Tire size, wheel Size & Regearing

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Lanlubber In Remembrance

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Always glad to see another GMT800 rig on here.
So if your truck has the holy grail of options with the 5.3L V8, Z71 package (which i see it does), and the towing package, it should have 4.10 gears from the factory. This means that the engine will still work in it's powerband all the way up to 35" tires. Granted you would need a 6" suspension lift or a 2" suspension + 3" body lift to fit those.

If it doesn't actually have the 4.10s it will have 3.73s which is fine for the 285/75/r16

In order to fit the 285/75/r16 tires, which is roughly 33", all you need is some leveling torsion keys which can be found for about $40-60

You could also get some small rear lift blocks so the truck isn't squatting with a trailer attached.

For the around town shifting that others have talked about, if you are worried about being in too high of a gear you could just hit the tow/haul button on the end of the shift stalk

Also towing can eat up the 4L60e transmissions sadly. I would recommend getting an upgrade transmission cooler and you should have no worries.

Hope all this helps
Amanda, FYI the serial number on your truck indicates what the factory gear ratio is for your truck. Also there is a metal tag attached to one of the bolts on the center carrier section of the differential that identifies the gear ratio of your truck. KNOW YOUR GEAR RATIO BEFORE YOU ENLARGE YOUR TIRES !

What @UltamaSanctus says about your gear ratio is true if you have his ASSUMED gear ratio. Only if your ratio is 3:80 or above would I not change gears for the taller tires. Anything less is marginal at best.

Transmission coolers are a whole other story and any good mechanic should be able to determine your trans cooler needs depending on your load and speed that you drive. Obviously slow (as in slow trail speeds and steep terrain) speeds cause more heating.

Most factory coolers are adequate, there is such a thing as a trans thermometer gauge you can install so you can know when the tran temp is to high. There is a gauge for everything if you are worried about something in particular.
 

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Dont worry about recalibrating your speedometer you will only be going from 3 to 5 mph faster just mentally note that. If you decide to go back to stock size you wont have to recalibrate again. If your really worried about your speed run a gps speedometer app on your phone.
 

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Dont worry about recalibrating your speedometer you will only be going from 3 to 5 mph faster just mentally note that. If you decide to go back to stock size you wont have to recalibrate again. If your really worried about your speed run a gps speedometer app on your phone.
Good idea on the gps although I like my speedo readings because it's reflected in the speedo mileage meter on the vehicle. With a 4 tire size change, I think the speedo would be off by at least 5 mph @ 65 mph. Just begging for a speeding ticket from the majority of law.
Personally I drive with my rpm meter for maintaining speed and gas mileage. A stead rpm will get you the best mpg if that is a concern and it also keeps your engine in the power range you want. Something Amanda will learn with experience !
 

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The auto trans will downshift automatically when going up hill. Very seldom have I manually down shifted with an automatic going up hill, but I do down shift going down hill as a breaking method. Your trucks computer shifts the auto trans as needed. If your truck has an OD, it dosent go into OD until you are 40-45 miles per hr. Always drive in D unless you are pulling a load and need 3 to keep your rpm's up so the engine does not lug (strain). That should not be necessary with an auto if your tranny is in proper working order.
Finally some proper transmission advice. Those Chevys shift off of RPM and throttle. Itll shift just fine on it’s own. DONT drive around in 3rd gear ‍♂. Down shifting coming off the freeway or something is a good idea but don’t leave it in third. It’s just going to be bogged down.
Thanks I start learning this, I was used to manual gears all my driving life, this is my first automatic !
I’m not sure where to start here. There’s a lot of very knowledgeable, experienced people on here that’s giving you advice that has worked for them but isn’t great for your vehicle.
A. Don’t change you wheels and tires until you do a lift.
• Stock wheels won’t fit on lifted Chevys. The offset is way too high.
• 16” is no different than 17” when is comes to ride. 16” is getting harder to find, especially if you need a used one. 17” has been the norm across the board for over a decade. Almost 2 decades by now.
B. 245 to 285 is not 4 sizes. And definitely not 4”.
• 245, then 265, then 285. Half a handful of manufacturers make a 275 (BFG is one. That’s how you can tell who’s brand loyal )
• 245’s are around a 31 1/2” OD (overall diameter). A 285 is just under a 33”
• The first number is the aspect ratio (tread face). 285mm. The second is the section height (sidewall) 75% of 285mm. The last is the wheel size. A 285/75-16 is the same as a 285/70-17. See? Rim size goes up, section height goes down, overall diameter stays the same.
C. The more you adjust the torsion bars, the stiffer you’re ride will be. Use the adjustment to dial your ride, just don’t crank them up for your sole means of lift. It also puts your front drive axles at a more extreme angle increasing wear.
D. The lift kits for IFS suspension on those models are permanent. No big deal but the “in case you go back to stock” comments are from guys that have never had one. The 6” is usually cheaper than the 4” because of popularity. 4” is a waste of time with the Chevys anyway. Fender trimming and mild torsion tweaking will get you a 285. A 4” that’ll run you over $1k will get you...a 285. Don’t bother. When Chevy got away from IFS and switched to struts was the best thing they did. $300 puck kit would get you a nice, fat tire.
E. Unless you’re rock crawling or towing a ton, you don’t need to re-gear. I’ve put over 100k on my lifted truck (35’s) with Stock gears. Re gearing is not a bad idea, but a lot of money that could be put elsewhere on a build.
Ultimately, do what you want. But, for right now, I’d get a good set of 3peak tires for this trip. When you get back and start your build, sell them. Used 16” snow tires in Canada will sell by the end of the day. Put that money into your wheel/tire/lift bill. Yes, all 3 of those need to be done at the same time on an older Chevy so start saving. Hope I didn’t bore you to death.
 

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Just a general rule of thumb I have found to be true. For every inch in tire size different than stock add or subtract 3% I’m using 33” tires instead of the stock 31” so 50 mph is actually 53 MPH at a 6% difference . 100 miles on the odo is actually 106 miles.. that’s not really enough of a difference to justify recalibration my speedo..
 

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285's are fine on those trucks with a small leveling kit. You'll have to clean up some of the plastic on the bumper and inner wheel wells in the front. Getting better shocks and going through the suspension as all the bushings and tie-rod ends and all that stuff is prob shot while you do a leveling kit is a pretty good way keep it stock-ish.

I have a thread with our 03' GMC Sierra thats lifted, on factory wheels with 285/75r16 tires. Its a great truck and it feels totally at home with that tire size and 3.73:1 gears in the rear axle.

They're great trucks, lots of parts and really easy to work on. Take care of it and it'll take care of you! Installing a larger transmission cooler is a good thing to do on these as well.
 
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Meh, with older style OD autos I have run I always run them in 3rd until up to highway speeds when I need overdrive at a sustained higher speed. It does help to keep things cooler.
Also all autos also sense road speed and use that to aid in shifting, and running in 3rd wont cause the truck to be "bogged down" the only thing it will do is prevent it from trying to shift into 4th gear (overdrive) at the set point of 45 (for GMs anyway), which 90% of the time is too low.
I mean you wont kill the trans just sticking it in D, but seriously, locking out over drive when you dont need it will help to lengthen the life of the transmission by keeping fluid temps lower by alleviating the chances of un needed up and down shifting and torque converter lock and unlock. So why not?
EDIT TO ADD: I should clarify that all of this is meant to be in the context of running larger tires ( or under a load). though I still manually shift out of over drive as I start pulling a grade ( I do this in any auto I am driving, in my GMC Terrain I typically pull passes in 4th or 5th (its a 6 speed auto) to keep the torque converter locked and the engine at peak power) I recommend that to all my friends, for the reasons I have described above.
 
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MOAK

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Meh, with older style OD autos I have run I always run them in 3rd until up to highway speeds when I need overdrive at a sustained higher speed. It does help to keep things cooler.
Also all autos also sense road speed and use that to aid in shifting, and running in 3rd wont cause the truck to be "bogged down" the only thing it will do is prevent it from trying to shift into 4th gear (overdrive) at the set point of 45 (for GMs anyway), which 90% of the time is too low.
I mean you wont kill the trans just sticking it in D, but seriously, locking out over drive when you dont need it will help to lengthen the life of the transmission by keeping fluid temps lower by alleviating the chances of un needed up and down shifting and torque converter lock and unlock. So why not?
EDIT TO ADD: I should clarify that all of this is meant to be in the context of running larger tires ( or under a load). though I still manually shift out of over drive as I start pulling a grade ( I do this in any auto I am driving, in my GMC Terrain I typically pull passes in 4th or 5th (its a 6 speed auto) to keep the torque converter locked and the engine at peak power) I recommend that to all my friends, for the reasons I have described above.
Agree 100%.. the sweet spot on my 4.5 FZE is between 2200 & 2700 rpms. When loaded with all our gear and trailer we scale out at 8,000 lbs. if I allow the trans to be automatic when pulling a steep grade it will upshift if I let off the throttle, only to downshift again. Sure, you could floor it and wrap it up to 3500 rpms, it will still upshift, bog down and downshift again. All transmissions do this, so instead of up down up down up down, I manually drop it into 2nd and pull the grade keeping the rpms at 2700. When loaded one should always be in the middle of the power band just as you head up the grade. Autos do not anticipate anything. Modern automatics in 18 wheelers operate the same way.. a good operator is more efficient than computers and sensor arrays that are incapable of anticipating grade changes, weather conditions, traffic, etc. I also find myself manipulating the cruise control to shift up into OD way before the trans would do it automatically.. hey, I’m doing something right, this rig has 315,000 miles on the original, and I’ve never had any kind of transmission or clutch failures ever, in over 40 years of driving multiple types of vehicles.
 
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These trucks are rated to tow close to 9k lbs, and its not because of the motor or the transmission. For the GMT800 trucks, the brakes and chassis are the biggest limiting factors. A bigger trans cooler helps if you're going to be off-road more IMO, but these trucks are stout and the engines in them are fantastic. The transmission can be a bit of a weak point if you abuse it, but there are MILLIONS of these trucks on the road still. Locking out OD is a big no-go on them as the motor will end up spinning at close to 3k and then you get down to single digit MPG numbers.

Also, all of this:
I’m not sure where to start here. There’s a lot of very knowledgeable, experienced people on here that’s giving you advice that has worked for them but isn’t great for your vehicle.
A. Don’t change you wheels and tires until you do a lift.
• Stock wheels won’t fit on lifted Chevys. The offset is way too high.
• 16” is no different than 17” when is comes to ride. 16” is getting harder to find, especially if you need a used one. 17” has been the norm across the board for over a decade. Almost 2 decades by now.
B. 245 to 285 is not 4 sizes. And definitely not 4”.
• 245, then 265, then 285. Half a handful of manufacturers make a 275 (BFG is one. That’s how you can tell who’s brand loyal )
• 245’s are around a 31 1/2” OD (overall diameter). A 285 is just under a 33”
• The first number is the aspect ratio (tread face). 285mm. The second is the section height (sidewall) 75% of 285mm. The last is the wheel size. A 285/75-16 is the same as a 285/70-17. See? Rim size goes up, section height goes down, overall diameter stays the same.
C. The more you adjust the torsion bars, the stiffer you’re ride will be. Use the adjustment to dial your ride, just don’t crank them up for your sole means of lift. It also puts your front drive axles at a more extreme angle increasing wear.
D. The lift kits for IFS suspension on those models are permanent. No big deal but the “in case you go back to stock” comments are from guys that have never had one. The 6” is usually cheaper than the 4” because of popularity. 4” is a waste of time with the Chevys anyway. Fender trimming and mild torsion tweaking will get you a 285. A 4” that’ll run you over $1k will get you...a 285. Don’t bother. When Chevy got away from IFS and switched to struts was the best thing they did. $300 puck kit would get you a nice, fat tire.
E. Unless you’re rock crawling or towing a ton, you don’t need to re-gear. I’ve put over 100k on my lifted truck (35’s) with Stock gears. Re gearing is not a bad idea, but a lot of money that could be put elsewhere on a build.
Ultimately, do what you want. But, for right now, I’d get a good set of 3peak tires for this trip. When you get back and start your build, sell them. Used 16” snow tires in Canada will sell by the end of the day. Put that money into your wheel/tire/lift bill. Yes, all 3 of those need to be done at the same time on an older Chevy so start saving. Hope I didn’t bore you to death.
Couldn't agree more.
 

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Amanda, I will try to give you my opinion but it may not be what you want to hear.
First, stick with the 16" wheels, you tire man is right about the cushion is better on 16" tires than the larger tires.
Second, your wanting to go up in size 4 tire sizes from 245 to 285. Keep in mind that a 4" taller tire will only raise your rig 2". I don't know the diameter of your 245 tire or the 285 tire but generally each size larger is about 1" taller than the lower number. 4" is a good guess for now. Your gear for the speedometer will have to be changed in the tranny to correct your MPH. At 65 MPH your engine RPM will drop several hundred RPM's, which means your factory cruising speed will change. (factory cruising speeds for MPG of gas is usually around 2200 to 2400 RPM on the tachometer.)
It would be a good idea to go out and check your driving speed with your tach so you will know before you change things what your engine performance is today.

The effect would be that your acceleration will not be as good as it was with 245 tires. To get the acceleration back to what it is now a lower gear ratio would need to be installed to compensate. If you buy the tires now I would also change the gears as well or you wont be happy with the performance. I know you wouldn't be happy with a smaller tire from the looks point of view so I wont even suggest doing that to avoid a gear change. Make the torsion bar height change if the man said it can be done. Don't forget, 4x4's can get stuck too, even with aggressive tires so don't go anywhere in snow country without tire chains. You didn't mention a locker differential but as long as you need a gear change I would put a locked in the rear at a minimum if it doesn't have one already. A posi-track is okay if you don't push it to hard and may be all you need. All of what I suggest will be needed when you add the extra weight of that camper. Even after the gear change you will notice a big difference in truck performance with what looks like a heavy camper. You truck is heavy half ton 327 ci v8 engine (5.3L) and to get all the performance you need you may want you visit a performance shop to see if they can get you another 100 horse power for off roading. That could wait until you get the camper installed on your rig.

The only other advise I can offer is to just go ahead and get a set of tire chains and head out. The truck is pretty capable as it is. The interstates are kept cleared for traffic and as long as your not going off road right now your tires will be okay for the trip to Washington. You will want all that I mention when you put that camper on so do it now or do it later.
Thanks LanLubber for your time and sharing your experience, i am soaking in everything and reacessing my next move.

Best wishes

Amanda
 

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These trucks are rated to tow close to 9k lbs, and its not because of the motor or the transmission. For the GMT800 trucks, the brakes and chassis are the biggest limiting factors. A bigger trans cooler helps if you're going to be off-road more IMO, but these trucks are stout and the engines in them are fantastic. The transmission can be a bit of a weak point if you abuse it, but there are MILLIONS of these trucks on the road still. Locking out OD is a big no-go on them as the motor will end up spinning at close to 3k and then you get down to single digit MPG numbers.

Also, all of this:


Couldn't agree more.
Aye, I beat on my '01 Sierra Z71 5.3L and it was tough as nails. Even with a turbo. :smilingimp:

 

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Ohh and don't forget the Sierra has Auto4wd. You can run it in Auto4wd (AutoTrac) any time unlike regular 4wd and you won't get steering bind. The drivetrain will run in 2wd until it senses wheel slip up front then it will act like 4wd/AWD. This is a huge help when driving in bad weather and 4wd is overkill.

I missed that truck. That's part of the reason I went to a GMC Canyon when it was time to get back into a truck. The new Sierra is huge!

'01 Sierra with 32s or 33s back in 2004. I forget. :)
 

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Aye, I beat on my '01 Sierra Z71 5.3L and it was tough as nails. Even with a turbo. :smilingimp:
Gebbus. The thought of that with a turbo makes my neck hurt just thinking about it.
 

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Axle ratio... block a front tire, shift into neutral.... lift one rear tire off the ground.... use a felt, mark the driveshaft/differential as a starting point.... tape a string to the driveshaft aligned with the mark..... mark the floor/tire and turn the wheel precisely 2 full revolutions.... count how many wraps on the drive shaft.

Just over 4, 4.10 ratio just under 4, 3.73 ratio 3 plus a bit, 3.07.... etc
 

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Axle ratio... block a front tire, shift into neutral.... lift one rear tire off the ground.... use a felt, mark the driveshaft/differential as a starting point.... tape a string to the driveshaft aligned with the mark..... mark the floor/tire and turn the wheel precisely 2 full revolutions.... count how many wraps on the drive shaft.

Just over 4, 4.10 ratio just under 4, 3.73 ratio 3 plus a bit, 3.07.... etc
You left out 3:90 to 1 between 4:10 and 3:73 :-) simpler way, turn wheel one full revolution and just count the # of times the drive shaft turns using a marker on the shaft and pinion housing.
 

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Just over 4, 4.10 ratio just under 4, 3.73 ratio 3 plus a bit, 3.07.... etc
[/QUOTE]

Hi everyone, I am trying to follow but you lost me, i am going to get something to eat, my head is spinning !!!!
 

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Gebbus. The thought of that with a turbo makes my neck hurt just thinking about it.
Then you should have ridden in it with the twins up front. Over 700rwtq. Just stupid. lol

Mine had 3.73s so there's a good chance @Amanda C does too. Honestly don't overthink this. Follow @rho advice and you'll be fine. She knows her stuff.
 

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Then you should have ridden in it with the twins up front. Over 700rwtq. Just stupid. lol

Mine had 3.73s so there's a good chance @Amanda C does too. Honestly don't overthink this. Follow @rho advice and you'll be fine. She knows her stuff.
Justin, was yours z71 or no? did you have the max tow package?
Max tow and z71 both get lower gears and g80s, our 2wd has the max tow which gave us 3.73 and a G80 in ours. I think the gear ratio breakdown is in the manual, I'll have to dig it up but im like 99% sure she's gonna have 4.10's
 

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It's a GM .. the rpo codes are an easier way to tell. It's in the glovebox
You mean if someone didn't remove them from the glovebox it is easy. I have never bought a used auto yet where even the manual was in the car. There is a metal tag on the diff cover that tells what it is.(unless some idiot removed it too.)