Tires and gear ratio

[da_garr99]

Hi everybody, could not find an exact answer so here I am and its alot sorry in advance. I have a 2009 dodge ram 1500 5.7 slt and am in need of new tires. Highway and city driving 50/50%, occasionally offroad, might tow a camper trailer in a few years. Stock tires are 275/60/20 (33"), currently running 33×12.5 20. I want to either run 285/75/17 (33.8") or a 285/70/17 (32.7") falken wildpeak at3. I do not know my gear ratio (will not have access to my truck for another few days so i cant check unfortunately). My questions are, what size should I run? Does anybody reccomend a different size instead of those? And if i go bigger how much stress will the 285/75/17 put on my driveline? I do not want to regear so have been leaning towards the 285/70/17 but I have no clue. I would like to have this truck for many years and I have been babying it alot. First time ever buying tires so I'm overthinking alot. Thank you for the help

 

[Dean2]

Welcome fellow Canadian.

The diameter differences you are talking about are completely irrelevant with respect to gearing. From 33" to 33.8" is 2.4% increase in diameter. You will get slight differences in your speedo readout but even that is not real significant, and can be adjusted to read correctly.

DIAMETER, ROLLOUT (CIRCUMFERENCE) AND REVOLUTIONS PER MILE CHART

Below is a chart to assist in vehicle drive-train calculations.

​ Diameter Rollout Revs/Mile* 17.5 55.0 1222 18 56.5 1186 18.5 58.1 1152 19 59.7 1120 19.5 61.3 1090 20 62.8 1061 22 69.1 960 22.5 70.7 938 23 72.3 917 23.5 73.8 896 24 75.4 877 24.5 77.0 858 25 78.5 840 25.5 80.1 823 26 81.7 807 26.5 83.3 791 27 84.8 776 27.5 86.4 761 28 88.0 747 28.5 89.5 733 29 91.1 720 29.5 92.7 708 30 94.2 695 30.5 95.8 684 31 97.4 672 31.5 99.0 661 32 100.5 651 32.5 102.1 640 33 103.7 630 33.5 105.2 621 34 106.8 611 34.5 108.4 602 35 110.0 593 35.5 111.5 585 36 113.1 576 36.5 114.7 568 37 116.2 560 37.5 117.8 553 38 119.4 545 38.5 121.0 538 39.5 124.1 524 40 125.7 517 45.5 142.9 453 46 144.5 448 53 166.5 388 53.5 168.1 384 54 169.6 381

 

[Jeepwalker]

Furthermore, there can be slight variation of *actual* measured tire OD size between mfgrs of the same size tire. Also, the difference in diameter of any tires you buy could be ~16mm (5/8") from brand new OD to the "time to replace" OD. So, good question, but the differences you're talking about are acomadated by the mfgr.

Interesting Note: For guys who DO consider quite a bit larger or smaller tires, the truck's computer can be easily re-programmed to accomodate tires of greater different OD size. There's a drop-down selection in the menu of many scanners when you link to the truck's computer and a long list of tire sizes which can be selected from. Depending on the tire size, it modifies the truck's fuel mapping, shifting programming and other parameters. Just an interesting side note. But in your case you shouldn't need to do anything.

Let us know how you like the Wild Peaks

 

[da_garr99]

Welcome fellow Canadian.

The diameter differences you are talking about are completely irrelevant with respect to gearing. From 33" to 33.8" is 2.4% increase in diameter. You will get slight differences in your speedo readout but even that is not real significant, and can be adjusted to read correctly.

DIAMETER, ROLLOUT (CIRCUMFERENCE) AND REVOLUTIONS PER MILE CHART

Below is a chart to assist in vehicle drive-train calculations.

​ Diameter Rollout Revs/Mile* 17.5 55.0 1222 18 56.5 1186 18.5 58.1 1152 19 59.7 1120 19.5 61.3 1090 20 62.8 1061 22 69.1 960 22.5 70.7 938 23 72.3 917 23.5 73.8 896 24 75.4 877 24.5 77.0 858 25 78.5 840 25.5 80.1 823 26 81.7 807 26.5 83.3 791 27 84.8 776 27.5 86.4 761 28 88.0 747 28.5 89.5 733 29 91.1 720 29.5 92.7 708 30 94.2 695 30.5 95.8 684 31 97.4 672 31.5 99.0 661 32 100.5 651 32.5 102.1 640 33 103.7 630 33.5 105.2 621 34 106.8 611 34.5 108.4 602 35 110.0 593 35.5 111.5 585 36 113.1 576 36.5 114.7 568 37 116.2 560 37.5 117.8 553 38 119.4 545 38.5 121.0 538 39.5 124.1 524 40 125.7 517 45.5 142.9 453 46 144.5 448 53 166.5 388 53.5 168.1 384 54 169.6 381

thanks for the help and nice to see a fellow Canadian! picked up the 285/75/17 today, hopefully getting them mounted this week or next. Currently rim shopping

 

[da_garr99]

Furthermore, there can be slight variation of *actual* measured tire OD size between mfgrs of the same size tire. Also, the difference in diameter of any tires you buy could be ~16mm (5/8") from brand new OD to the "time to replace" OD. So, good question, but the differences you're talking about are acomadated by the mfgr.

Interesting Note: For guys who DO consider quite a bit larger or smaller tires, the truck's computer can be easily re-programmed to accomodate tires of greater different OD size. There's a drop-down selection in the menu of many scanners when you link to the truck's computer and a long list of tire sizes which can be selected from. Depending on the tire size, it modifies the truck's fuel mapping, shifting programming and other parameters. Just an interesting side note. But in your case you shouldn't need to do anything.

Let us know how you like the Wild Peaks

Picked up the 285/75/17 today, will let you know in a few months how I like the wild peaks

 

[Dean2]

You may already know this but when rim shopping try to keep the offset on the new ones the same as your OEM rims. Also, going to 17s you want to test fit them before you buy them to make sure they clear the brake calipers properly and that there is room for balance weights.

 

[da_garr99]

You may already know this but when rim shopping try to keep the offset on the new ones the same as your OEM rims. Also, going to 17s you want to test fit them before you buy them to make sure they clear the brake calipers properly and that there is room for balance weights.

Very smart, I plan on going with a -12 offset. What happens when you deviate from stock offset ?

 

[Dean2]

The problem comes in when guys move the face of the mounting plate closer to the back of the rim. This increases negative offset. If the mounting plate was dead centre tou would have zero offset. As you move the mounting plate out board you get positve offset. Guys increase negative offset so wider tires clear at full lock and because they think it looks good This pushes the tire and rim further outboard of the wheel and wheel well. This means your wheel well may no longer cover the tire and it also puts a whole wack more pressure on bearings and front ends as well as rear bearings. The more leverage you apply the worse it gets. What was your factory offset. As long as the new negative offset is not greater you should have no issues.

 

[da_garr99]

The problem comes in when guys move the face of the mounting plate closer to the back of the rim. Guys do this so wider tires clear at full lock and because they think it looks good This pushes the tire and rim further outboard of the wheel and wheel well. This means your wheel well may no longer cover the tire and it also puts a whole wack more pressure on bearings and front ends as well as rear bearings. The more leverage you apply the worse it gets. What was your factory offset.

I do now know my factory offset, can check tomorrow night when I get home

 

[Dean2]

I do now know my factory offset, can check tomorrow night when I get home

Minus 12 is 12 milimeters so about a half an inch. The stock offset was usually from positive 20 to 25 depending on the exact wheel package. That means you are moving the tires outboard about an inch and a half, which is quite a bit.

 

[CanRebel]

Very smart, I plan on going with a -12 offset. What happens when you deviate from stock offset ?

Was bored, so I checked. Says 17-21. some list it as 19. As @Dean2 mentioned above.
You should check for yourself and make sure you really want -12 , as he said above, that is a big change.

 

[da_garr99]

Was bored, so I checked. Says 17-21. some list it as 19. As @Dean2 mentioned above.
You should check for yourself and make sure you really want -12 , as he said above, that is a big change.

I'm definitely not going with a -12 offset. Would you reccomend a 0 or a +1 ? My wife bought me bushwacker fender flares and she has made it her mission to get them on lol. So I want to try and have the tires flush or a little inward, but don't want it to look super tucked in. My rims say 20×8j +19.05 on the back of them. Ideally I would like to go with a 9 inch wide rim but are a bit concerned with my suspension and uca. Thanks for the help

 

[da_garr99]

Minus 12 is 12 milimeters so about a half an inch. The stock offset was usually from positive 20 to 25 depending on the exact wheel package. That means you are moving the tires outboard about an inch and a half, which is quite a bit.

My rims are 20×8j 19.05. What would you reccomend for offset ? Want to go with 9 inch wide rim (don't know pros and cons to a wider rim) but concerned about uca and suspension

 

[MAC830203]

Try this https://tiresize.com/comparison/

 

[Dean2]

Stock tires are 275/60/20 (33"), currently running 33×12.5 20. I want to either run 285/75/17 (33.8") or a 285/70/17 (32.7") falken wildpeak at3.

Going to an inch wider rim means it is 1/2" outside and in. Since you are at a -20 offset now on the stock rims a -8 would compensate fully for the extra 1/2". Based on your OEM stock tires being 275s, new tires 285, you are gaining less than a 1/2" in width, so the new tire is 11.2" versus stock at 10.8 and your existing tires at 12.5". So in effect you are going to a new tire that is 1.3" narrower than your existing tires. Personally, I would stay with the 8" or 8.5" rim and keep the stock offset.

 

[Dean2]

One final work of advice, a good tire or tire and rim shop should easily be able to get you into the right 17" rim using a factory or real close to factory offset of -16 to -20. Formula for tire with to Rim width is rim width x 1.33, so in this case ideal would be an 8.5" rim for the 285/75/17 tires, but 8" works too. Good thing about 17" tires is there is a way better selection and they are a whole bunch cheaper than 20 inch rubber. The biggest trick is to make sure nothing rubs and the rims clear your calipers, which on a 2009 they should as they came stock with 17s on some models.

Let us know what you end up with.

 

[Jeepwalker]

OP, Unless you have a particular style of aftermarket wheel in mind. But if not, are you open to a *stock* Ram 17" wheel? You might be able to find a good used set of take-off's on your local "used" listings (Facebook Marketplace/Craigslist ...or whatever is popular in your area). Perhaps a set that was removed years ago an owner now wants to get rid of. The benefit would be a good set of wheels for a lot less than aftermarkets. And around here (salt) aftermarket wheels don't typically hold up as well as OEM.

That's what I've done in the past on a few vehicles, when I wanted to get a better set of corrosion-free wheels. The previous owner of my Ram must have done that too by the looks of the build sheet.

Just a thought..

 

[da_garr99]

Going to an inch wider rim means it is 1/2" outside and in. Since you are at a -20 offset now on the stock rims a -8 would compensate fully for the extra 1/2". Based on your OEM stock tires being 275s, new tires 285, you are gaining less than a 1/2" in width, so the new tire is 11.2" versus stock at 10.8 and your existing tires at 12.5". So in effect you are going to a new tire that is 1.3" narrower than your existing tires. Personally, I would stay with the 8" rim and keep the stock offset.

That makes alot of sense, thank you for the help and very valuable knowledge.

 

[da_garr99]

OP, Unless you have a particular style of aftermarket wheel in mind. But if not, are you open to a *stock* Ram 17" wheel? You might be able to find a good used set of take-off's on your local "used" listings (Facebook Marketplace/Craigslist ...or whatever is popular in your area). Perhaps a set that was removed years ago an owner now wants to get rid of. The benefit would be a good set of wheels for a lot less than aftermarkets. And around here (salt) aftermarket wheels don't typically hold up as well as OEM.

That's what I've done in the past on a few vehicles, when I wanted to get a better set of corrosion-free wheels. The previous owner of my Ram must have done that too by the looks of the build sheet.

Just a thought..

I was looking at some fuel rims. Unfortunately where I am everybody thinks their beat up rims are worth 1k which is really stupid

 

[da_garr99]

One final work of advice, a good tire or tire and rim shop should easily be able to get you into the right 17" rim using a factory or real close to factory offset of -16 to -20. Formula for tire with to Rim width is rim width x 1.33, so in this case ideal would be an 8.5" rim for the 285/75/17 tires, but 8" works too. Good thing about 17" tires is there is a way better selection and they are a whole bunch cheaper than 20 inch rubber. The biggest trick is to make sure nothing rubs and the rims clear your calipers, which on a 2009 they should as they came stock with 17s on some models.

Let us know what you end up with.

Will definitely remember the rim formula. I would have loved to get some 2nd hand rims but people in BC price their rims stupid high and alot of tire shops out here are sold out on so much