Regear + bigger tires ; speedo correction riddle

Not sure why this is so hard for me to comprehend .

So , stock truck comes with P265/70R16 (30.61") tires and 4.3 gears

if one then removes and adds, 315/75R16 (34.6") tires and 5.29 gears....

it should be -6% off. In other words, the true speed is actually slower than indicated by the stock speedometer. So if it says 100mph on the speedo , the true speed is 94mph.

I am no calculus professor but I get +13% difference for tire change (34.6-30.61)/(30.61)
then -19% for the regear change (1/5.29 - 1/4.3)/(1/4.3)... add them up -6%?

What's wrong with this?

 

Gears do not factor into the equation because speed is measured at the wheel. Only the tire size affects the speedometer.

 

On another note, I think gear percentage is incorrect too. (4.3-5.29)/4.3

 

Tire calculator

 

Hummmm. I thought the speed sensor was at the transmission/ t case for manual transmission trucks ? If the speed sensor is at the wheel then ya , gears are irrelevant.

 

I was trying to have gear ratio as seen by drive shaft / transmission. 1 spin of drive shaft 1/gear ratio I thought .

 

Not on 3rd gens. 3rd gens are all measured at the wheels. You need to only make the adjustment from 265/70R16 to 315/75R16.

Also, expect it to read a bit fast when done, just like the factory bone stock speedo reads a bit fast (2-3 mph fast at highway speed). If you want it to read spot on, use 265/70R17 or 265/75R16 as the starting tire size, but then the odometer will register miles a bit slow.

 

Sweet , thanks !

 

Also, correct way to calculate gear % change is either new divided by old (5.29/4.30), or (new-old)/old ((5.29-4.3)/4.3).

Either comes up as a 23.02% gear reduction.

 

Actually hold on a second is the sensor just the ABS sensor and it’s using software to calculate it? that’s really efficient of Toyota, surprising. Can you then just use software to recalibrate the speed via software?

 

I don’t know about programming, but as far as I know there are no adjustments available.

All trucks come with the equivalent of a nominal 30.5-30.6” tire which measures about 29.5” real world. As such, every truck is calibrated to the same overall tire diameter. So they are able to just program all the speedos on all trucks to spit out the same speed info for a certain number of wheel revolutions and that’s that. No need for different adjustment options or anything else from factory.

 

Humm , if you had enough motivation you could inline modulate the abs output frequency signal to correct or recalibrate.

 

It’s a lot easier to just use the Rough Country or Hypertech unit lol. It’s essentially doing that.

 

Pishhh. I’ve learned so much today lol.

 

For whatever reason I was thinking you were playing around with a calibrator and not getting the results you wanted. I see now that you were simply wondering why your speedo is not off by the amount you thought it would be.

Just to elaborate, like I said earlier, the speedo is correct with 1” larger tires than stock, aka 265/75R16 or 265/70R17; 31.6” nominal (odometer reads a bit slow with that size). Your tires are 34.6”, so your speedo should be reading 9.5% slow. If you were competing to the stock tires that read a bit fast on the speedo, it would be a net 13% change.

But yeah as discussed, gears don’t affect it which is why you weren’t getting the results you expected to.

 

These are my thoughts as well. The speedometer correction devises I have heard mentioned on this site will correct the speedometer for tire size (they do this at the instrument panel), but not the cruise control display, or so I have heard. I would be interested in a device that changes the speed sensor output into the ECU.

Some people experience the factory speedometer being off from the factory, mine was spot
on, verified with two GPSs. Thus any change in tire size for me produces an inaccurate display. I am currently up to 33” tires and will be going larger.

 

Gearing won’t effect spedo but tire size will. With my 4.88 spedo dead on. With 33’s spedo needed correction so Rough country programmed and all good.

 

This would be the ideal way to fix it and have all systems on the truck then operate normally and with correct understanding of the vehicle's speed. I also believe this would correct the issue some have with VSC intervening unnecessarily after going to larger tires and different suspensions.

The raw wheel speed signals are only handled by the skid control ECU. After the skid control ECU everything else receives the vehicle speed as a CAN bus message including the combination meter assembly that displays vehicle speed.