215/85 16"verses 235/85 16"???

[pwr2tow]

The way I understand it is on a 99 dually the biggest tire I can put on is 235/85 16" before I run into problems with the tires rubbing on each other on my dually.



1. The question is how much taller are the 235/85 16" than my 215/85 16" Tires?



2. What would the differential ratio be, I have 3:54's now?



3. What would the RPM's be if I run in third direct, now at 65 MPH is approximately 2600 RPM's, what would RPM's be at with the 235/85 16" tires?



Reason I am thinking about changeing is I like the power in third direct but not the loud engine noise at that RPM or the lower fuel mileage. Just wondering if it is worth going to a taller tire.



Ron

 

[Ben Rumson]

Formulas

You ask what 3rd direct would be... I don't know. But the following formulas work once you're in overdrive.



Overall Ratio = (3. 54)(trans overdrive)

[transmission overdrives: Pre '94 auto 727 = 1. 0, auto 518 = . 69, 5 speed = . 77. Post '94 47RH = . 69, 5 speed = . 74]



Example: 3. 54 x . 69 = 2. 4426



MPH = (RPM)(Tire Diameter)/(Overall Ratio)(336)



Example: (1950 rpm)(31. 6" tire dia)/2. 4426 x 336 = 75. 08 mph @ 1950 rpm



RPM = (MPH x 336 x Overall Ratio)/ Tire Diameter



Effective Differential = (stock dia/new dia) X stock diff



Example: (30. 5" stock dia/31. 6" new dia) x 3. 54 = 3. 41 Effective Differential



another simple way is to go 31. 6/30. 5 = 3. 60% change

which is very close to 3. 54 - 3. 67% = 3. 41



Side Note If you change your speedometer transmission gear to adjust for different size tires, remember that if you go to a TALLER tire than stock, you need to go to a SMALLER tooth gear. There is an INVERSE relationship between the tire and the gear.



Example 31 tooth / 30 tooth = 3. 3% change in speedometer reading - which is closest to the above 3. 6% change in tire size.



Issue #17 page 34 and 35 has some examples but note that the editors not at the bottom of page 35 is NOT correct. He indicates that going to a larger tire means going to a larger gear. This is NOT true. Going to a larger tire, you need to go to a SMALLER tooth gear to properly adjust the speedometer.



If you have a porgrammable calculator, make a siimple program using the MPH formula. You'll have 3 unknown variables, the RPM, the Tire Dia. , and the MPH. Once you get your known tire diameter entered, you'll only need to input either the mph or the rpm value and have it solve for the other.



I went from a 30. 5 inch tire to a 31. 6 inch tire (3. 6% change) therefore I went to a one tooth smaller speedo gear 31 tooth / 30 tooth = 3. 33% (closest match, 29 tooth would be too much).

This dropped my effective differential from 3. 54 to 3. 41 = better mileage, lower rpm's - but not as strong getting from a dead stop to rolling. I haven't noticed any decrease in power since going to the taller tires, but I haven't worked it at GVW yet either. With the torque that my truck is putting out I'm not worried. It has power - period!



I'm using 235/85's vs my former 265/75's. I REALLY like the taller tires.



Note: I'm not a Dually. Just trying to help you with the formulas. I have a similar post from today to help out a friend with the tires on his Dually.



Issue #37 pg. 142 has a good article on Gearing and gear Ratios that discusses tires sizes and their affect on the differential ratio.



Good Luck

- Ben

 

[willyslover]

ron. . yours has electronic stuff... so you have to go to dealer for reflashed spedo..... or buy one of the electronic boxes to dial in the proper values... you can see one of these boxes online if you buy a JP or 4 wheel off road magazines. . they have them in the ads in the back and such.



one of these days I'll put them in my favorites as I tell losts of guys about them [the boxes]