Suburban Tire Size Change... thoughts invited from fellow diesel heads!

[knkreb]

Odd thing my speedo is off by 7 mph higher than gps, yet speed in PCM as seen with scan tool is spot on match with GPS and with those radar speed warning signs same as it was when I had the 265-75s it came with from factory, my height is a littler lower factory 1870# rated rims were 7" wide and now I have Classic MT 8" 3300# rated rims. But speedo with the MT rims 265 vs 235 off just the same as with the factory rims. Bill says some tucks the speedo needle was installed off just a little on the pin I could take it off and try to make it better, but it could also be worse.

I had that same problem all of a sudden one day with bus 1. I can't remember now if it was 5-10 mph off. It was spot on, then one day... boom. Dead on with OBDII recorder that I used to plug in.

Still gathering thoughts here on this tire size situation.... So, going with 235's is going to reduce rolling resistance, and going with the 85's is going to give me the extra circumference that I need to reduce engine RPM without having to re-gear the rear. Some shot gun math this morning while researching looked like it would effectively change it from a 3.73 rear to a 3.57 instead. 235's will be a smaller patch of rubber hitting the road, (better more wet than dry as I read it.) and raising up to a 85 I have more side wall, and a bit more slop in handling?

On a whole 'nother note... we have exited the bus era in our family last night. Bus 2 has found a new home in a new driveway. I have one very distraught son who hated to see it go. It was painful to see him so upset about it too. I too hated to see it go. There is only room for 4 vehicles in a two vehicle driveway.

The Suburban is still sitting loud and proud in the driveway. 168k on original IP too.... tick, tick, tick...

 

[nobby]

Knkreb
IMHO Based on mine with 3.42's a stock tune and always a load in the back you'd be fine with 265's if you like the looks. It's not as though I feel like my truck is seriously impaired by it just that you feel it so to speak.

Cheers
Nobby

 

[Dave01]

Knkreb, I'd sum it up this way:

235/85 and 265/75 are same diameter - both will lower cruise rpm, raise up the truck over stock 245's. Both will handle fine if you stay with E rated, tow great, no real negatives to either. My 265 Firestone A/T's were incredible in snow, the tire type you choose will make the difference in dry/wet/snow performance, much more than the difference in width.

The 235/85's will get better mpg, the 265/75 look great on these trucks. Can't go wrong with either! Hope this helps!

 

[jrsavoie]

I am completely lost when I get into any of this stuff. I read and I read but apparently my Evelyn Woodhead course did not improve my comprehension greatly.

I was under the impression that resetting the VSSB would reset your shift points and more than likely improve your milage.

VSSB mod.

Did I miss something here does an incorrect VSSB have that deeper effect on operation. I never really bothered with it because I always just mentally adjusted gauge readings according to having a taller tyre on the vehicle.

So other than possibly ABS which I never really got to the bottom off as to whether it is based on VSSB - what else does the VSSB have a noticeble effect on? My thought was and is whatever the VSSB is at with my setup she'll be doggy with the extra tyre/ a load and a 3.42 rear.

Of course a chip would be a good thing for this and planned, just never seem to find a spare $500 lying around. :rolleyes5:

Cheers
Nobby

 

[nobby]

Shift points

I recall when reading this a long while back that of course there was different opinion on this but I came to the conclusion that shifting is based on the trans input and output speed sensors and the throttle position sensor and should therefore respond to changes in gearing of its own accord. Lower rear gearing will then lead to more revs/higher speed before the shift which to my mind is what you want. Further to that I also now recall that I read that for the diesel the VSSB to PCM pin is not used so no signal to the PCM.

Any one have any input on this?

Cheers
Nobby

 

[nobby]

OK so I got curious for a minute, I cannot find right now what I read that said that the connection from VSSB to PCM was not used in the Diesel. However I did take a look at my year wiring diagrams and found that the VSSB speed output to the gauge cluster also goes to the PCM so that the PCM is therefore receiving a buffered speed signal. A look at wiring diagrams for the transmission finds no input from the VSSB just an input from the speed sensor on the input shaft.

A bit more reading finds that the manual states that an incorrect module will affect Speedo, ABS Module and Cruise Control.

Right that's enough procrastination for one day.

Cheers
Nobby

 

[DennisG01]

Not sure if this will help in your decision, but I went from stock to the 265/75 a number of years ago (probably about 5 or 6). The sole reason I did it was for aesthetics. I have absolutely no handling problems with the slightly taller tire (I usually run at least 65psi). I did the VSSB mod (although it was only off by about 3 mph at 70mph). However, it is now dead on at 70. I didn't keep nearly as good MPG records then as I do now, but my MPG definitely did not go up - that much I am positive. The only thing I can attribute that to is that the taller tire takes more torque to roll it.

I've actually contemplated going back to 245's - not because I'm concerned about MPG - but I wonder if it would make a noticeable difference in the trucks towing ability (don't get me wrong - it does real nice with 10K behind it and the stock turbo - but I still wonder, you know?). I also ponder going to a 4.10, but I'd hate to give up the lower RPM at 70 that the 3.73 affords.

Anyways, there's some more thoughts that will probably do nothing but cause confusion!

 

[rlb]

I have a 94' 2500. 5.7L and 4.10 gears. Currently I have 245/75/16 E rated tires. Thinking about going to 265/75/16 C rated tires. I don't tow a whole lot. Will I miss anything with the C load range? I figure if I tow anything large enough to need E rated tires, my 350 is going to be struggling.

 

[skhemer6.5]

I have a 94' 2500. 5.7L and 4.10 gears. Currently I have 245/75/16 E rated tires. Thinking about going to 265/75/16 C rated tires. I don't tow a whole lot. Will I miss anything with the C load range? I figure if I tow anything large enough to need E rated tires, my 350 is going to be struggling.

Load range E or 10 ply tires less likely to have flats than load range C or 6 ply I run dirt gravel roads with loaded trucks and wish there were more load range G or 14 ply options in 16 inch tires but if u just run hwy with no load u can get by w 6 ply load range D or 8 ply is a option u might look at if u dirt road a lot or pull light trailers

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[btfarm]

I sure wouldn't run anything less than D range on your truck but if you don't tow much and run mostly pavement then D is the better choice for all around good life. As long as you are rotating them every 15k or so.

 

[skhemer6.5]

My moms tundra runs hankook at/m 8 ply just for the fact I wont be having to go change a flat for her she is a school teacher and dont get her truck off pavement or haul more than a book their is bout 2000 ft of bluff between our place and town this time of year freeze and thaw rocks fall off bluff and lay in road till me or hwy dept clears them off and mom has never learned to play dodge rock so thats why I spec 8ply on her tundra we sold tires in the 80-90s and had wrecker service so I am like btfarms wouldn't even consider a 6 ply for your 3/4 ton truck but that is my 99 cents worth

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[JayTheCPA]

I have a 94' 2500. 5.7L and 4.10 gears. Currently I have 245/75/16 E rated tires. Thinking about going to 265/75/16 C rated tires.

The Burb had weight appropriate C rated AT tires when I bought it and they came off within the first 20 miles of my ownership; at max pressure, it was like the truck was on marshmallows. Given that the Burb was apparently a mudder for the PO, the C rated AT's made sense, but they simply did not have road manners that were acceptable to me. They are now living a happy life on a 1/2 ton.