So I just installed a set of 305/55r20 wheels. I had replaced my stockers at 275/65r18

I know this is about an inch in size difference so I was expecting my speedo to be off some. So a day before I did the install I wrote down some numbers at different speeds. I compared my trucks gauge to my gryphon pod gauge.

I wrote my rpm's and speed differences at 20,30,40,50,60,&. 66mph.

It was 1mph off all the way until 66mph. My gryphon read 64 mph. Every time besides this my gryphon was 1 mph slower.

I also wrote down what my rpm's were at these speeds as well.

Ok, so fast forward to tonight. I take my truck out along with my notes and every reading was the exact same right up to 66mph and the gryphOn read a 64 again.
The only slight difference was the rpm's and even that was minimal. Example at my trucks gauge of 60 mph the gryphon read 59mph and the rpm's were at an average of 1,655.
Tonight with my 305's trucks gauge read 60, gryphon read 59mph and my rpm's (gryphon gauge) read an average of 1,672 rpm's.


I'm not sure if this is correct or not?
I really didnt even notice any power loss either?
Shift points still fill the same too!


Just a little info on my truck. I'm running a 87 performance tune and am running above said new tire and wheels (305/55r20)
With a 6" lift. 3:73 Gears/5.4L Screw.

I was thinking for sure I was going to have to do some tinkering with my gryphon but I think I might just let it be?

Another thing I forgot to mention that I measured my old set and new set of wheels on the truck and my new tires are about 3/4 bigger.

What do you guys think, if I make an adjustment will I even feel a difference in performance, ie: shift points, rpm range?

I'm really surprised that I could barely tell any power loss with this wheel set up?

 

Well your post begs the question; Why haven't you DONE the obvious (DO the Gryphon tire size adjustment and re-do/finish the experiment you started) to answer your own question and then post the comparison results... just sayin'. (how are we supposed to know the answers to what your truck will/might do?)

 

Unless you reloaded the tune with the new tire size nothing will change, your truck does not know you changed size, it's not a mind reader. If you have a portable GPS unit use it to compare your speedo reading to what the gps says your doing. Then adjust the tire size in your gryphon until your speedo matches the gps. Once you compare your speedo reading with your actual speed, then you will know how far off you are.

Thats what I did when I put on my 35's and now my speedo is more accurate then it was with the stock tires.

 

I'd skip any electronic measuring of your speed as things like a GPS can be off by 10+%. Go to any interstate hiway where you'll find mile markers. Start at one with the tripmeter at zero and drive for the next 10 markers or 10 miles. How much the tripmeter is off is how much the speedo is off in percentage exactly.

 

Umm - the speedo and the Gryphon get their speed readings from the SAME sensor, that's why nothing changed! You need to use a GPS or manual timing against mileposts to find your TRUE speed, then apply corrections with the Gryphon.

Go out on the Interstate when there's no traffic, and set your cruise at exactly 60 mph on the Gryphon. It should take exactly one minute to go one mile. The math for the true speed is to divide 3600 by the number of seconds. A GPS *should* agree within 1 mph.

The Gryphon is going to be more accurate than the speedo - most Ford speedos read a bit high.

 

When the speedo says 60, you will be going 62.1 MPH
ASSUMING the speedo was right to begin with.

Mine was not accurate with the stock tires, when I went to a different size, just a C hair different to fit the wider wheels, the minute diff in tire size made the speedo sync perfectly to the 2 GPS devices in my truck.

When in doubt GPS it, there are free apps for smart phones if you don't have a true GPS or NAV system

when in doubt.... http://www.miata.net/garage/tirecalc.html

 

your speedometer is actually proably more correct now than it was stock. from the factory the speedometer shows 1-2 mph fast.

 

I've found the GPS to be VERY accurate when compared to mile marker timing. Typically it's within 1%, not 10%. I did do my test on relatively level ground, hills may affect readings.

 

Like everyone else has stated.. Test with a gps vs. your gryphon.

 

Thanks for the replys guys. I guess I didn't think it through enough about the gryphon and speedo getting there readings from the same source.

I just followed the gryphon tire change calculator and factored in a 2% squish factor as with a 55 series sidewall there will be less squish.

Came up with 2596, and plugged this in as the value in my gryphon.
Got the help from another thread.

This is about as close as I'm going to get for now.
I have a gps app on my iPhone so when I get the chance I'll check it.

I think I'm good with the value I got though (2596)

On another note, I hardly even felt a power loss?
I think I was expecting a big change and that's why it wasn't that noticeable.

Now with a gotts modd and my magnapack I should pick back up some of the lost ponies!