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Old 05-27-2022, 11:54 AM   #1
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tire/gearing question

need confirmation on the effect of upsizing tires on F450 with 4.30 gearing. I changed from Continental 225/70/19.5 (644rm, 33.1 tall) to Toyo M920 - 245/70/19.5 (619 rm, 34.6 tall). My objective was to get a less harsh ride and higher ground clearance, both of which were accomplished. What a difference in ride, fantastic. Now, I calculate with my limited/non-existant knowledge of gear ratio theory, the gearing has dropped to the 4.10 effective range. Can anyone assist in confirming this or correcting this. Many thanks
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Old 05-27-2022, 01:01 PM   #2
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Yep, it's all just simple math though it appears you've made a small error somewhere; in theory the difference should be between 31.91" tall and 33.02" tall which is about 3% so your 4.30's would behave like 4.17's . Take the tread width in mm's, divide by 25.4 to convert to inches, then multiply by the profile (70%), multiply by two (for both sides of the tire) and add the rim size = diameter...

or just plug the numbers into an online calculator;

https://tiresize.com/comparison/

Anyhow, it should make a minimal difference for your gearing and you get the improved ride. nice.

Dave
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Old 05-27-2022, 02:03 PM   #3
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Quote:
Originally Posted by Dave Pelletier View Post
Yep, it's all just simple math though it appears you've made a small error somewhere; in theory the difference should be between 31.91" tall and 33.02" tall which is about 3% so your 4.30's would behave like 4.17's . Take the tread width in mm's, divide by 25.4 to convert to inches, then multiply by the profile (70%), multiply by two (for both sides of the tire) and add the rim size = diameter...

or just plug the numbers into an online calculator;

https://tiresize.com/comparison/

Anyhow, it should make a minimal difference for your gearing and you get the improved ride. nice.

Dave
OK, gotcha. I bet the difference is that I used the specs (either height or Rev./mi) given by the manufacturer(s) for fully inflated tires [225/70/19.5 (sConti - 644rm, 33.1 tall) to Toyo M920 - 245/70/19.5 619 rm, 34.6 tall] to come up with the percentage difference and then applied that to the nominal 4.30 which gives a little bit different input to manipulate. Thanks for your input
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Old 05-27-2022, 03:35 PM   #4
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OK, gotcha. I bet the difference is that I used the specs (either height or Rev./mi) given by the manufacturer(s) for fully inflated tires [225/70/19.5 (sConti - 644rm, 33.1 tall) to Toyo M920 - 245/70/19.5 619 rm, 34.6 tall] to come up with the percentage difference and then applied that to the nominal 4.30 which gives a little bit different input to manipulate. Thanks for your input
No problem; I was wondering if your calcs were based on the Manufacturer's data which may include more accurate revs/mile based on load and inflation pressure.....in that case, my math is right but your numbers are more accurate because the weight of the load effectively changes the running diameter. Works out to pretty much the same thing; about 3% difference.

Cheers,
Dave
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Old 05-27-2022, 06:54 PM   #5
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Dave, thanks again.
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Old 05-27-2022, 11:53 PM   #6
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Originally Posted by grantrekker View Post
need confirmation on the effect of upsizing tires on F450 with 4.30 gearing. I changed from Continental 225/70/19.5 (644rm, 33.1 tall) to Toyo M920 - 245/70/19.5 (619 rm, 34.6 tall). My objective was to get a less harsh ride and higher ground clearance, both of which were accomplished. What a difference in ride, fantastic. Now, I calculate with my limited/non-existant knowledge of gear ratio theory, the gearing has dropped to the 4.10 effective range. Can anyone assist in confirming this or correcting this. Many thanks
Your numbers are off according to tiresize.com.
225/70/19.5s are 31.9" tall.
245/70/19.5s are 33" tall.
You've gained barely more than 1/2" in ground clearance,
but reduced your per-mile revs by 22, which will, regardless
of rear-end ratio, slightly increase your MPG on the highway.
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Old 05-28-2022, 10:01 AM   #7
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Quote:
Originally Posted by Zulu Kono View Post
Your numbers are off according to tiresize.com.
225/70/19.5s are 31.9" tall.
245/70/19.5s are 33" tall.
You've gained barely more than 1/2" in ground clearance,
but reduced your per-mile revs by 22, which will, regardless
of rear-end ratio, slightly increase your MPG on the highway.
Zulu, thanks for your comment. Not to be contentious, BUT. I believe you have based your comment relying on information that is at best generic as presented on tiresize.com. I used the actual manufactures' respective specs for the calculations and would have more confidence using those numbers. If it were of material consequence, I would prefer that methodology. I did measure the actual gain in clearance and got 1" height gain at front air dam. I attribute the difference between the calculated and this observed clearance to be not only size related, but also tread depth as I did the gain measurement after the Continentals were used 25,000 miles and the Toyos were new. At any rate, the ride is unmistakably superior and no more chipped teeth.
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