my box stock 05 6 speed 4x4 qcab,truck has about 7500 miles on it now.............with 245`s got 16.5 all day long pulling boats and such,only tank checked without any pulling got 18.2 mpg........now,ive put on h2 wheels,and 35x12.50/17 mastercraft courser tires.as far as watching others speedometers going down the highway,appears to be about 7mph off at highway speeds.where it used to run 2200 or so at 70,now it runs 1850-1900 or so..........last tank fill up went 365 miles on 25.12 gallons..so what does anyone think its actually getting now??..........thanks for input,bama

 

OK, I will try. First are you sure you had 245s, most are 265/70R/17s on 4x4s that I have seen.

Is the 365 actual miles or Odometer miles. The speedometer difference between 245/70R/17s and 35x12.50x17 is 13.99% slow. I assume that the Odometer is going to be off about the same percentage.

If that is the case and the 365 are odometer miles(assuming again), then you should have actually traveled about 453 miles. 453/25.12 = 18.0 mpg, hard to believe. I lost at least 1 mpg with the 315s.

If you had 265/70R/17s the speedometer difference is about 10% slow. Same assumption on odometer. You should have traveled about 401 miles. 401/25.12 = 16 mpg. That is more realistic. Just my feeble attempt at math.

 

Well I found that the stock 2005 with 3.73 and new 6 speed manual turned 200 more rpm at 60 mph in 6th then my 1998.5 with 4.10 auto in 4th lockup. So I'd say you could pull much bigger tires and improve milage greatly.

 

my truck was a st model and came with 245`s,however,i did notice that the speedometer read a little fast with the 245`s.i think the speedo`s are probably calibrated for 265 size tires regardless of what size came on them........i just wasnt sure if the differences were constant or if they exponentially grew as you got faster when going to a taller tire........bama

 

Well the percentage of difference is constant.

 

I worry about guys from Alabama that use the word "Exponentially" in a sentence about their trucks

 

FWIW, my 265 equipt SLT is within 1 mph of what my handheld gps says. You have my permission to use this info as an exponent.

 

thats what i love about Top,,i can always count on him

 

Consider your "Exponent" exposed

 

I had 245s on my truck, with 4.10s and the 6spd. My speedo was dead on according to my GPS. When I put on my 315s my speedo/odo is exactly 13% slow, so at "50mph" I'm actually going 56.5mph.

You need to get a GPS, or at least borrow one and just verify it.

 

Well that kinda verifies my limited math capabilities. I calculate 245/70R/17 stock compared to 315/70R/17 speedometer difference should be 12.65% slow. I do believe that the correct "Pinion Factor" (RevsPerMile) should be programmed. Since this is part of the ABS programming it could effect the ABS breaking.

 

i finally ran it against a gps......10% diff.....50 is 55,60 is 66,70 is 77.........so the 16 mpg would be the number........bama