Speedo Correction after Tire Diameter Change

[Triple H]

Hello All:

I just ordered my Pro Tuner (Haven't received it yet) and curious where I should start reading to fix my first issue with my 2004 Silverado 2500HD 4X4 with a 6.0

I swapped out the stock 245 70 16's for 295 70 16 BFG Mud Terrains and want to have an accurate speedometer and proper shifting.

I hope this is a good point to start playing with the new toy.

Thanks

[EC_Tune]

Here's the best way I have found:
This was for an F-body so the tire diameter doesn't apply to your vehicle but the math does.

1st:
Check your roll-out in inches.
Mark the tire & the ground (at nomal tire temps after driving).

Rotate one or two revolutions (divide that value in inches by 2 if you do 2 revs ). Then mark the ground where the tire mark makes it's 1st or 2nd revoltion. It should be ~88.18" If it's different, time to rescale.

Here's the math on how to do the speedo rescale:

Tire revolutions/mile = Inches per mile / Tire Circumference Inches
Tire Revolutions/mi = 63360 / 88.18 --- based on (28.07" tire * Pi)

Trans revolutions/mile = Tire revolutions/mile * Gear ratio
Your values:
(63360" per mile / 88.18" tire circumference) * 4.56 gear = 3276.33

Take that value and multiply by the Rev-Trans value (40 in *most* cases) to get the VSS Pulses/mile = 131053

Put your values in the appropriate places and your speedo's dead on. Check with a GPS if you have doubts.

EC

[Triple H]

Thanks EC_Tune

Here's my start and I haven't gotten my Kit yet. I can't wait to figure out how to extract my program from the ECM and look into the code.

So my '04 Silverado probably produces 40 pulses per Rev.

I should see a value calculated off the old 245 70 16's in the code based on the 40 pulses and this would confirm the pulse count. Then follow your excellent advice and calculate the new total using you example and enter this data and reflash the EEPROM with the modified program.

I travel like 300 days a year and this is one of those times I can't wait to get home and play with the new Toy!!!

H3

[EC_Tune]

Done and done!

Only "extra" tools you need are something to mark the ground (pennies work pretty good!) and a tape measure to get the rollout in inches.

[cpelton]

what about using the tire size feature? click on the unit conversion menu in the editor, enter your tire size in as 295/70r16, you'll see the spot to enter it, now that will give you the tire diameter. Click on speedo in the editor and there is a spot to enter in your new tire size, copy over the existing gear ratio from the greyed out box next to it and enter in your new tire size and click commit. It will change all transmission shift points automatically to correct for your tire size as well as correct the speedo for u

[Triple H]

I'll look at the that and have the software calculate and also perform the calculation as Ec has explained. it will be interesting to see how close the software gets to the real world measurement. I got my delivery date recently and can't stand waiting to get my tuner. I have downloaded much reading material. This is going to be awesome!


Sit'n in the Hotel Bar checkin out the forum, I love it. Can't wait to get home.

[EC_Tune]

Problem is, the calculators don't take into account tire "squish" that is there when you put 1000+ pounds on each tire.

[cpelton]

ahh i gotcha, ya i wonder how much of a difference it would make, would be interesting to figure out. I just got hptuners 2 weeks ago so im still early in the learning process!

[EC_Tune]

This ought to give you some idea:
2003-2006 Hummer H2 uses a BFG 315/70/17 tires (34.5" according to BFG)
in the stock speedo cal they are 33.69"
That's almost an inch of tire squish.

[Triple H]

Hi EC:

I received my PRO Unit today and couldn't start the software, says invalid activation key. I emailed customer support. Once iget that functioning I'll download my trucks bin file and edit the Tire info. Thanks for the formula and method.

I came up with 102123.24 pulses. I listed the wrong tire size, they are really 295/75/16's and with the real world measurement at the tire pressure I run it was 101.75 inches per tire revolution (approx 33.3 inches diameter)


Thanks
Triple H

[Triple H]

Hi EC Your the Man

I have tried to type three replies and hit the wrong buttons and blown the responses!!!!!

I used your info from your replies to me and they worked excellent!!!!!!

Refcer to my responses previously and my Speedo is spot on!!!!

[EC_Tune]

Congrats & my thanks to Chris for explaining how all this mess works in the first place.

[blownta]

what is the Rev-Trans value? i noticed you said most of the time its 40.
i am trying to calibrate mine, i figuired out how to do it but not sure on the trans value= 40. is it 40 on a 99 trans am. 3.23 gear.
thanks

[blownta]

nevermind, got it.
thanks.

[Blacker]

On my way to work, the police put a radar box along the side of the road. I found my speedometer was off by 2 mph at 60mph. Finally tweaked the Tire/Gear Ratio until the speedometer matched the radar box. My new ratio is 6.879 (25.66"/3.73) and my old number was 6.70 (was to account for weight on the tires).

[TwoFast4Lv]

I never thought there would be some thing so simple I would like about edit and Cats over HPT but this is it!

[8850]

On my way to work, the police put a radar box along the side of the road. I found my speedometer was off by 2 mph at 60mph. Finally tweaked the Tire/Gear Ratio until the speedometer matched the radar box. My new ratio is 6.879 (25.66"/3.73) and my old number was 6.70 (was to account for weight on the tires).

Were you reading 58 or 62 mph? Forget which way to go but if I remember right it backwards than what you would envision.

[Programmer]

I never thought there would be some thing so simple I would like about edit and Cats over HPT but this is it!

Absolutely !