gearing questions for larger tires

[James S]

Hey Everyone,

As I have said, I am getting a 2003 2500HD crew cab long box. If I put on 285 tires which I am planning to, my gearing is changed from 3.73, to 3.46. What does this do. Does it alter my towing capacity? If so, by how much up or down? Will it impact my acceleration, by how much up or down, in terms of 0 to 60 times, will it be slower or faster with larger tires? I gather I will have to tie a couple of places back, that is fine. The turning diameter will not change, correct??? [This truck is hard enough to turn] Everyone is saying to go with Michelin tires. Which all season one is good, and what is the load capacity? Also, I have been told that fuel milage is not affected. Is this true? I would think that with the change in RPM that it would be. Will my milage increase or decrease with bigger tires? I feel more secure in a truck that rides high, but I do not want to put a lift on, just increase the tire size. What about the 255/85 tires? What will they do to my gearing, towing, acceleration, and milage, and ride height? Thanks for your responses. Right now I am somewhat confused, particularily over the gearing changes and their affects. I do not understand the 3.46, 3.73 numbers. DETAILED explanations would be greatly appreciated. Thanks.

James

[SoCalDMAX]

James,

This is only my opinion, and you may get others, so keep an open mind.

A 285/75-16 tire is basically a 33" dia tire. You've already done the gear ratio calc. and I'll just assume it's correct. This means that as you're going down the road at 2,000 rpm, you're speedo is going to read 70mph as it did before, but in reality you're traveling ~76mph. This means that you're turning lower rpm for a given speed and lower revs per mile covered.

On level ground, all else being equal, I would think you'd get maybe 1mpg better mileage just from the lower rpm, if you drove at the same speed you normally drove at. Your speedo would read lower and your odometer would read ~9% lower.

Empty, your acceleration would seem to be slower, but since a diesel has more torque than hp, I wouldn't worry about it. If you're the type who likes to fly low, you'll be buying performance add-ons anyways, and they'll add more than the tires took away.

Towing: here, I'm guessing. Since shift points will be incorrect by 9% and 4th gear will actually take you 9% faster, you may not experience the "4-5 shift busyness" as much as some have experienced. (This shift issue is inherent in all automatics when travelling at certain speeds that are kinda "between gears", especially when a lot of weight is involved.) Once again, if towing a large trailer, a power add-on will do wonders to move the load, and keep it in 5th gear. The tires will keep the rpms down, perhaps giving even more fuel savings per mile than empty driving.

I'm running 255/85-16 (same 33" dia.) and feel it's the best of everything.

1. The wheel well is filled a little better, looks good.

2. Lower rpm at favorite cruising speed, better mileage (IMHO).

3. Larger dia. tire rolls over bumps smoother, has more sidewall and flexes slightly more for a smoother ride. (flex is dependant on load range, not necessarily tire size.)

4. If you're getting a 4x4, 285s are really gonna help the truck with traction.

These trucks look great with a 285 under it, no lift needed. Look around and click on member pics to see some really awesome rims. Boyd Coddington, Centerline, Weld... you name it. Go with an 8" wide rim with the right offset. I'd recommend Discount tire for their service and prices. Ask for Ride-matching and the rims/tires will be mounted based on high/low spots and a great deal less weight will balance the tire to perfection.

Regards, Steve

[James S]

Hey Steve,

Thank you for that response. I have read that a completely stock Duramax will run 0 to 60 miles, in about 9 seconds. What do you think the 0 to 60 ratings would be with the 285 tires. I am not going to add any power mods. If it is going to affect the acceleration that much, I will become concerned that a F**d or Do**e will have better acceleration than I would, and that is just wrong!!!!!!! What are your 0 to 60 times with the 255/85 tires on? How much higher does the truck ride? Would the 285 tires ride about the same height as your 255 tires? I am not going to do vary much towing, but I want a 2500HD to have the versitility of a crew cab, long box, and the trailer capacity, so I can throw anything at it, and not worry about whether the truck can handle it. Thanks for your reply, and previous response. If I could have more responses from others, I would appreciate it.

James

[shft22]

James,
I have 285

[SoCalDMAX]

Hi James,

The 255s are the same dia. as the 285s, just slightly narrower, thus the 255s fit on the stock rim. I agree with shft22, the taller tires make the truck feel like it's geared better. You might notice a very slight change in the seat-of-the-pants dyno, but it's got so much torque it'll run pretty much the same as the stock tires.

the hard part, really, is deciding which rims and tires to buy.

Regards, Steve

[bluenote]

I've considered a taller tire once the original 245's wear out. Since my truck is 2wd/no off-roading, I'd probably go with the 255's and keep the stock rims. For those who have done this (or the 285's), how much higher did your truck bed sit? We pull a fairly low sitting fifth wheel, and clearance is an issue. If there is little difference in bed height between the 255's and the 245's, I would be more than happy to have a taller tire on the truck.

[hdmax(mike)]

The 245/75/16`s are 30.4" tall. The 285/75/16`s 32.4" tall. The 255/85/16`s are 33" tall. So they will raise the truck approx 1"-1 1/4" . Wheel width will play a small part in the over all height. But not more then maybe 1/4"-1/2"Most likely less then that.

[mdrag]

Topic moved to 2500HD/3500 HD Trucks & Drivetrain forum.

mdrag

[imported_]

Has anyone stolen the gears (4.10) out of a gasser and put them into a dmax (3.73) equiped truck. It should be a direct swap with no mods. Just need to find the gears. Correct?

Now if I could only figure out how to make that 6th gear in the allison work I would have an even better on/off road truck :D

[DmaxMaverick]

Big Blue

My brother has an 02 2500HD with the 8.1L/Allison. We have tossed around the idea of swapping his 4.10 with my 3.73, front and rear, both are 4 X 4. He would rather have the 3.73 gearing. He would be able to easily recalibrate, and mine with 285's would be right on. We just can't get ourselves to do it.

If we had the 6th gear in the Allison, I wouldn't hesitate. Although, if he had 6th, he probably wouldn't want to trade. What a situation. I really like the OD I have with 33" tires.

[imported_]

DmaxMaverick

Go ahead and swap, add a lift, juice it and jump up to 35s. My simple swap just got $$$$$ I am going to keep my eyes-n-ears open for a crashed up gasser to steal the gears out of.

If you do swap let me know how it worked out.

Cheers

Brad

[Joe Duffiney]

Correct me if I am wrong ....... but according to my calculations 4:10's with 33's should be close to the same rpm's as stock with 3:73's and 245's . I was thinking of doing the same thing cuz I didn't want to lose the grunt down low with the taller tires.

[DmaxMaverick]

Joe Duffiney

Exactly my train of thought. GM won't reprogram for my tire size, so the only way is to "lie" to the computer. I've put much thought into it. Doing it is another thing. It works so well as it is and I love it. The speedo and odo is just off. Can't justify fixing what ain't broke. The larger tires don't cause any loss of "grunt". It is an illusion do to the speedo going slower. The "seat of the pants" says it all. IMO, it feels quicker after the start. I have no problem getting 12K moving up a 12% grade from a dead stop.