Speedo and Odo Incorrect After Tire Size Increase...Fix?

[Ferrugenfish]

Evening ladies.  I imagine I'm not the only one to wonder this, and I'm hoping there is some easy fix like a TPMS sensor adjustment... but for the speedo/odo.  

 

I swear my MPG increased by roughly 1.5 mpg after I put on larger tires on my new (to me....only 5k miles) 2019 NV3500 SV V8 Hightop.  ... but that shouldn't happen... doesn't make sense, so it got me wondering if the Nissan NV judges speed by assuming you're using stock tires, counting one tire rotation as a set distance (odometer), then multiplying the number of rotations/time to get speed.  If that's the case it would explain why I think I'm getting more MPG (because the tires now have a wider diameter, causing more distance to be traveled per single rotation.  This would also explain why, when I told my brother to set his cruise control to 60 in his Honda CRV and matched his speed with my van, my speedo said I was going about 64 mph.  It seemed that the faster we went, the larger the discrepancy was (When brother went 80, I was going "86").  

 

So that's my theory... that the NV vans don't (accurately) know how fast we're going, or how far we're going, unless we keep stock sized tires.  Am I wrong?  If I'm right... we're all speeding in front of cops unknowingly... and we're all putting more miles on our odometers than the meter shows.  This is good for re-sale, but I'm sure a man is curious how many miles he's REALLY driven the rig, especially if you're getting up there in miles and start to get proud... but the pride is erroded by this post from a guy saying that if you got bigger tires then you're putting more miles on your rig than you know. 

 

I could totally be wrong and someone will chime in and school me about how I don't know basic stuff... but I welcome it because maybe I can correct it (the speedo/odo).... or at least someone can talk some sense into me?

(put on KO3s at 285/70/17 I believe.... Method wheels but same 17 as stock).  Also lifted vehicle 3" and put side steps on)

 

Do all vehicles figure speed the same way?.. is that same way the way I described or am I wrong and I'm just imagining "better" gas mileage improvements after getting bigger tires (and 4x4 conversion, and side steps at the same time). 

 

Thanks for your thoughts and insults. 

[crusty]

You are correct.  Like every production car I have ever seen the speedometer gets input from the rotational speed of the wheel, driveshaft, etc.  There is a calibration in that speedometer based on the rolling distance the tire will cover per revolution.  In the olden days you could go in and get a different gear (different number of teeth) to correct for different tire sizes as well as different axle ratios.  That was the driveshaft based speed.

Modern stuff runs wheel speed sensors.  That feeds an RPM signal into the ABS system.  The ABS knows how many revolutions the tire turns per mile, and a little basic math will give speed.  There is another speed sensor for the output shaft of the transmission.  That feed back to the transmission so it can monitor that the gear ratios are correct and the transmission isn't slipping.  Just like the old transmission speedometer days but this has no gear.  It is done with software. 

 

Fun bit, if you change axle ratios and the cruise is on, you can get an engine code for something like mismatch of speeds that will keep the cruise control from working until you turn it off and start it back up again.

 

I don't know of any real good way to correct a speedometer for oversize tires in the vans.  A cheat sheet (or stand alone GPS speedometer) is the easy way to really know.

[Ferrugenfish]

Ahhh thank you so much!  Gosh darn computers making my odo dumb... but at least maybe it's helping my lady lie about her wear, over time.  That was an amazing and detailed explaination, and a big part of me can now rest knowing that I'm not the only one "lying" about my vehicle mileage