Ok, so being my own worst (best) critic, I have an update to the tire size / gear ratio "thing" we've been discussing.
So, to regroup, as my prior example had a couple assumptions known to me but not specified clearly, it looks a bit muddy. So, thanks for the comments and here I try again:
2012 Seneca
Final drive ratio 4.78:1 according to Freightliner, on the sticker on my visor. Tires 245/70R19.5, same reference.
245mm is the tread width at widest point. Aspect ratio is 70%. Wheel diameter is 19.5". These numbers are stamped on the tires.
Multiply tread width by 70% (245mm x 70% = 171.5mm) to get sidewall height from rim to tallest part of tread
171.5mm / 25.4 = 6.75” This is the distance from the rim (bead) to the extent of the tire distant from the center of the wheel
SO, wheel is 19.5 inches PLUS
Bottom sidewall (closest to ground) is 6.75” PLUS
Top sidewall (wheel to top of tire) is 6.75”
Therefore, diameter is 19.5 + 6.75 + 6.75 = 33” (Independently verified by tape measure.)
Considering the “3% rule” for air pressure variations, that means 32.01 to 33.99 is the range of diameter overall.
2018 Seneca
Final drive ratio 5.13:1 according to Freightliner, on the sticker on the visor.
Tires 275/80R22.5, same reference.
275mm is the tread width at widest point. Aspect ratio is 80%. Wheel diameter is 22.5". These numbers are stamped on the tires.
Multiply tread width by 80% (275mm x 80% = 220mm) to get sidewall height from rim to tallest part of tread
220mm / 25.4 = 8.66” This is the distance from the rim (bead) to the extent of the tire distant from the center of the wheel
SO, wheel is 22.5 inches PLUS
Bottom sidewall (closest to ground) is 8.66” PLUS
Top sidewall (wheel to top of tire) is 8.66”
Therefore, diameter is 22.5 + 8.66 + 8.66 = 39.82”
Considering the “3% rule” for air pressure variations, that means 36.63 to 41.01 is the range of diameter overall.
Ratio 5.13:1
Tires 275/80R22.5
Considering tire diameters, this exercise tells me that changing to 22.5’s in rears would reduce my final drive ratio to 3.96. That means I’d have to change the ratio to 5.77:1 to get the same characteristics. (You can fool around with those numbers here:
https://tiresize.com/gear-ratio-calculator/)
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. Still interested in any comments or thoughts.
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