I am the 3rd owner of a low mileage F150 HD SCREW. I have a diablosport predator programmer and used it to change the revs per mile but my MPH are still off. It is like the computer doesn't recognize the change.

I measured the actual height of my rear tires while on the truck and it comes out to 651 RPM (20168/31) and that is what I input and verified as such. Yet, when my speedo reads 70, I am actually moving at 65 MPH.

Stock tire size is 275/45-20 and the tires I am using are 305/50-20.

Is it possible a previous ower attached a hard code chip device or something that is overriding the predator changes when it comes to tire size? If so, what might it be and what might it look like? That is the only solution I can think of - but I don't know where else to seek advice.

Thanks!

 

How many inches from the ground to the center of the axle (radius)?

 

Try and check your tire size again. I figure your 305-50-20 tire size at 32". Plug the numbers(20168/32)= 630 this number has your wheels not turning as much in a mile, and should bring you closer to what you need. If that does'nt work, keep pluggin in a lower number<630 untill you get to your desired speed.
I also run the diablo, and had to mess with tire/gear change settings to get the correct speed vs rpm. also, I got lucky enough to have one of those speed trap carts parked down the road, so i could use that to figure out correct speedo settings.
Hope this helps

 

305/50-20 with 5% squish comes out to about 642 Rev/Mile. If you leave out the squish it is 630, as wonderboy suggested.

Also, keep in mind that the Speedo needle will ALWAYS read 2 MPH faster than wat the vehicle's PCM is actually reading, so technically you are only about 3 MPH off.

Bill

 

Thanks folks. I am glad you mentioned the extra 2 mph, as I had already figured that in as I just always reduce it by 2. The exact reading on speedo was 72 vs 65. I reprogrammed to 630 to see what happens and will have my wife follow me tonight to see if it is correct.

Without the weight of the truck the diameter measures 32 inches, but while on the truck I only get 31 inches from the ground to the top of the tire.

Again, thanks to each of you!

Mac

 

This is why you measure the radius. The tire is only loaded from the axle to the ground.

 

Or, even more precise - put a mark on the tire that is at the center of the contact point on the ground. Mark the ground there too. Then, back up or move straight forward until that tire mark is EXACTLY at the center of the contact point again after one revolution. A helper is useful here and doing it on a fairly level concrete surface helps too. Mark the gound at that point too.

Now, measure the distance between the two marks. If you can do it in centimeters, you've just measured the EXACT distance covered in one tire revolution. If you have to do it in inches, multiply the measurement by 2.54 and again, you've got an EXACT distance per tire revolution.

You should do this using the front tire, I think, since the speedo and PCM should be getting it's data from there. The rear wheels are more subject to "loading" and may have a bigger variance in revs per mile. (The reason we don't use 4WD on hardball - they don't spin at the same rate).

A radius measurement is going to have a slight error, and when you muliiply that by 2 x pi, you multiply the error.

- Jack

 

I took the advice and executed both measurements. Radius to center of axle (both sides of vehicle) is 15 inches - this calcs out to 672 revs per.

I also marked my tire with a thick stripe of white latex paint across the tread, backed up two full rotations and measured the distance between marks for two rotations = 97.5 inches per full rotation and that calcs to 650 revs per.

It seems I should be somewhere between 630 and 672, but why such a variance? While I surely can manipulate the revs to force my speedo to be correct - it is driving me crazy.

It seems stock gears for my truck should be 3.55. What would be the implications if it actually has 3.73? How can I tell? Would this cause my variance? It does seem that no matter which set of tires I run (I have 3 sets: winters 235/70-16, Harley stockers at 275/45-20, and the awesome 305/50-20 aftermarket chomes) that I am always moving slower than the speedo says. I can tell by the large number of people that pass me up on the hwy. They aren't passing me when I am driving another one of my vehicles.

Here is a nearly complete list of the mods I have done just in case someone spots something that might contribute to my problem:

K&N CAI (SS)
Halo Plugs and Excel cops
SS Brush Guard
Wolverine Bug Shield
BedRug
Roll-up tonneau cover
Westin W2W SS steps bars
305/50-20's Toyo Proxes S/T
DiabloSport Predator programmer
HellWig RASB
Energy Suspension Front endlink bushings
SkyJacker Nitro 8000 Shocks
Energy Suspension 1.5 inch coil spacers
Husky Liners

Oh, and when I bought the truck 6 months ago it had a staggered stance with 295/45-20's on the rear and 275/45-20's on the front - if that matters.

Again, thanks for additional wisdom!

Mac

 

macmanks,
When you program the new numbers in, be sure you enable the revs per mile change in the diablo. that could have been looked over. It should be in the same place as the axle ratio change/ tire revs per mile.

 

Well, the correct answer was......672. Thanks to everyone, but especially i.ride.suzuki because it was the radius measurement to the center of the axle that proved closest to accurate.

God Bless each of you!

Mac