I thought my new bigger.....

[White Ram1]

I thought my new bigger TIRES would drop my RPM's.



I went from stock size to 35's, but at 70 MPH I'm still dead on 2,000 RPM's before and after the change of tires.



I'm new to the Cummins game, maybe there is something in the computer I need to modify to drop the RPM's???????



Any input???

 

[Cummins4WD]

Did you have the dealership set your revolutions per mile to 601 with their DRM tool... There is a setting for tire diameter which you should have changed.

 

[White Ram1]

Is 601 rev's per mile the number it needs to be set at for 35" tires?



I take it in Monday for the Mopar remote starter install, so I want to have them set the rev's per mile if I can find out the number it needs to be set at.



Thanks for the info and help!

 

[Big_Daddy_T]

Its says your doin 70 but you aint. You are going prolly over 80 mph.



I didnt do the math but it appears he did. 601 would be the revolutions per mile with a 35 is what he stated.

 

[XJSuperman]

If you have the BFG 315's you would be at 601 revs per mile.



If you h ave an actual set of 35's (which are about 1/4" taller than 315's) you would need to call the tire manufacturer OR check their website for the correct revs per mile.



A BFG 35 is more around 594 revs per mile compared to the 315's 601. BUT check with the manufacturer... an MT vs AT would be different... and a different tire make may be different as well.



I posted in the 'non-engine' forum as to you speedo recalibrating but I'll say it here too.



Right now when your speedo says 70 you are running 78-80.



Once you recalibrate it at 70 your RPM's would be at about 1750.

 

[4xquadrod]

I put on procomp 35" tires and took the truck to a speedo shop to see the actual speed.
When my speedometer indicated 70 mph, actual speed was 73 mph.

 

[dhill]

Why don't you spend a couple hundred $ for a good GPS and use that for your spedo leave the truck computer alone and extend you warrentee by 10-12 percent?



Dave

 

[White Ram1]

The tires I've got are Dunlop Radial Rovers and they are marked 35x12. 5x17, however like most tires they are not a true 35, they are only 33. 5" tall. I have no idea how to figure the rev's per mile based on tire size.



If they adjust the rev's per mile I assume that will correct the speedo. Is that correct?

 

[XJSuperman]

Yes that is true... the adjustment makes the speedo read accurately.



The only dunlop specs for revs per mile I could find was on the radial mud rovers. For the 35X12. 50-17 it lists them as 637 revs per mile. And they are a short tire... also... if this is what you have they are only a load range C tire???

If you have another radial rover... . unless someone else can find it... . you;d have to call dunlop and get the revs per mile.

 

[Luke Warmwater]

I went from stock size to 35's, but at 70 MPH I'm still dead on 2,000 RPM's before and after the change of tires.

Someone tell me this is not a funny thread lol.

 

[White Ram1]

XJ,

They are the Rover RV, not the mud. They are C rated. Tire shop said they are plenty for my towing needs, 36 foot gooseneck with a racecar, or a Viper in it. I haven't ever towed anything heavier that that. Does that sound right?

 

[White Ram1]

Someone tell me this is not a funny thread lol.

Sorry if these are dumb questions Luke. I'm a total newbie to the Dodge CTD world. Trying to learn and figure out things that I have no idea about. I thought that was one of the bennifits of this website.

 

[ak_fleming]

The tires I've got are Dunlop Radial Rovers and they are marked 35x12. 5x17, however like most tires they are not a true 35, they are only 33. 5" tall. I have no idea how to figure the rev's per mile based on tire size.



If they adjust the rev's per mile I assume that will correct the speedo. Is that correct?

here is the math get out your calculator.



33. 5" diameter tire converted to feet for easy math = 2. 792' Diameter.



2. 792' X 3. 1416 (pie) = 8. 7702' tire Circumference.



5,280' (ft in 1 mile) / 8. 7702' = 602. 032 rotations per mile.



hope that helps

 

[White Ram1]

AK,

Thank you for the info!





XJ,

Same thing, Thanks for your help too.

 

[XJSuperman]

No problem.

... . but I don;t think 602 revs is right. When I had my 4x4 I had the 315 BFG's that measured 34. 5" tall and they were set at 601 revs per mile per BFG and my speedo was accurate per GPS once reset.

How could a tire a full inch shorter be 602 revs? Incorrect math... ??? I say call dunlop and be sure so you don;t have to have it reset 2 times... "Do it right or do it twice"... . a 33. 5" tall tire should be near 615-620 revs per mile so again I say call dunlop to be certain.





As for the load range C... I don;t tow much (yet) with my truck and I wouldn;t put C rated tires on it. D at the very least. A C rated tire is meant more for a half ton truck or SUV. And I;d bet a 36' trailer with a car on it weighs a lot. A lot of folks here will even say a D rated tire isn;t enough BUT I know of several people who tow with D tires and are fine... but even in my limited experience I wouldn;t go with a C... no way.



Not trying to burst your bubble... and with luck the tires may be fine. But in my opinion they could get too hot under a load like you tow with especially with a 7-8,000 lbs truck on top of them all the time plus you may h ave a lot of squirm in the rear end of the truck with that much sidewall and that much weight on them... even at full pressure... . BTW... whats the max recommended pressure and load capacity (in pounds) at that pressure?

 

[White Ram1]

I'll call cust service of Dunlop in the a. m.



Squirm is my biggest concern at the moment.



Trailer and car together are about 5,000 to 6,000 lbs max. Not much tounge (neck) weight in the bed.



Even though I was a Ford PSD owner I also have towed with a Ford F-150 Supercrew with true 33" tires C rated. I ended up towing a 28 foot enclosed with the racecar and a few times with a Viper which is over 3,100 lbs, not a single problem with the tires, not even on a 1,100 mile one way trip. But maybe I just got lucky?



The tire guy told me they are rated over 2,500 lbs each, but didn't tell me the max air for towing. I imagine the hitch (neck) weight in the bed is about 800 to 1,000 lbs, just taking a guess. If I end up being wrong, which I have been plenty of times before, I guess it's new tires sooner rather than later.

 

[ak_fleming]

Tire size

No problem.

... . but I don;t think 602 revs is right. When I had my 4x4 I had the 315 BFG's that measured 34. 5" tall and they were set at 601 revs per mile per BFG and my speedo was accurate per GPS once reset.

How could a tire a full inch shorter be 602 revs? Incorrect math... ??? I say call dunlop and be sure so you don;t have to have it reset 2 times... "Do it right or do it twice"... . a 33. 5" tall tire should be near 615-620 revs per mile so again I say call dunlop to be certain.





Your right there is a big difference between 33. 5" and 34. 5".

the 34. 5" = 584. 588 rpm and the 33. 5" = 602. 032 rpm.

that is just how the math works with the measurments provided.

I don't no if the measurments were taken correctly, ie. weight on wheels and properly serviced.

 

[McCRam]

THe max weight the tire will hold is on the side of the tires in little letters next to the rim, it will have the max weight at max pressure. If your tires are rated at 2500 each, then you should be fine with those tires. Larger tires typically have higher weight ratings over the smaller size tires. i. e. the 285 /75 D rated tires I have now, have about a 300lb higher weight rating than the 265 E I had before.

 

[matches]

I agree with SUPERMAN your C rated tires aren't heavy enough for your towing needs. The front tire alone will wear excessivly because of the weight of the engine. I wouldn't tow a tent trailer behind your truck with those tires. Sell them fast and get atleast a D rated tire.

 

[White Ram1]

McCRam,



You are right, it is on the side wall of the tire. 2,450 lbs and max press 35psi.



5,000 - 6,000 lbs isn't much, hell a 1/2 ton gasser v8 towed my Viper to Dallas and back with 33's that were C rated, 3,000 lb car and a 2,500 lb enclosed trailer, and I drove 80 mph to 90 mph most of the way.



We'll see what happens when I get around to towing something.



Main thing is I want to get my rev's per mile set right first.