Speedo way off on '06 CTD

[Smoked!]

Got the new revision in my 7131 and I've noticed that my speedo is way off when I input the correct stock tire size in (what's on it), roughly 3-4 mph.

I have a 06 CTD with a 6spd and 3.73 gears.

Are the rear axel and tranny ratios accounted for in the tuner? If I reload the tune with out the tire size feature, will this correct it?

[johan]

Did you change your tire size from the stock size?

[Smoked!]

They are the stock size. 265/70R17s to be exact.

My Rpms are noticeably lower at 65 mph with the tuner set for the stock size vs stock tunning.

[johan]

Hmm, i never tried changing the tire size on a truck with the stock tires, it is possible the calcs gets thrown out of whack when that is done.

[Smoked!]

The reason I had tried it was because the speedo was a good 3 mph off when it was left alone.

I reloaded the stock programming and reloaded the tuner. I made no changes to the tire size portion, and the speedo is still 3 mph off. It dead on with stock programming.

Rather than adjusting by tire size, I would rather see the adjust by revs per mile (much more accurate, especially with the differences between tire mfger's), like the Predator on Ford gassers could do. This was really handy with a GPS unit as one could get it dead nuts.

At any rate I need to do something as I'm not excited about being 3 mph slow on a leased vehicle.

Any suggestions?

[johan]

I would try to adjust it via the tire height, I have seen it being more accurate than the side wall height.

[tiger094]

When I put bigger tires on my 06 ctd I used the tire height from the manufacturer. the tires I have are 35s, however the actual height stated by the manufacturer is 34.5 inches. Thats the same # I plugged into the predator using actual tire height option. You can look this info up online. The actual sizes vary from one manufacturer to another.

[Dodgezilla]

I just took a trip through Pa where the highway construction sites all have those radar readouts telling you how fast you are going. I was surprised to see that I was always going 2-3 mph less than what my speedo was showing!! Is it possible that the Predator may have changed the tire size???? I have a Dually w/ 235s on it and it seems that it is now calibrated for 265s....

[johan]

Unless you went in and modified the tire size the Predator will not alter it.
I did notice one thing here recently on a truck a i had here with 35's, the tire measured 33 inches in actual height but to get speedo 100% accurate (on dyno) i had to input 32.5 inches. Anything higher than that the speedo was too fast, i suppose you have to take tire squish into consideration as well when setting up the tire size.

[Dodgezilla]

No, I did not change it. I'm not too worried about it because that's what size tires I will be going to eventually.

[tim_r73703]

My cousin is a highway partol so i had him clock me after i changed the tire size for my truck. The first time i used the size on the sidewall and when he clocked me i was going about 3 to 4 miles an hour faster than what my speedo showed, the second time i measured the tire from the ground to the top of the tire and i was almost dead on i showed 65 he showed 64.5 almost perfect. Hope this helps

[madmanwithmouse]

I use my gps to calabraite the spedo on my 07 . With the stock tires my speedo was 1.5-2mph slow from the factory . With 245/70-19.5s it was 2-3 mph high. I also noticed that inputing the sidewall size with the tune on u7131 caused it to be off about the same as stock tune/tires. I have not yet tried actual height.