Tires profiles and sizes - the pros and cons

[oldtamiyaphile]

Originally Posted by mcrews

Trucks are normally geared for mpg on the stock tire. so going TOO tall can be counter productive.

I'd say commercial vehicles tend to be geared low so they can deal with a hefty load, the heavier the rating you go for the lower the gearing will be. Even something as small as my Renault will pull a light load in 5th at 21mph (~3250rpm at 60). Load it up to GCM though and you'll appreciate the low ratios.

[mcrews]

I'm not talking about 'commercial vehicles' I'm talking about a personal pickup truck. One that has a mileage sticker on the window at retail purchase.
For example, Ram (previously dodge truck) is touting 29 mpg on the highway. They are doing that to meet the stricter CAFE standards.

[Longsnowsm]

I have been doing some digging and found that JR here on the forum also has a Canyon and he also went up one tire size and recorded the fuel economy improvement. So I think that is the direction I will be going also. One size up from the default OEM configuration. The stock tire is 225/75R15, and it looks like a 235/75R15 is probably the direction to go with this. I will know more after the drive back on Sunday. :-)

[oldtamiyaphile]

Originally Posted by mcrews

I'm not talking about 'commercial vehicles' I'm talking about a personal pickup truck. One that has a mileage sticker on the window at retail purchase.
For example, Ram (previously dodge truck) is touting 29 mpg on the highway. They are doing that to meet the stricter CAFE standards.

It's still a commercial vehicle. Even the baby Ram 1500 is rated to tow 10,000lbs, which suggests it could be re-geared/ tyred quite a lot for light load economy. Having said that, today's 9 speed autos change the game a bit, you can have a 5 speed while towing and a 9 speed for economy. Almost like having a dual range box.

[redpoint5]

Originally Posted by SDMCF

Why? (From an MPG point of view)

I have not researched this idea myself, but some say that the wider (and shorter in length) contact patch forces the tire to deform less while providing the same total area of contact. In other words, all other things being equal, a wider tire might reduce rolling resistance because the rubber doesn't have to bend as much to provide the same square inches of contact with the road.

The trade-off is that the wider profile is less aerodynamic, which becomes an increasing factor with increasing speed.

[oil pan 4]

Check those 4 sizes see which one is cheaper.
I went with slightly taller tires for one car, the price difference was about $12 per tire.
Turns out the taller size is the more popular of the 2.

The only way you are going to see any improvement by going to a smaller tire is if you regear to numerically lower ratio.
I think the only way you would see any improvement just going to a smaller tire and changing nothing else is if you delivered mail and all your stop and go covered the distance of some ones yard.

[Longsnowsm]

Well after all that research on tires for my truck it turns out that the decision was made for me! The truck needed tires, and the dealer stated as much so I have been trying to figure out the best tire based on the OEM sizes that were used on the truck to replace the existing tires... What I didn't know is that the wheels on the truck had been replaced with a 16" wheel instead of the stock wheel. And the dealer after talking with me decided that he didn't want to see me driving that truck with those existing tires through part of 7 states to get it home... So he put new tires on the truck for me. The good news is that he appears to have chosen a decent tire. The bad news is that it isn't exactly the tire I would have chosen for longevity or LRR. The tire is bigger than the tire that I was going to put on it in a 15" size, but only slightly. The tires are a 235/70R16 so they are slightly taller than the 225/75R15 that I had planned. So that worked out well as far as RPM and economy.

The bad news is it is more of an All Terrain tire, and not what I would have preferred. But I got new tires in the deal and that is always a good thing. Leaves more money for other projects and mods that are needed. So overall I am pretty happy, RPM's at 65mph are about 2150. Of course the speedo and odometer are off. The factor I am using is 1.0833 as the multiplier for miles and economy. So far it looks good. Thanks for your input. I like having the larger wheels on the truck and the larger tire helped on the highway where I will use the truck most often so it is a great fit.

[oil pan 4]

Don't let anyone else pick your tire for you.
They are likely going to pick the tire that benefits them the most.
Go some where like tirerack get the exact tire that meets your needs and have a tire shop put them on.

If you don't think you need an AT tire don't get them. I put Yokohama 31x10.5R15 AT/S tires on my suburban back in 2010. They are a great tire, but they are more than I need.
I wished I had gone with a highway tire. (yoko HT/S in that size)

2100RPM at 65mph sounds high. Unless its a tiny 4 cylinder.

[oil pan 4]

Originally Posted by Katmandu

What is BSFC ?

Brake specific horsepower consumption.

[user removed]

Brake Specific Fuel Consumption.

https://en.m.wikipedia.org/wiki/Brak...el_consumption

regards
mech