Speedo Calibration With Larger Tires - Why?

[PungoteagueDave]

We see discussion on this forum about folks using FORScan or having their dealership recalibrate their truck's speedometer after changing to larger wheels/tires. Vehicles used to have a mechanical connection cable between the transmission and the speedo, so any change in diameter would alter the speedo and odometer accuracy, but most modern cars use electronic sensors, correct?

I put Michelin Agilis LT 285/60R-20's on my KR Power Boost, replacing the OEM Pirelli Scorpions 275/60R20's. The original tires were 33" and the replacements are 33.5". The new tires are much heavier (57 lbs) as actual truck tires, needed for heavy towing, compared to 38 lbs for the tires I took off. Weight isn't part of this issue, but just noted as I was looking at this question - and a key reason my empty MPG has dropped from 23.9 to 20.9.

The change in tire diameter resulted in a new circumference 105.24" or a 1.51% increase over the 103.67" circumference for the OEM tires.

Given the discussions here about folks working to dial in their speedometers, I expected my speedo to be off after changing tires. However, using GPS (either google maps or Waze), my speedometer was dead on accurate before putting on the larger tires, and remains accurate to the exact GPS-indicated miles per hour afterwards. So what's up with this issue? Is it just a problem for models without NAV systems?

Sponsored

 

[UGADawg96]

I'm surprised you lost 3mpg with such a subtle change in tire size and just 80lbs.

 

[PungoteagueDave]

I'm surprised you lost 3mpg with such a subtle change in tire size and just 80lbs.

They are wider too. The 50% tire weight change (19 pounds per tire going from 38 lbs to 57) is a lot in rolling weight given the sensitivity that hybrid/electric systems have - they are finely tuned and every element is critical to the balance that creates the magic numbers. A PowerBoost's MPG falls off a cliff when asked to do anything truck-like - accelerating even a little bit, tipping into the turbos or towing and all of a sudden it gets worst-in-class MPG (my 9,750-pound boat/trailer drops MPG to 5.9 on average on level ground at 65 MPH, half what my F-350 got for the same rig with diesel).

 

[UGADawg96]

yeah, mine is on order, but was planning on going with LT275/65r20 to add 1" of height without changing the width. Was looking at 53 lb tires in that size. Not sure how much effect that would have.

 

[Doyouevenboost]

Our trucks transmissions have an output shaft speed sensor. With the “known” data that is programmed into our trucks through witchcraft (axle ratio and tire circumference) the vehicle speed is calculated. Not through satellite or gps.

I just don’t think jumping only a half inch in diameter on your rig is going to show a drastic change like some of the guys running bigger setups.

 

[PungoteagueDave]

Our trucks transmissions have an output shaft speed sensor. With the “known” data that is programmed into our trucks through witchcraft (axle ratio and tire circumference) the vehicle speed is calculated. Not through satellite or gps.

I just don’t think jumping only a half inch in diameter on your rig is going to show a drastic change like some of the guys running bigger setups.

Okay - but I would have thought it would be off by 1 mph if originally accurate, and I increase the distance per wheel revolution 1.5% (making my speedo read too slow and odo run low). Doesn't seem the case, but I accept your explanation.

 

[Rich1982]

Okay - but I would have thought it would be off by 1 mph if originally accurate, and I increase the distance per wheel revolution 1.5% (making my speedo read too slow and odo run low). Doesn't seem the case, but I accept your explanation.

I changed from 275/60r20 to 275/65r20 and now my miles/speed appear to be very close to accurate - only checked once prior to the tire change over a 10 mile distance and it appears m speedometer/odometer were reading slow from factory setup -

 

[Zyvin]

Bump up your psi to 80, then you’ll see a difference.

 

[jeffcrum]

We see discussion on this forum about folks using FORScan or having their dealership recalibrate their truck's speedometer after changing to larger wheels/tires. Vehicles used to have a mechanical connection cable between the transmission and the speedo, so any change in diameter would alter the speedo and odometer accuracy, but most modern cars use electronic sensors, correct?

I put Michelin Agilis LT 285/60R-20's on my KR Power Boost, replacing the OEM Pirelli Scorpions 275/60R20's. The original tires were 33" and the replacements are 33.5". The new tires are much heavier (57 lbs) as actual truck tires, needed for heavy towing, compared to 38 lbs for the tires I took off. Weight isn't part of this issue, but just noted as I was looking at this question - and a key reason my empty MPG has dropped from 23.9 to 20.9.

The change in tire diameter resulted in a new circumference 105.24" or a 1.51% increase over the 103.67" circumference for the OEM tires.

Given the discussions here about folks working to dial in their speedometers, I expected my speedo to be off after changing tires. However, using GPS (either google maps or Waze), my speedometer was dead on accurate before putting on the larger tires, and remains accurate to the exact GPS-indicated miles per hour afterwards. So what's up with this issue? Is it just a problem for models without NAV systems?

Back in the day, if your tire size difference was significant, you would replace the cog (I guess) at the end of the cable that went into the transmission.

 

[sswalters]

We see discussion on this forum about folks using FORScan or having their dealership recalibrate their truck's speedometer after changing to larger wheels/tires. Vehicles used to have a mechanical connection cable between the transmission and the speedo, so any change in diameter would alter the speedo and odometer accuracy, but most modern cars use electronic sensors, correct?

I put Michelin Agilis LT 285/60R-20's on my KR Power Boost, replacing the OEM Pirelli Scorpions 275/60R20's. The original tires were 33" and the replacements are 33.5". The new tires are much heavier (57 lbs) as actual truck tires, needed for heavy towing, compared to 38 lbs for the tires I took off. Weight isn't part of this issue, but just noted as I was looking at this question - and a key reason my empty MPG has dropped from 23.9 to 20.9.

The change in tire diameter resulted in a new circumference 105.24" or a 1.51% increase over the 103.67" circumference for the OEM tires.

Given the discussions here about folks working to dial in their speedometers, I expected my speedo to be off after changing tires. However, using GPS (either google maps or Waze), my speedometer was dead on accurate before putting on the larger tires, and remains accurate to the exact GPS-indicated miles per hour afterwards. So what's up with this issue? Is it just a problem for models without NAV systems?

Great post. I too just added a 2" level and bigger tires to 295/60/20. And driving home I noted that my Waze GPS was dead accurate to the speedo. I was expecting to have to add a component to adjust the speedo such as Hypertech device that even the dealer installs when needed. But I don't understand the need to do this if my speedometer is dead accurate to phone GPS speeds. Did you wind up having anything adjusted?

 

[wolfnards]

They are wider too. The 50% tire weight change (19 pounds per tire going from 38 lbs to 57) is a lot in rolling weight given the sensitivity that hybrid/electric systems have - they are finely tuned and every element is critical to the balance that creates the magic numbers. A PowerBoost's MPG falls off a cliff when asked to do anything truck-like - accelerating even a little bit, tipping into the turbos or towing and all of a sudden it gets worst-in-class MPG (my 9,750-pound boat/trailer drops MPG to 5.9 on average on level ground at 65 MPH, half what my F-350 got for the same rig with diesel).

Well that’s really disappointing to hear… I got the PB because of the better MPG. Does the non-PB 3.5 Ecoboost also suffer from the same issue when towing?

Sponsored