Interesting Discovery

[04QC4X4]

When I was reprogramming my truck to disable the MDS, the programmer makes you look at all the current vehicle settings.  I noticed that the tire height was set to 31.5 in which is the stock 17's.  I have 20's that are 33 in tall.  I thought that maybe this was just what the programmer was set to from the factory, so I went ahead and changed it to 33 in.  I drove the truck to work today and noticed that my rpm's at 70 mph are now 200 lower than before.  I wonder if my truck was programmed for 17's from the factory.  If I run my rpm's up to where they used to be at 70 mph I'm now going about 74 mph.  What do you guys/gals think?

[lilfroger]

Yes there is a default tire size set at the factory in the computer. It is used for lost of different calculations. When changing it will adjust the shift points (on an auto), change the speedo, there are others but I'm drawing a blank

When we set ours to the correct size I noticed my lie-o-meter didn't lie so much either.

It uses the size to set the Revolutions per mile of the tire. Here is the brake down for the stock tires:


P245/70R17 (WRANGLER SRA-GOODYEAR) 685
LT245/70R17 (LTX A/S-MICHELIN) 675
LT265/70R17E (LTX A/S-MICHELIN) 657
LT245/70R17 (RUGGED TRAIL T/A-BF GOODRICH) 684
LT265/70R17E (RUGGED TRAIL T/A-BF GOODRICH) 658
LT285/70R17D (ALL TERRAIN T/A-BF GOODRICH) 632
P265/70R17 (WRANGLER SR/A-GOODYEAR) 657
P265/70R17 (WRANGLER GS/A-GOODYEAR) 661
LT275/70R17E (WRANGLER-AT/S GOODYEAR) 649
LT275/70R17E (TTL-GOODYEAR) 649
P275/55R20 (EAGLE LS-GOODYEAR) 655
P275/60R20 (WRANGLER HP-GOODYEAR) 636
LT235/80R17E (WRANGLER SRA-GOODYEAR) 649
LT235/80R17E (WRANGLER GSA-GOODYEAR) 649
LT235/80R17E (AMERITRAC-GENERAL) 649
LT235/80R17E (AMERITRAC TR-GENERAL) 648

[and just as a side note....if you're not on stock tires and would like to see what your Revolutions per mile are of your tires you can go to http://www.csgnetwork.com/tireinfo4calc.html and it will calculate it for you]

[Cumminalong]

It's not your RPM's that are lower, it's your actual road speed is lower.

When you made the tires a larger size, it makes the speedometer read faster.

If you told the computer you had smaller tires, it would make the speedo read slower than the true road speed.

You NEED to use a GPS when adjusting the tire size. It's the only way you'll get it really accurate.

For What It's Worth.....

My speedo wasn't correct until I ran my 33's, but had the height set at 31.6".

[Kilch123]

Great site Jo - that's ones now in my favorites!

I've been trying to set my Adrenaline, and still can't get it right!  My speedo shows about 5 MPH below GPS, but the Adrenaline only adjusted like 3 MPH of that

[Mitch]

How do we look at the current settings and make changes? I have been all through my overhead display deal and I never saw that.

[Cumminalong]

The only thing it will do with the Edge is display the speed on the Attitude.

It won't correct your speedo.

It's under Juice Configuration.

[Parts/Service]

Here is the way to figure out how many revs per mile you tire does.  Its really complicated but really simple...

There are 5,280 feet in a mile.
Take a piece of string and wrap it around the tread of the tire to determine the distance around the tire. 
Measure that in inches'
Then we take 5,280 feet  and multiply that by 12 (inches) which gives us 63.360 inches in a mile.  Then we divide your tires inches into 63,660.  And that gives us how many revs the tire turns in a mile

Example:

I know from experience that a 315 tire is 108 inches around
so 63,360 divided by 108 equals 586 revs per mile.  Simple enough right?

Any questions, just ask...

[Mitch]

I got the Edge figured out.

I am talking about the OP discovery when he was reprogramming his truck. How are you reprogramming your truck?

[lilfroger]

Mitch he has the Superchip Flashpaq for the Hemi's it works differently than the Edge.

[Mitch]

Ohhhh I didn't catch that part.

[big.bryant]

a buddy of mine threw on some bigger tires on his cummins and got a ticket for speeding because his new tires were 13% off from stock (making his speedo read 13% slower)... remember when you change tire sizes, change the speedo or you can risk getting tickets and your mileage won't be accurate any longer...

[04QC4X4]

I guess I forgot to mention that I bought my truck brand new with the 20's.  They were on the factory sticker, so I don't think the dealer added them, but it was obviously programmed for the 17's.

[Jr. Mechanic]

a buddy of mine threw on some bigger tires on his cummins and got a ticket for speeding because his new tires were 13% off from stock (making his speedo read 13% slower)... remember when you change tire sizes, change the speedo or you can risk getting tickets and your mileage won't be accurate any longer...

Yeah my speedometer is off by about 10%, but im too poor to buy a new speedo gear from Dave 

[Scott91370]

Example:

I know from experience that a 315 tire is 108 inches around
so 63,360 divided by 108 equals 586 revs per mile...

What aspect ratio and wheel size is that at though? A 315/75-16 (35 tall) is smaller around than a 315/80-16 (36 tall) and a 315/75-17 (36 tall).

[NickC]

You NEED to use a GPS when adjusting the tire size. It's the only way you'll get it really accurate.

For What It's Worth.....

My speedo wasn't correct until I ran my 33's, but had the height set at 31.6".

i was just coming in here to share my finding with you all. i found out exactly what rich did, you HAVE to check with a GPS. i first thought i could just set me tire size to 33 after i got new rubber and started driving around. well to say the least, it did not feel "normal" so i checked it with a HPS and found out it was not.

i cant remember exactly what i end up setting it to but i know it was somewhere around the same as rich... 31 inchs.