bigger tires, now down on power?

[SSell]

Need some opinions----went from 265/70's to 285/70's before last week's camping trip. Previous trips to same same place (7000 ft/ mountain grades) my truck had no problem holding 60-65 MPH uphill pulling my 24' toyhauler (call it 10,000lbs). This time was struggling to do 45, pedal on the floor, transmission selector in drive (3), tow/haul mode on. Tach would sit just above 2,000 rpm. Just flat, no extra power, no acceleration. Shifting into (2), tach jumps to nearly redline, but truck would slowly accelerate to 50-55MPH. Stock truck, not reflashed for new tire size. Don't think its a transmission problem, not running hot, seemed to run fine when I charged up smaller hills. How critical is it to update for tire size? Used to have 325/60/18's on it and ran fine... Any Ideas?

 

[SDoumit]

Need a power pod to get the power back. If not a pod you would have to change the gearing in rear ends. the larger the tire the more power is lost.

 

[cerberusiam]

You have another problem somewhere. The tire size change will not effect power like that. Even going to 33" tires it won't have that effect. Not setting the tire size messess with the shift points but not power.



Check for codes, air filter, fuel filter, etc.

 

[HBarlow]

Your signature indicates you have gauges. Was full throttle boost normal? How about EGT? If boost and exhaust temp were normal the power loss is probably due to power loss to spin the larger tires.

Changing the tire size in the PCM will only correct the speedometer/odometer reading, won't compensate for power loss.

 

[SSell]

You have another problem somewhere. The tire size change will not effect power like that. Even going to 33" tires it won't have that effect. Not setting the tire size messess with the shift points but not power.



Check for codes, air filter, fuel filter, etc.

Dont have gauges, new (mopar) airfilter. Fuel filter has about 5,000 miles/ 4 months on it----is it likely to clog that quick? Truck seems to run fine except it just runs out of @ss on hills that it had no problem with last summer.

 

[SDoumit]

He went from 31's to 33's It make a difference. There may be some other things going on too but tire size can change things.

 

[HBarlow]

Sorry, I guess I looked at the wrong post and read gauges.

It's hard to know when you don't have gauges to monitor boost and egt. A bad map sensor will cause low boost and low fueling.

You didn't reply to cerberuseum's question about codes. Do you know how to look for codes? Turn the ignition from off to on and off again in three times in quick succession and then return to the on position and it should display any codes in the odometer. Sometimes it is tricky to get the sequence just right. An Autozone and maybe other auto parts stores have the scanner and will usually come outside and read codes for you by plugging in the connector under the dash.

Other than those suggestions, your next alternative is to take it to a dealer to read codes then run a performance diagnostic.

 

[AClayton]

He went from 31's to 33's It make a difference. There may be some other things going on too but tire size can change things.

Bigger tires will lower your power output some but at 2000 rpm's,your Cummins should pull 2 trailers of that size up the grade with relative ease. On a gas motor,I agree that tire size and proper gear ratio are extremely important,but on a diesel,the 1 size jump shouldn't make that significant of a change.



Put the smaller tires on the back and try your trailer if you think it is related to the tire sizing.



I cringe whenever a member suggests adding more power to compensate for large tire size increases. The proper method is to lower the axle ratio... . more power means more heat and higher egt's. Among the few exceptions would be a Smarty Jr. In either case,285's is rare to cause this much of a change in power.



Alan

 

[NavyDood]

Ditto the others. There is something else going on. Going from 265 to 285 will NOT dog it down like that. Buddy of mine did the exact same tire change a while back. He hasn't seen any change in performance or mileage towing his 26' power cat boat loaded out with trailer that weighs around 11,000 lbs.

 

[Regcabguy]

Actually,I felt the power loss going from 285-70-17's to 285-75-17's, a height gain of 1. 3" and a weight gain of 14# ea. My 2000# camper is on all the time and even with the G-56 with the . 79 final drive(equivalent to an auto with 4. 10's) it's less peppy. If it wasn't for my Smarty set on the lowest setting,I'd have to return to the "70" series again. If I ever go to 35's it'll take a new clutch also. I'd still like to see skinnier,tall tires out in an "E" rating similiar to the sizes offered in 19. 5 configuration.

 

[tkerrigan]

Torque converter slipping?

 

[SSell]

Sorry, I guess I looked at the wrong post and read gauges.



It's hard to know when you don't have gauges to monitor boost and egt. A bad map sensor will cause low boost and low fueling.



You didn't reply to cerberuseum's question about codes. Do you know how to look for codes? Turn the ignition from off to on and off again in three times in quick succession and then return to the on position and it should display any codes in the odometer. Sometimes it is tricky to get the sequence just right. An Autozone and maybe other auto parts stores have the scanner and will usually come outside and read codes for you by plugging in the connector under the dash.



Other than those suggestions, your next alternative is to take it to a dealer to read codes then run a performance diagnostic.

I'll see if I can pull a code, I see nobody jumped on the clogged fuel filter theory so I'll leave that alone for now. Will keep posted when I figure it out---thanks

 

[SSell]

just to clarify, tach looks right @ approx2200 rpm (pretty sure something would have gone up in smoke if it was slipping) , no go with the gas pedal. feels EXACTLY like our old ('96) 12 valve truck... . (bone stock, just put a brick on the pedal and you'll get up the hill sooner or later). Used to run 325/60/18 (almost 35's) tires and it would feel like the truck was willing, just the gearing was just off enough that the transmission would hunt, seemed happiest in 2nd gear on a big hill. This is different---truck just sits in 3rd gear & doesnt "try" to accelerate, wont shift to 2nd unless I manually shift from 3 to 2. got some ideas to start with... thx again!

 

[GaryOwen]

If you were going into a headwind that could easily make the difference. Toyhaulers are usually pretty bad in the wind. Also, check all your boost connections.

 

[cerberusiam]

just to clarify, tach looks right @ approx2200 rpm (pretty sure something would have gone up in smoke if it was slipping) , no go with the gas pedal. feels EXACTLY like our old ('96) 12 valve truck... . (bone stock, just put a brick on the pedal and you'll get up the hill sooner or later). Used to run 325/60/18 (almost 35's) tires and it would feel like the truck was willing, just the gearing was just off enough that the transmission would hunt, seemed happiest in 2nd gear on a big hill. This is different---truck just sits in 3rd gear & doesnt "try" to accelerate, wont shift to 2nd unless I manually shift from 3 to 2. got some ideas to start with... thx again!

Ummmmm... ... . gotta ask it are you SURE it was in drive and not stuck in OD???



Are you sure the TC was locking?



What speed were you reading when this happened?



At 2200 rpms it should have upshifted unless you were really pulling hard when it drops to drive you are gonna bounce off redline but drop once the TC locks again.



Fuel filters are tricky, 5k is not much but if you got a shot of bad fuel... .....

 

[Gary - K7GLD]

Closest I can come to this, is my own move from 265's to 285's on the rear, towing our 5er in the hills - with the 265's, O/D was too high a gear to pull when curves required speed down around 45-50 MPH - but 5th gear (NV-5600) put the RPM well up against the power band at 50-55 MPH, so lots of shifting was needed between O/D and 5th gear.



The 285's ended that - NO perceptible loss of power, either empty or towing within the power band, BUT, I can easily cruise up the steepest grades from 45-60 mph in 5th gear - exactly what I want!



An added plus, is a reduced engine RPM when cruising out in the flats, either towing or empty - and the MPG is great too! :-laf

 

[birddog]

low power

As a fleet mechanic I will say absolutely positively without a doubt you could have fuel filter issues. The bigger tires are not the problem. A fuel filter is an easy test to perform. also knowing your boost and if your holding boost in a low power situation is a must.

 

[SSell]

cool, will try new fuel filter. Checked for codes, boost hardware prob, doublechecked new airfilter, loose connectors, etc..... nothing. I know it wasnt in OD, gear & RPM looked right, just no go & slowly dropped speed when I should be passing tankers RV's, not getting out of their way!

 

[cerberusiam]

cool, will try new fuel filter. Checked for codes, boost hardware prob, doublechecked new airfilter, loose connectors, etc..... nothing. I know it wasnt in OD, gear & RPM looked right, just no go & slowly dropped speed when I should be passing tankers RV's, not getting out of their way!

Do you remember how much boost it was reading?



In drive with 10k it should have accelerated easily even with the TC locked. It just sounds like a fuel delivery problem, or a situational gremlin. Hard to tel though without being there.

 

[AClayton]

Do you remember how much boost it was reading?



In drive with 10k it should have accelerated easily even with the TC locked. It just sounds like a fuel delivery problem, or a situational gremlin. Hard to tel though without being there.

OP said he doesn't have gauges..... it does make it hard to diagnose with no gauge readings. I still think bad fuel!!



Alan