Small tires lead to bad fuel mileage ?

[CEnzien]

My 2001 ram 2500, 4x4, 3:55 rearend, automatic, 235 hp engine

has 245/ 75/r16 tires on it. This is only a 29" tire. I'm told these

trucks usually come with 31" tires like 265/85/r16.

I have been told that the 2" height difference in these tires can mean 2 to 3 mpg difference. I'm only getting 15mpg now.

IS THIS TRUE ?

Also I'm told that if i go to a 31" tire from the 29" tires on it that this will not screw up my odometer, speedometer or tachometer because the electronics in the engine and transmission will automatically compensate for the difference in tire height. IS THIS TRUE ?

 

[Mschevey]

Some items true, some not.



Smaller tires mean more rev per mile this in turn means more rev. per min. for the engine to go the same speed. Higher rpms lower fuel milage. Generally speaking.



Automatinc compensation for larger than stock tires, NOPE. The compuer can be reprogramed to compensate for up to two size larger tires by your dealer. (Not automaticly. ) Or you can get a 'little black box' that will recondition the signal for the speed sensor. Then you calibrate the box to any size tire you have on.



Mike.

 

[klenger]

Go to http://www.miata.net/garage/tirecalc.html and compare tire sizes between the 245-75R16 and 265-75R16. You'll notice that one the 245 is 30. 5" dia and the 265 is 31. 6" dia. The % difference is 3. 9%. The tire difference brings your effective axle rato to ~3. 67. Not much of a change. You somewhat poor milage is probably due to the engine not being broken in, not due to the tire size.



Do you tow? If so, the smaller tires provide better torque and therefore better performance while towing. I have 4. 10 axles, and don't find that they hurt the milage all that much, and the towing performance is great.



Ken

 

[Posm]

I drive a lot of miles, 100K since May 2000, and have went through the change you are considering. I had the 245/ 75 tires on my truck stock and put 45k on them. I switched to a 235/ 85 tire which is the same about the same height as the 265/75 only a litle bit more narrow. I got NO increase in fuel mileage with the switch. I did get a lowering of RPMS by about 100 across the highway speeds and that puts me out of the tourque band when running 55 to 60 MPH. I have had my truck to the dealer three times to get the speedo recalabrated and have had no luck. I adjust the miles by 5. 03% to correct for the tires. I arrived at this figure bu doing many 200 mile plus interstate runs and comparing with the mile markers. I had also checked it with the stock tires and it was right on. I have found after checking over 200 tanks of fuel for mileage that speed is the biggest factor not RPMs. I get over 20 MPG when I run under 70 for a full tank and I get 16 when I run 75 plus. I get 13 MPG when I pull a gooseneck with a GCVW of over 22K, but only get 11 MPG when I pull a tall hay baler with a GCVW of 13K. Wind and speed make more diffrence than RPMs. Do a search on MPG and I think you will find as I have that the people with 4. 10s get as good of mileage as those with 3. 54s. I like the look of the taller tires, but don't like the loss of launch tourque. I need new tires in about three months and I have not decided whether to go back to the 245/75s or stick with the 235/85s. My ideal combo would be 4. 10s with taller tires or maybee just a set of 3. 73s. Are you on the line DC. Listen UP. I do not think the 3. 73s would provide better mileage, but I think it would allow for less stressfull launches and more tire lattitude.

 

[CumminsAholic]

Going from the stock 245/75's to 255/85's I gained about 2mpg. But you have to take into consideration that I have 4:10 gears. Also I like to drive fast. Without getting my speedo corrected I can get the same amount of miles on a tank of fuel. Say I have 500 miles and it takes 30 gal of fuel. With the stock tires that's 16. 7mpg. With the 10% added for the taller tires, making it 550 miles, that's 18. 3 mpg. I'm now going to 295 75's, about the same height as the 255/85's but wider. I'm interested to see how much this lowers my mileage because of the width and weight difference.

Corey

 

[Posm]

I believe Cumminsaholic and I may both be right. If CA is driving fast ( over 75 MPH) then his truck would be out of the " sweet spot". There was a write-up in the TDR several issues back that showed the "best RPM" at a constant load. If I remember right, the best RPM for 24 valve engines was about 1900 to 2500 at a constant load. That is not to be confused with the diffrence between driving in high gear at these same RPMs. The load is much greater at 2500 RPMs because of wind resistance and mechanical ineffeciences of the drive line. This RPM comparison would only be valid on a pump or genarator that has a constant load. I would guess that Cumminsaholic is doing about 2850 RPMs with the 245/75s and about 2600 with the 255/85s. I find that I am on the low side of the sweet spot more than I am on the upper side with the 235/85 tires. Oh, DC here my call for a 3. 73 rear end. That would allow for tire size flexability and still keep the engine where it should be at all practical speeds.