Effect of 215/85's Tires on Dually

[Web]

I was shopping '04 D/A Dually and noticed they come with 215/85R16 tires instead of the 265/75R16's on the 3500 SRW. I assume this is because with Duals you don't need the weight carrying capacity of the larger size tires.

However, I believe the smaller tires change the final drive ratio and shifts the optimal towing speed. My current '03 2500HD D/A towed my 13K fifth-wheel nicely at around 2000rpm which was about 62mph.

With a higher final drive ratio, 2000rpm might equate to a speed in the fifties. Does anyone know how to translate the smaller tire size into the final drive ratio and effective speed?

Bottomline, I don't intend to slow down my driving mph and I don't want to discover after the purchase that the dually has to run at a higher and less efficient RPM.

Hopefully, I'm just worrying about a non-problem and you guys can just add to my education...

Thanks in advance

[90LX]

MPH = (rpm/rear gear) X (tire height/336)

215/85r16 are about 30.375" tall

265/75r16 are about 31.5" tall

If you have overdrive, use:

rear gearOD = rear gear X overdrive ratio

in the above equation.

[Craig M]

The reason for the 215's jon duallys is width, not load capacity. To get two tires with a 1 inch or so separation between them, the narower 215 are specified. The older dually used 235's, but 265's were to wide.

[Heartbeat Hauler]

I suspect it's more than just rear tire spacing. As in the initial post, I believe GM is trying to keep the Durmax in the correct RPM range and based on gear ratio (trans)& rear gear ratio the tire size is like the fine tuning for proper RPMs at a given speed. Plus, GM is not really enthusiastic about allowing it's customers to customize their rigs for fear of warranty related issues. Anyways, that's my opinion.
JP

[MartyB]

Actually, a 215-85 and a 245-75-16 are for teh most part, the same diam and revs per mile( IIRC the 215's are 2 RPMs less). So if comparing to the 2500HD, there should be no difference in engine RPM's at any given speed. Compared to 265's, about 3% difference.

[plowking35]

I have an 04 dually LLY. The 215's are a bit small, and 80 is about the max you can go on the interstate. But at 2200 rpm you are pretty much right at 65 mph.I will check today and see what rpm I am at when going 60.
Dino

[Craig M]

You should like this one. Talked to a Duramax owner at a recent dyno day event. He had installed the 19.5" Alcoa classic wheels (via adaptor) and 19.5 tires. Fit nice but I think he said about 6.5% longer tire od. GM dealer would not let him recalibrate the vehicle speed sensor. So now he can go 106,500 miles and still have his 100,000 mile waranty.

[plowking35]

1900 RPM at 60 mph
Dino

[cowboywildbill]

I just purchased six new 215x85x16 Michelins LTX's. I paid $158.00 apiece. However when SEARS was putting them on my Dually, The tire tech took a Hubcap pry tool and ripped my left front stainless wheel simulator cover off of the truck. I ran over to try and stop him, he just looked at the two pieces(stock hub center cover and wheel simulator) with a puzzled looked on his face. He stripped the plastic lugs that hold the center cover on and bent and dented the simulator. I tried to keep my cool and explain to him that you have to take a socket and turn the fake plastic lugs on the factory center caps,off by hand first"just like you would any time"and then the simulator would just fall off. He said OK, then when I walked back in, he had just taken his air ratchet and run the other plastic lugs off. He spun them so hard they stripped, he kept spinning them thinking they would come off like a lug. But they are supposed to stay in the cap. I was there 7 hrs to get two tires mounted and balanced, I had previously dropped off 4 factory rims and they had already mounted and balanced those. Sears is going to buy me new simulators and they come with the plastic lugs. I guess that guy was out sick the day they had lug nut school at Sears. Go figure! PS the good news is I took a look at my brake pads and the front look almost new, and the rear look not quite half worn. Not bad for 43,000 miles and a lot of towing.

[Web]

Thanks for all the input....

Web

[cowboywildbill]

You are welcome, I took a copy of the invoice for the new wheel simulators that will replace the damaged one, I had to buy a whole set because you can't buy just one. Sears cheerfully refunded me the full amount off of my tire install. They surprised me. I will probably buy my tires from them again because of the way they handled it. But I sure will watch them when they install them next time.