit since the mileage shown on the odometer would be less than the true mileage the truck travelled.

It may only be 3.24%, but that's an extra 3,240 miles your truck really travelled for every 100,000 miles shown on the odometer. Another way of looking at it is when your speedometer says your travelling at 65 mph, you're really going 67.2 mph.

I went through this before when I had two different sized sets of tires for my 4Runner. I made a little sticky note with two columns: "Speedometer Reading" and "True Speed" and filled in half-a-dozen values to remind me I'm going faster than what the speedo says when I had the larger tires on the truck.

People should also factor this in when planning routine vehicle maintenance which is usually scheduled by mileage.

[TMS2U]

With 4.88 gears and 35's my speedo runs 10% over actuall mph. I got a Dakota Digital unit to correct this. Haven't done the install yet tho

[JeffD]

When I was running 33's I installed a speedo recalibrator and raised the speedo about 8% to get an accurate reading via gps. I don't remember what brand I got (it was not dakota) my only complaint was that the speedometer didn't move as smoothly as it did stock, it seemed to only move in .5 second increments.