How much does tire size change affect ratio?

[Darrel]

Comparing a 3:55 rear end ratio running 245x75 l6" tires to a truck with 4:l0 rear end ratio running 265x75 16" tires. What would the effective ratio be with the bigger tires compared to the truck with the 3:55 ratio? I know it will be less than 4:10 but I don't know how to calculate it.

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Darrel A
1999 24v,Qcab,Sbed,2wh drive,Auto,BD Brake,Auto lock,EGT Boost Trans Temp Gauges

 

[KatDiesel]

<font color=blue>

Well it depends, if the 265's on the 4. 10 truck are "stock" then its still 4. 10.


If the 4. 10 truck had "stock" 245's then when changing to 265's it will have a 3. 96 ratio.

If the 3. 55 truck had stock 245's then going to 265's it will have a 3. 43 ratio.

Does that answer your question?

 

[TREEMAN]

Kat , howdya work the ratio out ,I have an 01 with 295,s & a 4:10 rearend. Cant remember what the stock tires were as i never saw them,my dealer gave me a good deal on the swap & have had the 295,s from day 1. I figure i,m around 3:73 am i far off base?

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2001,2500 q-cab,auto 4x4,4. 10 anti-slip rearend. All options except leather seats. Line-x bed & tool box. Slide-in -removable rack,BFG 295,s ,Fender flares. Westin nerf bars,billet grill & fog inserts. Rancho,9000,s,Isspro gauges,1994 2500 4x4,12 valve,5 speed, bone stock for now.

 

[Neal Cassady]

TREEMAN,

I'll give it a crack, your truck probably shipped with 265's.

265/295 = . 8983

. 8983 * 4. 10 = 3. 683


Dave D

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2001. 5 QC, LB, 4x4, Auto, 3. 54, Everything but Leather, Bright Silver Metallic, ARE K Series, Diprocal EGT, Boost and transmission Temp in an Autometer Dash Pod, Westin CPS Platinum Step Bars - Black on Stainless

 

[KatDiesel]

Originally posted by TREEMAN:
I figure i,m around 3:73 am i far off base?

If you still have your window sticker it will tell you what size tires came on your truck.

If you had 245's then when going to 295's with 4. 10's you now have 3. 74 ratio.

If you had 265's then when gonig to 295's with 4. 10's you now have a 3. 88 ratio.

This is assuming all the tires are ***/75/16 of course.

Does that help? I use a handy "calculator" to figure it.

 

[klenger]

Dave D:
I could be wrong here, but I think your math is defecient (as well as my spelling). Refering to the Michelin web site at http://www.michelin-us.com/us/eng/tire/guide/lesson4.html , the first part of the tire size does not tell the whole story of the tire diameter. The first number is the width of the tire in MM, the second number is aspect ratio in %(height/width), and the third number is the rim size in inches. Note that tire width is in MM and rim size is in inches, so some metric to english conversion is required. The height of the tire from the rim to the tread is the tire size in mm * aspect ratio. Therefore, the total height of the tire is twice this number, plus the rim diameter. Tire_height = tire_width_mm * . 039 * aspect_ratio * 2 + rim_dia. For a tire with a 75% aspect ratio and 16" rim, this formula can be reduced to tire_width_mm*. 0585+16". For a 265/75R16, the height is = 265*. 0585+16=31. 5". For a 295/75R16 height = 33. 26". The ratio between the two sizes = 31. 5/33. 26 = . 947; therefore, the final drive ratio difference = 4. 10 * . 947 = 3. 88. The reason that all of this additional math is necessary is because although the tire size is proportional to the tire total tire height, the rim size is constant for all sizes. Wow, I haven't done algebra for years.

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Ken Lenger
On Order - 2001. 5 2500 QC 4X4 SWB SLT, ETC/DDX 5 Speed, 3. 55 (no antispin), camper group, trailer folding mirrors, RBR CD/Radio, LT265/75R16E AT OWL tires, trailer tow group, Patroit Blue / Agate,
Replaces 98 Jeep Grand Cherokee V8 which is for sale.
Trailite 21' Travel Trailer, ~4000 lbs loaded.

 

[WeirCummin]

This may be what your looking for.
http://www.4lo.com

 

[Neal Cassady]

Dooooohhh!

Sorry for the bum steer TREEMAN, thanks Ken, I thought my version seemed way to easy. Now I have a program to calculate tire heights.

Dave D

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2001. 5 QC, LB, 4x4, Auto, 3. 54, Everything but Leather, Bright Silver Metallic, ARE K Series, Diprocal EGT, Boost and transmission Temp in an Autometer Dash Pod, Westin CPS Platinum Step Bars - Black on Stainless

 

[spitty]

Originally posted by KatDiesel (Eric Howard):
If you still have your window sticker it will tell you what size tires came on your truck.

If you had 245's then when going to 295's with 4. 10's you now have 3. 74 ratio.

If you had 265's then when gonig to 295's with 4. 10's you now have a 3. 88 ratio.

This is assuming all the tires are ***/75/16 of course.

Does that help? I use a handy "calculator" to figure it.

Could you tell me what the original tire size that came with the truck has to do with the gear ratio? Are you saying that a truck with 245's has a different ratio ring and pinion than the the truck with the 265's?

 

[B.G. Smith]

What they are talking about is "what if". The ratio does not change but the distance traveled per revolution of the rear tires does shange with tire diameter change. They are calculating what the ratio would be to get the same distance traveled with the original size tires. Now you got me confused? bg

 

[SOLER]

spitty & B. G. Smith
The ratios in the diff's will be the same, either a 3. 54 or 4. 10. What changed with the 245s or the 265s was the correction in the speedometer from the factory. Trucks that came with 245s had a lower (numerically higher) final drive ratio than the 265-equipped rigs.
So, if your truck came with 245s and 3. 54s, you still had 3. 54s, but your engine will be turning faster @ 65 mph than the guy that got 265s and 3. 54s on his truck.
I hope this helps. #ad


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Shane Oler
'01 2500 4X4, ETH/6 speed, ISSPRO Pyro (pre-turbo)and Boost Pillar mount, Dark Garnet/Camel, 4" exhaust (still in the box... It's freaking COLD in Oklahoma!), Kenwood deck w/10 disc changer, NON-OEM Infiniti speakers x 4, 10 inch sub, two 150 watt amps, 20K Reese 5er hitch

Other: '70 Olds Vista Cruiser 455 (future resto project)
30' Sandpiper 5er Super Slide bunkhouse(about 10k lbs. )

 

[Darrel]

Thanks for your overwhelming answers. Kat Diesel why do you say the ratio is the same if the vehicle came stock with the 265 tires? The reason for the question is my friend bought an auto with 410 and 265x75 16 tires. I have an auto with 355 and 245x75 l6 tires. We both will be pulling the same fiver weighting 9500 pounds. I wanted to know when pulling at 2300rpm in direct gear(third)what will the speed differences between the two vehicles be. I appreciate all the input.

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Darrel A
1999 24v,Qcab,Sbed,2wh drive,Auto,BD Brake,Auto lock,EGT Boost Trans Temp Gauges

 

[TREEMAN]

Thanks for the imfo guys,i guess i,m running with a 3:88 ratio with the 295,s. I,ve often wandered ,still not sure how far my speedo is off though. Ihad the speedo recalbrated at the dealer when i bought the truck,& have yet to find a freeway with the mileage markers . Gotta luv this site you learn something new everyday !!

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2001,2500 q-cab,auto 4x4,4. 10 anti-slip rearend. All options except leather seats. Line-x bed & tool box. Slide-in -removable rack,BFG 295,s ,Fender flares. Westin nerf bars,billet grill & fog inserts. Rancho,9000,s,Isspro gauges,1994 2500 4x4,12 valve,5 speed, bone stock for now.

 

[Craig DeYoung]

Guys,

A good number to use in calculating the effective differential ratio is the tire's 'revolutions per mile'. This number will take into account the tire's actual rolling radius. Any good tire manufacturer website will list these data. Here is an example: A bridgestone Deuler A/T 265/75-16 goes 672 rev/mi. A Deuler 35x12. 50-15 goes 618 revs per mile. Let's say you have the 265s with 4. 10s, and want to know what your effective ratio is with the 35": 618/672= x/4. 10, x = (618/672)*4. 10. x = 3. 77:1. So having 35"s with 4. 10s is like having the original 265s with a 3. 77 gear. Your new speed is simply a ratio of the two rev/mi numbers: 672/618= 1. 09 or 109% If you change to the 35s and don't compensate, you will actually be going 9% faster than what is displayed. 65 will be 65*1. 09=71mph.

Craig

 

[klenger]

One of the best ways to check speedometer calibration that I know of (short of attracting the attention of a cop with a radar gun)is to check your speed with a GPS, which is quite accurate since the DOD turned off selective availability last year. I suspect that GPS speed is well within 1 MPH. If you can borrow one from someone, it's quite easy to check as no GPS programming is requred. Just turn it on, and once the GPS acquires enough satalites to be in 3D mode, correct speed should be available.

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Ken Lenger
On Order - 2001. 5 2500 QC 4X4 SWB SLT, ETC/DDX 5 Speed, 3. 55 (no antispin), camper group, trailer folding mirrors, RBR CD/Radio, LT265/75R16E AT OWL tires, trailer tow group, Patroit Blue / Agate,
Replaces 98 Jeep Grand Cherokee V8 which is for sale.
Trailite 21' Travel Trailer, ~4000 lbs loaded.

 

[KatDiesel]

Originally posted by Darrel:
KatDiesel why do you say the ratio is the same if the vehicle came stock with the 265 tires? The reason for the question is my friend bought an auto with 410 and 265x75 16 tires. I have an auto with 355 and 245x75 l6 tires.

I said that because 245 and 265's are both available as "stock" tires. Meaning I have a 4. 10 truck, it came with 265/75/16 tires... ... in your post I wasn't sure exactly whaat you were asking.

If your friends truck was stock with 265's and 4. 10's then that is his ratio. If he had 245's and then changed them to 265's then it would change the ratio.

Does that make more sensE?

 

[Hale on wheels]

OK guys, to calculate your effective ratio for a speedo calibration if you change tire sizes, you should calculate your MPH for each set of tires (at a set RPM) with the formula:

MPH=(RPM*tire diameter*. 002975)/Gear ratio.

Then you can calculate your new "effective" ratio by:

Eff Ratio=(MPH old tire/MPH new)*gear ratio.

Example... Say you have a 3. 55 rear end with stock 245s and use 2000 RPM (in direct drive, no overdrive) for all calculations.

MPH(245)=2000*30. 3*. 002975/3. 55=50. 7845 MPH
MPH(265)=2000*31. 5*. 002975/3. 55=52. 796 MPH

Effective ratio=(50. 7845/52. 796)*3. 55=3. 415

 

[spitty]

KatDiesel,
The number 4. 10 is the ratio of how many rotations the pinion has to make for 1 rotation of the ring gear. With 4. 10 meaning the pinion will make 4. 10 revolutions to the ring gears 1 revolution. It has nothing to do with the original size tires that came on the truck. If a truck came with 245's and a 4. 10 and you changed to 295's, the final drive ratio will be the same as that of a truck which came with 265's and a 4. 10 and you changed to 295's. This is my understanding of how it works. Please correct me if I am wrong.

 

[Hale on wheels]

I feel like a college professor... my calculation above is a complicated drawn out way to take the ratio of the circumferences (or diameters) of the two different tires and multiply that ratio times your gear ratio, which is really what Craig is saying.

Spitty, I think Kat means the same thing by effective ratio that I do, it is only in regard to what your speedometer will read.
Your actual gear ratio doesn't change, just the speed as related to RPM.

 

[MikeR]

Soler's post is right on. I had a '99 with 3. 54's and 245/75/16,and then bought an identical '00 with 3. 54's and 265/75/16, and the truck with the 265's turned exactly 200 RPMs less then the truck with 245's.

Darrel,
In my on the road experience in comparing 4. 10 trucks to 3. 54's,your buddy's truck will turn about 140 RPM more than your truck at the same speed.

-Mike