speedometer adjustment

[tcaron]

I'm running BFG 33 x 12. 50R 16. 5 LT tires and my speedometer is showing faster than I'm traveling. Is there an adjustment you can make?

 

[DieselDoghouse]

Possibly, the dealers DRB tool can reset your PCM computer to a better size IF one already exists in the software loaded. If not then the next choice is an Abbott's or Truspeed electronic speedo corrector which can be set to the most appropriate revolutions/ mile (that info should be available from your tire MFR). Since I believe '96 all our speedos went digital so you can't change a gear and be done. The electronic gizmos seem to work well, but they're modifying a diigital signal so they hafta be wired in better than 'just ok'. Soldering and auto-grade connectors are best

 

[rayplumas]

just bought a 25oo it had a lift and big tires, went back to stock.

the 20 in tires were showing around 10% more speed and miles than actual, verified by GPS. went back to stock tires and no lift same problem speedo shows 85 mph, actual is 74 mph. Good suggestions would be appreciated

 

[AH64ID]

Woah, 22 year old post!

What are the specs on your truck @rayplumas?

 

[rayplumas]

Stock 2500 TD 4x4, 4In pipe

 

[AH64ID]

Stock 2500 TD 4x4, 4In pipe

Year?

 

[rayplumas]

2012

 

[bcbender]

Im not sure about the 4th gens, but I know the 3rd gens had a limited range for modifying the axle factor,... I went to 34s and was at the top end to re-program for the larger R/mile. (around 5mph difference). Seems odd that your indicated speed would not change with a difference in R/mile due to tire circumference.

 

[BigPapa]

Like pulling teeth……

 

[AH64ID]

2012

Depending on how tech savvy you are, or are willing to try, a programmer like JScan or AlfaOBD can adjust that. The dealership can also set it back to stock.

There are also one trick programmers like the Rough Country tire calibrator.

 

[rayplumas]

My shade tree mech has a programmer that we are going to try. He is also perplexed on the zero change from wide 20 in tires back to stock. I also found cruise control stops at 86 mph indicated, 76 mph GPS..

 

[AH64ID]

My shade tree mech has a programmer that we are going to try. He is also perplexed on the zero change from wide 20 in tires back to stock. I also found cruise control stops at 86 mph indicated, 76 mph GPS..

Tire width and rim size have nothing to do with speedo calibration.

What size were the previous tires?

What size is on there now?

 

[rayplumas]

20 in Nittos. current tire is factory stock tires and rims.

 

[AH64ID]

20 in Nittos. current tire is factory stock tires and rims.

That doesn’t say anything.

There are dozens of tire sizes that would fit a 20” rim for these trucks, and there is more than one stock size.

@BigPapa like pulling hens teeth.

 

[rayplumas]

The Nittos were 20x12.5 R20. stock LT275/70R18

 

[BigPapa]

Only thing worse than no info is wrong info….

 

[AH64ID]

The Nittos were 20x12.5 R20

That’s not a legit size…

 

[rayplumas]

35x12.50R20LT

 

[AH64ID]

35x12.50R20's have rev/mile of ~600 and the OE size of LT275/70R18 has a rev/mile of ~626. That's only a 4% change from new, so I am not surprised you don't perceive a difference with how far off your speedo it. If the 35's were worn down at the 275's new the difference would be much smaller.

Your speedo is ~11.6% fast, or calibrated for a tire with about a 554 rev/mil which means the previous owner likely had it calibrated for 37's.

 

[rayplumas]

Hopefully the tuner we have can get it adjusted?? thanks for the help..