2nd Gen Non-Engine/Transmission And another bigger tire question

[kozowski]

OK, if I go to bigger tires, what needs to be done, if anything to the ECM/PCM? How about adjustments for the speedo?

 

[Ladders]

Just a couple of ans --

My 0. 015¢, if you search for this, you can find out the dealer can reset the 'puter to certain sizes based upon revs per mile/size. There are also boxes avail that can make adjustments for different sizes.



Need to know current size/revs per mile and then the new size/revs.



Hope this assists you.



SOTSU!!

\\BF//

 

[gsbrockman]

kozowski :



Think twice before "adjusting" your speedometer. I replaced my crappy Michelin LTX A/S's (265/75/16) with LT285/75R16 Bridgestone Dueler A/T's at 330 miles when my truck was brand new. My odometer SHOWS 32,000 miles but after correcting for the OD of the two tire sizes, my truck actually has around 33,157 miles... Think of it as an extended warranty at no additional charge. And at 36,000 miles, I'll actually have received 37,302 miles of use out of the truck. {all this translates to 3. 61818 percent difference} My effective axle ratio is now 3. 42 as well--a really nice cruising gear on the interstate, I might add... .....

 

[emjay]

Joseph, Oregon, huh? I passed through there about twelve years ago -- what a beautiful town!



Anyway, I'll post this again, it should answer all of your tire/gearing questions:



Here's two simple (if you have a calculator ) formulas for figuring RPM and speed based on O/D ratio, R/P ratio and tire diameter:



MPH = (RPM/OD/RP) x TIRE_DIAM x 0. 002975



... and... .



RPM = (MPH/0. 002975/TIRE_DIAM) x OD x RP



Tire diameter is in inches. So, at 3000 RPM with 30. 5 inch tires, you get 111 MPH with a 3. 54 RP and an OD of 0. 69. With 4. 10 gears, your speed at 3000 RPM is 96 MPH.



Note that "what-if" comparisons are easy as scaling one parameter, such as tire size. For example, say you wanted to switch to 35" tires from the 30. 5" tires in the first example. All you have to do is divide the new tire size by the old tire size and multiply by the speed; 35/30. 5 = 1. 15, then 1. 15 x 111 MPH = 127 MPH. You can use that same scaling factor derived from the differences in tire sizes to determine effective gear ratio, for instance 4. 10/1. 15 = 3. 56. So 35" tires and 4. 10 gears are nearly equivalent to 3. 54 gears and 30. 5" tires.



Since you must have a computer to read this, you may already have a spreadsheet program that you can enter this stuff into (like I did) so you only have to think hard once