Engine/Transmission (1998.5 - 2002) Top Speed w/ 3.55 vs 4:10's

[BigDan]

Q. For everyone going faster the 114mph, are you positive that is the true speed, not just the speed displayed by the spedo. Plus did you alter the gear ratio or tire size since it was built.



Q. Does D/C have an automatic speed control that reads TRUE speed no mater what you do to alter the engine RPM(tire,gears) or just what ever your truck was built to have.



Q. So you are all telling me that if I put 35" tires under my truck(which now has 265's) will go 120+mph with out defueling or "hitting a brick wall"



Q. Does ECM defueles based on engine RPM and nor MPH



Thanks

Got lot's o' questions

 

[emjay]

BigDan,



Check my post earlier in this thread. Putting larger tires on will make the actual speed faster, but the indicated speed and RPM will be the same. If you put 33" tires on in place of 30" tires, you will be going 10% faster than your speedo says, so when it reads 60, you're really doing 66.



I've heard that vast changes in tire sizes upset ABS, but I don't understand why.



Another example: If you had 3. 54 gears and swapped them to 4. 10s and replaced your stock tires with 35", the ratio change and tire diameter change would pretty much cancel each other out. The math behind that is:



4. 10 - 3. 54 = 0. 56 (the difference in ratios)

0. 56 / 3. 54 = 0. 158 (15. 8 % increase in ratio)



So 4. 10 is 115. 8% of 3. 54.



Take 115. 8% your original tire diameter to get the tire diameter that will cancel out the ratio change:



30. 25 * 1. 158 = 35. 0295 (pretty close to 35")



The tire diameter varies on the LT metric tires slightly between different manufacturers and models. The best way to determine the rolling diameter of a tire is to place a chalk mark at the 6 o'clock position of the tire and mark the street at that point , too. Roll the vehicle forward once until the chalk mark on the tire is at the 6 o'clock position again, mark the street, then take a tape measure and measure the distance between the marks. That distance is the approximate circumference on the tire. Divide that value by PI (3. 1415926), and you get an accurate rolling diameter. Divide by two again, and you have the rolling radius, which is the distance from the axle centerline to the ground.



For instance, for a distance of 95", your tire diameter is about 30. 25

95" / 3. 1415926 = 30. 25 (rounded to thirty and a quarter)



... and a distance of 110" your tire diameter is 35"

110" / 3. 1415926 = 35. 0 (rounded to thirty five)



And if you check, 110" is 115. 8% of 95"

 

[emjay]

OOPS!

I forgot that the tone ring on the ring gear is used for speed measurement (in the old days, it was the output shaft speed of the trans) DOH!



Oh well, that means that the speedo calibration will change with a tire size change, but not a ring & pinion ratio change. That must be what screws up the ABS. Did I answer my own question correctly?

 

[Mundgyver]

A question of what speed you are going can be answered by a garmin GPS III+. It will record your top speed to be viewed later, or when every you like Tire size maters not to the GPS.

 

[JHerrlich]

With my truck, the fastest I've hit was 112 by the GPS. Gets there pretty quick but hits a wall when I get there.

 

[WadePatton]

Topped out at 119

I tried a top-end run last night and she pulled hard up to 119 then no more.



I was too busy checking the EGT's, speedo, and road to look at R's.



That's with 3. 54/6-speed/265's UFM-race on level 5- speedo IS accurate. But might check with GPS next time.



Not sure if I was in 5x3 or 5x5. Spur of the moment sort of deal.

 

[kaptn]

Just my . 02. My truck with 4. 10 is very fast off the line and I raced a stock Powerstroke who told me there was no way I could beat his extra 2 cylinders. I Did hehehe. 3:55 I bet has a higher top end than me. I dont care, do to I dont have much room to go over 100 without a copper getting me. I plan on Drag Racin in the future and I have only raced so far in the 1/8 mile. I dont know if I could reach top end in the 1/4???? I guess I will find out. PS I wouldnt trade my 4. 10 for a 3. 55.