Speedometer error - Where did I go wrong? -UP DATE

[ThumperF150]

2024 RCSB 5.0 10 Speed 4X4 with 3.73 gears.

I leveled and then mounted 35X11.50/17 tires. 35s are 10% larger than the 31's (265X70/17) that came stock.

As EXPECTED, my speedometer was off by 10%. 40 mph on the dash was 44 GPS, 50 was 55 GPS, 60 was 66 GPS. I used a couple of GPS devices to double check those numbers.

As I had planned, I then swapped out the 3.73 gears for 4.11 gears. 4.11s are 10% lower than 3.73 so for any given RPM, the axles shafts should be turning 10% slower than stock but when you add the 10% bigger tires, according to my calculations, my speedometer / actual speed should be darn near back to stock and accurate.

Not even close. I am still a solid 7-7.5% off. 65 mph on the dash is 70 on the GPS.

I figured even taking into consideration that not all tires measure exactly the same, even if everything was a full 2% off, that would only be 1 MPH at 50.

So where did I screw up?

Math? Nope, no way, even Tiresize.com verified my numbers.
Logic? + 10% on tires - 10% on gears should balance out to 0.
Lack of understanding of gear ratios and tires? Maybe. If so, where did I go wrong?

Sponsored

 

[SilverPigeon]

Hi Rich. Speedo can be corrected in FORScan if you have it. But let's have a sanity check first. With your current setup (35s, 4.11 fdr) in 10th gear at 60 mph, what is your tacho saying (as accurately as you can read it)?

 

[ThumperF150]

Hi Rich. Speedo can be corrected in FORScan if you have it. But let's have a sanity check first. With your current setup (35s, 4.11 fdr) in 10th gear at 60 mph, what is your tacho saying (as accurately as you can read it)?

Today I was showing about 1,700 rpm at 69/70 actual. 64/65 on the speedo. But I was in Normal mode so there is a chance trans was in 9th or maybe 8th?

I’ll need to do it again in Sport mode to be sure of trans gear.

Does Forscan work on the 24s yet?

 

[SilverPigeon]

...Does Forscan work on the 24s yet?

Not sure as i have a '23, but worth a try?

 

[ThumperF150]

Not sure as i have a '23, but worth a try?

Maybe. I was hoping to not need to mess with computer and programming.

So what are your rpms at 60 in 10th?

 

[SilverPigeon]

1,652, with 3.73s, at 110 kph, so in mph yours seems normal as far as the driveline is concerned... if it wasnt in 10th, id expect over 2,000. I think adjustment of tyre size in forscan is in your future

 

[ThumperF150]

1,652, with 3.73s, at 110 kph, so in mph yours seems normal as far as the driveline is concerned... if it wasnt in 10th, id expect over 2,000. I think adjustment of tyre size in forscan is in your future

Thanks.

Guess I’ll need to do research on Forscan and buy a PC laptop and cable and figure out all those modules and codes.

 

[SilverPigeon]

Just FYI, this is the math I used for 3.73:

Final drive ratio: 3.73
Tyre Size: 275/60R20 OWL all-terrain
Distance in km for 1 wheel rotation: (838mm x pi)/1,000,000 = .00263265km
Wheel rotations in 110km = 110/.00263265 = 41,782.92
Wheel rotations at 110km/h for 1 min = 41,782.92/60 = 696.38
Driveshaft RPM = 696.38 x 3.73 = 2,597.50
Engine RPM in 10th gear = 2597.50 x .636 = 1652.01 RPM

So your math for 4.11 and 35s should be:
Final drive ratio: 4.11
Tyre Size: 35in (diameter 889mm)
Distance in km for 1 wheel rotation: (889mm x pi)/1,000,000 = .00279288km
Wheel rotations in 110km = 110/.00279288 = 39,385.93
Wheel rotations at 110km/h for 1 min = 39,385.93/60 = 656.43
Driveshaft RPM = 656.43 x 4.11 = 2,697.93
Engine RPM in 10th gear = 2,697.93 x .636 = 1715.88 RPM

110kph = 68.351 mi/hr so you're not going crazy ?

 

[whizkidder]

Have you compared the Revolutions per Mile specification of the two tire brands/sizes? Differences in sidewall stiffness may cause the diameter calculations to be off compared to the actual revs per mile.

 

[ThumperF150]

Just FYI, this is the math I used for 3.73:

Final drive ratio: 3.73
Tyre Size: 275/60R20 OWL all-terrain
Distance in km for 1 wheel rotation: (838mm x pi)/1,000,000 = .00263265km
Wheel rotations in 110km = 110/.00263265 = 41,782.92
Wheel rotations at 110km/h for 1 min = 41,782.92/60 = 696.38
Driveshaft RPM = 696.38 x 3.73 = 2,597.50
Engine RPM in 10th gear = 2597.50 x .636 = 1652.01 RPM

So your math for 4.11 and 35s should be:
Final drive ratio: 4.11
Tyre Size: 35in (diameter 889mm)
Distance in km for 1 wheel rotation: (889mm x pi)/1,000,000 = .00279288km
Wheel rotations in 110km = 110/.00279288 = 39,385.93
Wheel rotations at 110km/h for 1 min = 39,385.93/60 = 656.43
Driveshaft RPM = 656.43 x 4.11 = 2,697.93
Engine RPM in 10th gear = 2,697.93 x .636 = 1715.88 RPM

110kph = 68.351 mi/hr so you're not going crazy ?

That’s above my paygrade. Even ForScan is complicated to me.

 

[ThumperF150]

Have you compared the Revolutions per Mile specification of the two tire brands/sizes? Differences in sidewall stiffness may cause the diameter calculations to be off compared to the actual revs per mile.

No I have not. But I have a hard time believing the fudge factor would be more than 1-2%.

 

[ThumperF150]

I may just go with the Hypertech. Says it works for a 2024 and does both gear and tire size. Plugs in between truck and speedo and does the correction there so no messing with the truck's computer.

 

[jasonkosi]

Just a tip if you ever want to see what gear it’s in, just hit the + button on the shifter. The gear display will show even in normal mode.

 

[ThumperF150]

Just a tip if you ever want to see what gear it’s in, just hit the + button on the shifter. The gear display will show even in normal mode.

Thank you Sir.

 

[HammaMan]

I'd try setting both your new ratio and your tire circumference (per their actual specs, x .967) in the BCM and everything should be happy again. 21-23 says axle ratio isn't used (however mine is correct at value 0175 hexD, translates to 373 decimal, also checked a 24s and it is used). However the tire circumference can be used to properly adjust your speed by substituting a value appropriate to the distance you're actually covering vs the distance your truck said you traveled.

The 4 stars of 726-15-01 (positions 5-8 should be changed to 019A) If you had 373s it should read 0175

Go find your tire's exact specs, convert the circumference to mm, multiply by .967 (rolling diameter correction), take that value and enter it into a decimal to hexadecimal converter to find your 4 digit value. Here I chose a random number of 2527.4 mm circumference, * .967
2527.4 * .967 = 2444

As an example, while the gear number is what you need, you didn't post the exact tires you have so I can't provide you the numbers that fill out your BCM values

Sponsored