Ok i put some 32's on my 94 150 4x4 5.8 e4od, you could see they were bigger then the stock size on the truck.

So bigger tires make you go faster then speedo actually reads, because it is measuring rpm of trans or axle, and rpm's are same, but you are going farther per revolution.

But I get out on the road, and fire up the gps, and it says I am running the opposite. If speedo says 65 mph gps says 61. The faster i go the more the spread.

So can the speedo be that far off? If i went back to smaller tires, it would be worse.

Of course i am assuming that the gps mph is correct, it seems dead on in every other detail. it also seems that the gps speed is on with traffic, and even portable radars.

Just curious about any ideas on this, for discussion purposes.

 

Go to article /spects above tells how toi rest your POSOM to the new tires.

 

thanks for the info on that, i will look into that.

However, if the tire size was the issue, shouldnt the speedo be low, ie speedo says 65 actual speed is 68?

Mine behaves like I have small tires on it, speedo says 65 actual is 61.

I had battery cables off, and left them off and touching each other all night, in the middle of a long long road trip, for anouther reason, but it was still the same after.

I may actually be getting better mpg then i thought too.

 

Try this: Reseting Recalibrating your PSOM for the Correct Speed

 

I think that chart is off. My current setting is 918, which matches the artcle on here for the factory 265/75r15's . The article you gave says it should be 889? The aricle here lists my bfg's at 914.

Looking over these chart however, the trend is the same, bigger tires lower number. I am thinking i could goto higher number and get better readings.

I will reset it once and try it, then wait til i get home where i can get the sheriff to radar me and see what that says. I might tweek it then.

I will look up on bfg site, and also use the numbers from the site you listed to come up with a number to try.

thanks

 

HOW TO RECALIBRATE THE SPEEDOMETER ON 92-96 FORD F-SERIES AND BRONCO .: Articles
here is the link I used for my 95. It is pretty much spot on for the 33's on my truck.

 

more info this weekend, they have a chassis dyno in tucson, where i am visiting with Army right now. I am just going to go there and run it and see how far it is off, then make a change, see effect and so on.

As a plus, i will do a pull as is, then go back after i finish insulating my intake line, bypass coolant to TB lines, and maybe an EGR plug at manigold and insulate. See if these common mods really DO anything.

 

Bigger tires make your speedo read slower...
Smaller tires make your speedo read faster...

 

I have done the PSOM thing with my 93 w/32's. There are generally a lot of different opinions about tire sizes, so this is what is true--for me, for Bridgestone APT's and I have done a lot of measuring, not just talking.

First of all, the diameter and Rev per Mile figures (which are directly related), are generally not accurate (Michelin, Bridgestone) maybe not for all tires or mfg's. I have found that the standard method for interpreting tire diameters ie: 32X11:50:15 does not mean the tire is 32" tall, which directly impacts the circumference and RPM figures. My bridgestones (and michelin) are about 1 in shorter (31") than the figures say they should be. That is true for the metric rating for tires as well (ie 235X60X16 etc). Maybe these numbers represent what size the mfg's targeted or wanted to sell, but they "aint" all that accurate. Neither is the Revs per Mile figure shown on the MFG's chart.

The PSOM chart directly relates the RPM figure to a setting in its software. The method for figuring a circle, circumference/diameter/radius is a well known constant. The simple fact is, we dont loosely apply the figures to the formula. In short, its all in the "tale of the tape". If your tire only rolls 100.5312 inches in 1 revolution, it is 32" tall, not 33, not 35, not 28. That figure is is directly related to the Revs per mile. That accurate measurement is what you need to enter into the PSOM. Look down the chart to find that # and enter it.

I have measured ("tale of the tape") my 32's from 50 psi(max rating) down to 10 psi (side rollover) and there is less than one inch difference in circumference. Soon as I get some of these alligators that are nipping at my Asp "kilt" I can post those figures. My suggestion is for you to actually measure the tire as it rolls--dont try to get out at 60 mph to measure

 

I just put bigger tires on my truck but my truck is a 87 f250 do I ajust for that?

 

I think you are going to have to use a different speedo gear at the tranny. If you are out of range, a correcting adapter (will involve some cost).

My opinion is that, if you are concerned, you should learn a procedure to calculate the speed then try to make the equiptment match.

I am fond of using % for a lot of calculations. It will roughly convert most parts correctly. If you speed (measured) is off by 10% over, then 10% under is the gear needed.

I like to use several miles on a flat turnpike etc and measure time at 60 mph, sometimes difficult to hold at perfect 60. Some people use the GPS. I havnt done that yet, but am a little sceptical (?).

I have done a reverse calculation that will be accurate except for a small percentage over converter slip. Measure actual rollout of tire in inches. Do the calculations of gear ratio of rearend and tranny ratios or 1:1. You can ultimately wind up with a correlation to RPM or current speedo, if you are out of range of available speedo gears.

I have set up a chart in MS EXEL for tire sizes where all I have to do is enter the advertised size and get all the figures (I am aware that the figures may be slightly off). Do the same for all the reverse engineering of tire sizes and present gearing. All the effort is only worth it if you really care. Worst part is figuring it all out---the first time! LOL