Mismatched tire diameters... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

AllGoNoShow · 09-09-2021, 06:55 AM

Is there any issues with running slightly different tire heights/diameters front and rear on these trucks? I'm researching wheel/tire combos and just as a function of what I am looking for, to get the width I want the front tires are going to be 1/4 of a shorter height then the rears. Will anyone even notice besides my OCD self?

Looking at 275/40r20 in the rear which are 28.7 height and I want to run a 255 (40s) in the front which will be 29" high. If I went down to 245s I could get the exact same height, but I just want the wider tires! Any issues you guys foresee?

**Palf70Step** · 09-09-2021, 07:08 AM

As long as the axles are the same size no issues. Your close to same height anyways. Normally tallest tires are in rear, at least with my experience. BTW, a 255/40x20 would be 28.1 tall by my research.

**dagnabbitt** · 09-09-2021, 08:58 AM

Should be fine.

**1970 CST Short Wide** · 09-09-2021, 04:09 PM

Quote:

Originally Posted by Palf70Step

As long as the axles are the same size no issues. Your close to same height anyways. Normally tallest tires are in rear, at least with my experience. BTW, a 255/40x20 would be 28.1 tall by my research.

Is there a formula to figure tire height. I’m thinking of going with 20’S but need to have a 28” height for some clearance
Thanks

**dagnabbitt** · 09-09-2021, 05:00 PM

Quote:

Originally Posted by 1970 CST Short Wide

Is there a formula to figure tire height. I’m thinking of going with 20’S but need to have a 28” height for some clearance
Thanks

I always use this site: looks kind of hokey but I am on it whenever I need to make a tire or wheel decision.

https://www.tacomaworld.com/tirecalc

**72afr** · 09-09-2021, 09:16 AM

I'm running 275/60/15 on the rear and 255/60/15 on the front. Those are about an 1" difference and no issues.

72c20customcamper · 09-09-2021, 09:31 AM

No issue at all with 2wd 4wd totally different situation as it will tear up the transfer case. Cars and trucks been running bigger rear tires for years

geezer#99 · 09-09-2021, 04:31 PM

Simple math done in metric to confuse Americans!
Convert the width in mm to inches by dividing it by 25.4.
There are 25.4 mm to an inch.
Multiply that result by the aspect ratio. Aspect ratio is the percentage that the sidewall is to the width.
That number is the sidewall height. Double that number because you have sidewall top and bottom of tire.
Add that number to tire diameter.
Pretty easy, eh!
Try it.
Tire is 255 40 20.
255 divided by 25.4 equals about 10.
Times aspect ratio. 10x.40 eguals 4. Double that equals 8. Added to tire height. 8 plus 20 equals 28.
You’ll be doing it without a calculator in a couple tries.

Honestly likely easier to find a chart.
Or go to tire rack and look for a common tire and you’ll find added tire specs that will give different dimensions.
Like searching for a 275 60 15 radial t/a.

sick472 · 09-09-2021, 04:58 PM

Great lesson on the math Geezer...thanks. I do have to say that I have messed around with a couple projects tires sizes and found that the charts and equations don't really match up with reality. I even found charts that gave "loaded" and "mounted" diameters for different rim widths. The charts and such are not to be trusted to the decimal...that's for sure.

For what the OP is doing, this is not too important, but if trying to figure out speedometer gear changes, that's a different story.

**pjmoreland** · 09-09-2021, 05:01 PM

The lazy way is to use a calculator like the one on this webpage.

https://www.tciauto.com/speedometer-gear-calculator

Scroll down to "Tire Diameter Calculator".

geezer#99 · 09-09-2021, 05:14 PM

It’s quite easy stuff!
Now, I’m still having a need to figure out the metric time conversion. That ones a little harder to do in your head.

**pjmoreland** · 09-09-2021, 05:31 PM

Quote:

Originally Posted by geezer#99

It’s quite easy stuff!
Now, I’m still having a need to figure out the metric time conversion. That ones a little harder to do in your head.

Ha!

Then there's the manual method of measuring out the distance the tire travels in one revolution. I did that on my truck and it came out at 26-1/4", even though the calculated diameter is 27". I used the measured diameter for selecting my speedometer gears. I tested my odometer over a 10 mile drive on the freeway, and it ended up dead on. Hopefully it will stay that way for a while, depending on how heavy my foot is.

Warrens69GMC · 09-09-2021, 05:49 PM

Easy Peasy-

https://tiresize.com/tiresizes/

09-09-2021, 06:55 AM	#1
AllGoNoShow Registered User Join Date: Feb 2020 Location: Rochester, NY Posts: 200	Mismatched tire diameters... Is there any issues with running slightly different tire heights/diameters front and rear on these trucks? I'm researching wheel/tire combos and just as a function of what I am looking for, to get the width I want the front tires are going to be 1/4 of a shorter height then the rears. Will anyone even notice besides my OCD self? Looking at 275/40r20 in the rear which are 28.7 height and I want to run a 255 (40s) in the front which will be 29" high. If I went down to 245s I could get the exact same height, but I just want the wider tires! Any issues you guys foresee?

09-09-2021, 07:08 AM	#2
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 47,082	Re: Mismatched tire diameters... As long as the axles are the same size no issues. Your close to same height anyways. Normally tallest tires are in rear, at least with my experience. BTW, a 255/40x20 would be 28.1 tall by my research. __________________ Bill 1970 Chevy Custom/10 LWB Fleetside 2010 Toyota Tacoma PreRunner SR5 Double Cab - DD Member of Louisiana Classic Truck Club (LCTC) Bill's Gallery Life isn't tied with a bow, but it's still a gift. Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God!

09-09-2021, 08:58 AM	#3
dagnabbitt Senior Member Join Date: Dec 2014 Location: Edmonton Alberta Posts: 1,400	Re: Mismatched tire diameters... Should be fine. __________________ DAVE Edmonton, Alberta 1959 Apache 1967 K20 1968 C20 1970 C10 1972 GMC 2500 1981 C10

09-09-2021, 09:16 AM	#6
72afr Beep! Beep! Join Date: Nov 2003 Location: Rural, MO Posts: 280	Re: Mismatched tire diameters... I'm running 275/60/15 on the rear and 255/60/15 on the front. Those are about an 1" difference and no issues. __________________ 1972 Chevrolet C/10 shortbed, stepside, 350, HEI, Fire Engine Red, Black Interior, Former Army cargo transport 2003 Chevrolet 2500HD CC 6.0 4wd 2004 Jeep Rubicon some aftermarket stuff... 2019 Jeep Unlimited Rubicon (the wife's baby hauler)

09-09-2021, 09:31 AM	#7
72c20customcamper Registered User Join Date: Jul 2018 Location: Catskill Mountains,NY Posts: 8,626	Re: Mismatched tire diameters... No issue at all with 2wd 4wd totally different situation as it will tear up the transfer case. Cars and trucks been running bigger rear tires for years __________________ Mark 72 c20 custom camper Husky edition, 66 SS396 Chevelle 1964 Hawk, 63 Avanti,62 lark 1969 AMX , 1968 c20 stepside ,85 K20 1977 Suburban sold 68 anniversary.

09-09-2021, 04:31 PM	#8
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,706	Re: Mismatched tire diameters... Simple math done in metric to confuse Americans! Convert the width in mm to inches by dividing it by 25.4. There are 25.4 mm to an inch. Multiply that result by the aspect ratio. Aspect ratio is the percentage that the sidewall is to the width. That number is the sidewall height. Double that number because you have sidewall top and bottom of tire. Add that number to tire diameter. Pretty easy, eh! Try it. Tire is 255 40 20. 255 divided by 25.4 equals about 10. Times aspect ratio. 10x.40 eguals 4. Double that equals 8. Added to tire height. 8 plus 20 equals 28. You’ll be doing it without a calculator in a couple tries. Honestly likely easier to find a chart. Or go to tire rack and look for a common tire and you’ll find added tire specs that will give different dimensions. Like searching for a 275 60 15 radial t/a.

09-09-2021, 04:58 PM	#9
sick472 Registered User Join Date: Jul 2015 Location: Sedalia Mo. Posts: 1,131	Re: Mismatched tire diameters... Great lesson on the math Geezer...thanks. I do have to say that I have messed around with a couple projects tires sizes and found that the charts and equations don't really match up with reality. I even found charts that gave "loaded" and "mounted" diameters for different rim widths. The charts and such are not to be trusted to the decimal...that's for sure. For what the OP is doing, this is not too important, but if trying to figure out speedometer gear changes, that's a different story. __________________ He who is without oil shall throw the first rod. Compressions 8.7:1 1972 C10 1976 C10 (parts truck) 1985 K20

09-09-2021, 05:01 PM	#10
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,747	Re: Mismatched tire diameters... The lazy way is to use a calculator like the one on this webpage. https://www.tciauto.com/speedometer-gear-calculator Scroll down to "Tire Diameter Calculator".

09-09-2021, 05:14 PM	#11
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,706	Re: Mismatched tire diameters... It’s quite easy stuff! Now, I’m still having a need to figure out the metric time conversion. That ones a little harder to do in your head.

09-09-2021, 05:49 PM	#13
Warrens69GMC Registered User Join Date: Jan 2003 Location: Safford,AZ Posts: 3,612	Re: Mismatched tire diameters... Easy Peasy- https://tiresize.com/tiresizes/

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