Can you calibrate speedo for anything bigger than 20" rim setup? - Diesel Forum

rambunctious · 06-23-2008, 07:13 AM

Can you calibrate speedo for anything bigger than 20" rim setup?

I have 325's ( 35") on 20 rims and dealer said " it is already calibrated for the largest tire, the 20" rims..."

well, i got one ticket already. I would like my speedo to be accurate,
Also, my shift patterns may be better if calibrated properly.

thanks

Ramb

RRose · 06-23-2008, 09:22 AM

The wheel size has nothing to do with the recalibration of the gauges.... its the overall size of the wheel/tire combo that is what you recalibrate for. In your case, you have to recalibrate for your 35" tires.... and 35" is the biggest you can calibrate for.

rambunctious · 06-23-2008, 09:49 AM

i understand,
just their comment on the "20 wheel" (ie my optional wheel and tire package that is stock...... ) is all they can calibrate to,
so they did not change anything,

but my rubber went from 33" to 35"
so I need to know if it CAN be changed. sounds like it can

thanks

Ramb

RRose · 06-23-2008, 03:57 PM

Ya... it can be done... IMO... its hard to find a dealer that knows how to do it. I'd just get a programmer for the truck that allows you to do it yourself... or go to a speedo shop.

Stymie · 06-23-2008, 09:59 PM

your dealer can, a shop can, or there are various calibrators. your tranny shift patterns wont change.

timid1 · 06-23-2008, 11:30 PM

Quote:

Originally Posted by rambunctious

I have 325's ( 35") on 20 rims and dealer said " it is already calibrated for the largest tire, the 20" rims..."

well, i got one ticket already. I would like my speedo to be accurate,
Also, my shift patterns may be better if calibrated properly.

Calibration is done on your overall TIRE SIZE, not your wheel size. Your wheel size has nothing whatsoever to do with your tire size. Google for speedometer calibration and you'll find NOTHING on wheel size.

F350FINALLY · 06-24-2008, 04:56 PM

The dealer can only calibrate for originally available tire and wheel packages. In this case, the largest tire happens to be available on the 20" wheels. The Goodyear Wranglers are ~34.5" tall. That is the largest tire they can calibrate. You will need an aftermarket programmer that will allow you to put in whatever size you like. This will run you around $200. Accupro make these....

dt75flh · 06-28-2008, 05:08 PM

superlift also sells a tire recalibration unit...called true speed....I also have the 325 tires on the factory 20" rim theyre about 35.5 in tall.....

I changed out my gears to 4.10 and my speedo is now correct within 3 mph at 80 mph....you can also get a gps and verifiy your ground speed at various rpms....

i found the tires were about 10 % off before changing my gear ratio...so at 70 mph on the speedo I was really going 77 mph

Stymie · 06-28-2008, 05:47 PM

Quote:

Originally Posted by F350FINALLY

The dealer can only calibrate for originally available tire and wheel packages. In this case, the largest tire happens to be available on the 20" wheels. The Goodyear Wranglers are ~34.5" tall. That is the largest tire they can calibrate. You will need an aftermarket programmer that will allow you to put in whatever size you like. This will run you around $200. Accupro make these....

wrong and wrong. the aftermarket calibrators can only go to 35's on 08's without throwing and abs light...

HeavyAssault · 08-28-2008, 12:29 PM

Discuss with your dealer reflashing the ABS module and use 601 as the value for the new setting.

SmokeyWren · 08-28-2008, 04:17 PM

Quote:

Originally Posted by rambunctious

Can you calibrate speedo for anything bigger than 20" rim setup?

I have 325's ( 35") on 20 rims and dealer said " it is already calibrated for the largest tire, the 20" rims..."

Your dealerperson was mostly right. The tallest tire the dealer can calibrate accurately has 601 revs/mile. The LT275/65Rx20 tires that came on 20" rims have 605 revs/mile. There is not enough difference between 601 and 605 to mess with.

Quote:

well, i got one ticket already. I would like my speedo to be accurate,

LT325/60R20 tires have 586 revs/mile. That's 3.1 percent difference. So if you don't calibrate the speedo for the taller tires, your speedo will read 3.1 percent slower than it did with the LT275/65Rx20 tires. So if your speedo was accurate with the 275s, 70 MPH indicated with the new tires will be 72 MPH on Officer Bob's radar.

If you want the speedo more accurate than that, your only option is to spend about $200 street price for a SuperLift TruSpeed calibrator or one if its competitors.
Click here for Superlift TruSpeed Calibrator

Rear axle ratio has zero to do with speedo accuracy. Changing the rear axle ratio will NOT affect your speedo. The computer program that manages your speedo, odo, and tripmeter uses only tire revs/mile for calibration.