Hi,

I am due for a tire replacement and I am looking at downsizing from a 265/70 R17 to a 275/55 R17. This will give me the effect of gearing closer to 4.10 instead of the 3.73s I have now. I checked out the weight of TV + Hitch weight versus the tire ratings and it looks good. I will need to get the speedo reprogrammed but other than that does anyone see a problem with this setup. I am trying to get a bit more power to climb the mountains with my 5775 lb TT.

Gary

 

I do not know the answer to this question. Just recommendations and a guess or two.

Be sure to stay with the same load range and rating. For example if you have Load range E tires, don't drop down to range D. Likewise with the max weight rating. Don't drop from say 4,000 pounds to 3,500 pounds unless you know for sure what weight per axle you are carrying.

Most wide profile tires are car tires and are NOT Rated for the load a tow vehicle will require. Be carefull that you have a real truck tire here. Having a tire blow out because you exceeded it's rating is an exciting thing when towing, you really don't want to try this. Been there done that, been really lucky it was a little trailer and not the truck tire that blew.

Shorter tires will have the effect of raising the numerical gear ratio which is better for towing but will affect mileage. A wild guess, the wider tires, especially if they are not truck tires may generate more heat which increases your risk.

Be sure to get the same size spare, or close to it, especially if you have limited slip. This also allows you to rotate 5 tires for a little more mileage.

Inflate your tires per the towing recommendations, usually door sticker or manual. I think it is roughly 80-90 psi front and rear when towing, and down around 60psi when not towing.

Good Luck,

Jim Henderson

 

I think the first number is the height of the sidewall in millimeters. Looks like you are contemplating going from a smaller 265 tire to a 10mm taller 275 but 15mm narrower tire, both fitting a 17" rim. This will lower the numeric ratio to (guessing) 3.60, the opposite of what I think you're looking for.

-Ed

 

Tire specs are as follows... say 235/75 r15 The 235 is the tire width in millimeters, measured from the bottom of the bead to the bottom of the bead, the 75 is the sidewall aspect ratio, the ratio of sidewall height to tire width at the tread (indicating that the sidewall height is 75% of the tread width), and the 15 is the wheel rim diameter in inches.

 

So he is going to a shorter, wider tire. My mistake, thanks for cvorrecting me!
-Ed

 

Hi,
Thanks for the responses. I have had a difficult time finding alternatives to the P265/70 R17 tires. The rear axle load rating of the 2003 Expy is 4128 from the door plate. The current tires have a max load rating of 2535 lbs. each. If you derate them by .91 for light truck use as recommended then that gives me 2306 lbs rating for the current tires. I have found only two alternatives and they are 265/65 SR17 (2271 lbs.,effective ratio of 3.90, Bridgestone AT Revos) and 275/55 HR17 (2271 lbs., effective ratio of 4.10, Goodyear Fortera Silent Armor). I have been unable to find a truck tire (LT) in these sizes but maybe I am not looking in the correct places. Any suggestions?
Gary

 

Going off your numbers, it kind of looks ok to use what you have found, as the rating is 4542 with those tires, above the manufacturer rating for the axle. It doesn't give as stiff a ride, which can lead to a bit of wobble when towing, so be aware of that. I doubt you will be towing regular, so it isn't as absoute that you need to have the LT tires. Also, only 577.5 pounds will be riding on your hitch, and rear axle. The rest will be on the trailer axle.

 

Hi,
I only tow maybe 12 times a year on long weekend trips to the mountains. I have a WD hitch so the 600 or so lbs is distributed partially to the front axle as well. I may slightly lower mileage however my daily commute is short and I may even get better mileage towing since I won't be downshifting quite as much. I am hoping this will help in the mountains as the 5.4L looses a lot of power at 8 to 10,000 feet! Thanks for the feedback.
Gary

 

Hi,
I would be interested if anyone knows if a manufacturer offers 17 inch tires in an LT version. I haven't been able to find one yet and I would much prefer an LT tire if possible.'
Thx,
Gary

 

I had the same thoughts regarding my truck and tire sizing.

From the factory it had 235/85-16's. When I got it it had 245/75-16's installed which, by using ratio and tire size calculaters - http://www.differentials.com/calc.html , gave it an "effective ratio" of IIRC ~3.70 in place of the installed 3.55's. When I replaced the tires I went back to the 235's and was concerned about what kind of difference it would make. We recently did a 2200 mile trip to southern Colorado (yes, altitude kills off a LOT of horses!) and for all practical purposes saw little or no difference. That surprised me.

Since returning we've bought a new-to-us camper that is a little taller, a little longer and a little heavier. While talking to my "carb guy" (~30 years experience) I expressed a little concern and he suggested the first and most effective thing I could do would be to change axle ratios, suggesting something around 3.73. I stated my experience with tire sizes and "effective ratios". He said, "... it just isn't the same although you would think it should be." He started talking about friction, gear drag, different ways of rating HP & tq and other factors I don't remember, but I came away thinking he was probably right. Maybe someone else can elaborate.

Clyde's primary mission in my life is either hauling or pulling and not much else. I've talked to a lot of people, read a lot of posts and after some thinking have decided there's a set of 4.10's in Clyde's future. More than a few have suggested I may even see a slight mileage increase because the engine won't be working as hard and I won't have "my foot in it" as much or as hard.

Roger

Clyde S Dale, my chestnut brown & tan 5th wheel workhorse is a
1984 F250HD XLT RCLB 4X2 8600 GVW
460 C6 3.55's
67,500 original miles (and counting)

 

One other thing....might not be a big factor. Tires "squish" on the bottom, so the axle-ground distance is less than axle-top of tire. Effectively makes the tire "smaller". My observations (based on trucks, cars, etc, and playing with numbers like that myself) is that a taller tire will squish more, so going to a shorter sidewall actually won't make much, if any, difference. Not to mention the variation in sizes between different manufacturers....I try to keep all matched tires on my vehicles. I've noticed up to a half-inch difference between two same-sized tires of different brands, and those were small tires (185/70-14).

 

Thanks mikebon08, good point. I am trying to go by the manufacturers revs per mile rating for the tires and not the actual sidewall label. I have noticed that there is a bit of a difference, as you mentioned, between manufacturers for the same size. Just like you said, about a half inch or so. The tread alone accounts for almost an inch change in diameter over the life of the tire!
Gary