Quote:
Originally Posted by Robbbyr
Good point Dananu, taller tires will have the same effect as having a numerically lower axle ratio which will likely reduce "pulling" performance. How much, I can't say.
RayBo, check your driver's sunvisor to see if the label lists your axle ratio as mine does. For comparison, my 2014 has a 5.13:1 ratio. Might help with guesstimating the change larger tires might make.
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All good points. I did dig into it. It reduces the final drive ratio too much, in my opinion. I'm not going to do it. (Besides, since mine's a 2012, I might like a new one next year :-)
Anyway, because of the way my brain works, I had to go do research. Here's some info (maybe way more than anybody wants, but somebody might:
2012 Seneca
Ratio 4.78:1
Tires 245/70R19.5
2018 Seneca
Ratio 5.13:1
Tires 275/80R22.5
This tells me that changing to 22.5’s would reduce my final drive ratio to 3.96. That means I’ll have to change the ratio to 5.77:1 to get the same characteristics. It also tells me that the newer Senecas (Freightliners) have dropped the gear ratios to get more “pull” and makes me wonder about fuel economy a little. (That’s a topic for another day.) Probably I’m just going to change the front wheels so that the wheelwells don’t look so empty and so that the steering and handling are much better.
Anyway, because we “share” here, I thought I’d post the info for anyone looking to learn more about tire sizes and gear ratios. Never can have too much information, and that’s what forums are all about…or so I think.
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2012 Seneca
Tire size/gear ratio relationship notes:
Locate the O.E. tire size. This is found on the sidewall of the tire and will look similar to:
245/70R19.5
Note the width of the tire from the first number in the tire size. In this case,
245 represents the width across the tire, in millimeters, from the widest point.
Calculate the tire sidewall height. In the example tire size,
245/70R19.5, the number “
70” represents the height of the tire’s sidewall, from rim to tread, as a percentage of the tire width. In this case the tire’s height would be 122.5 millimeters (50 percent of 245), or 4.82 inches. This is also known as the tire’s “series.”
Locate the O.E. wheel diameter; this is the last number in the tire size. In the example,
245/70R19.5, the number
19.5 represents the wheel diameter, in inches, on which the tire is intended to be mounted.
Calculate the overall tire height, in inches, using the following formula ((Width/25.4) x Sidewall Percentage) x 2. Therefore, a 245/70R19.5 tire would have an overall tire height of 9.64 inches: ((245/25.4) x 50%) x2.
Combine the overall tire height with the wheel diameter. Using the example of, 245/70R19.5, you would add 9.64 to 19.5 for a final diameter of 29.14 inches.
Using the 3 percent variance rule (tires “balloon” or “sag” depending on air pressure,) you would be able to install a new tire and wheel combination that was between 28.27 inches and 30.01 inches in total diameter.
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2018 Seneca
Locate the O.E. tire size. This is found on the sidewall of the tire and will look similar to:
275/80R22.5
Note the width of the tire from the first number in the tire size. In this case,
275 represents the width across the tire, in millimeters, from the widest point.
Calculate the tire sidewall height. In the example tire size,
275/80R22.5, the number “
80” represents the height of the tire’s sidewall, from rim to tread, as a percentage of the tire width. In this case the tire’s height would be 137.5 millimeters (50 percent of 275), or 5.41 inches. This is also known as the tire’s series.
Locate the O.E. wheel diameter; this is the last number in the tire size. In the example,
275/80R22.5, the number
22.5 represents the wheel diameter, in inches, on which the tire is intended to be mounted.
Calculate the overall tire height, in inches, using the following formula ((Width/25.4) x Sidewall Percentage) x 2
A 275/80R22.5 tire would have an overall tire height of 10.83 inches: ((275/25.4) x 50%) x2.
Combine the overall tire height with the wheel diameter. Using the example of, 275/80R22.5, you would add 10.83 to 22.5 for a final diameter of 33.33 inches.
Using the 3 percent variance rule, you would be able to install a new tire and wheel combination that was between 32.33 inches and 34.33 inches in total diameter.
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One peculiarity that sticks with me in tire sizing: We use metric to describe width, a percentage to describe height, and inches to describe the wheel. Kind of wacky if you ask me, kind of like buying a pound of hamburger, weighed by the gram and then paying for it in ¥en.
PS if anyone finds flaws in my math, please correct me…I don’t get offended so easily and want things to be correct. AND, it's 6:30am and I have not had my coffee yet.
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