[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?