All About Tyre Sizes: Is Bigger Really Better ?

[Frederic]

In this topic i am trying to go a little bit deeper into the whole concept of tyre sizes for offroading. Often, we start off with the stock 31inch tyres and find ourselves upgrading to a 32-33- or even 35 and more inch tyres, which might give benefits, or not ?

First of all, we need to split into two categories:

1. Offroading in sand/dunes aka dune bashing.

2. Offroading in mountains aka rock crawling or wadi bashing.

It is important not to confuse both, and they are almost opposite in terms of wheel size and i will explain this further below.

We will not be discussing the fuel economy in relation of tyre size in below topic, nor the effects of the odometer reading, but rather focus on the performance aspect  

 

Category 1: Offroading in the sand/dunes

What are the key parameters that determine the performance for driving in the dunes ?

* Tyre weight (relation with overall size). 

* Surface area of each tyre on the sand when deflated.

* Rolling Resistance

* Tyre profile (aggressive knobby vs smooth HT tyre).

Looking at above parameters, we would conclude that we are looking for a light tyre. The tyre profile in sand is not so important while driving but deflation is critical, and an AT tyre can be deflated further down safely compared to an HT tyre. We also noticed big differences during self recoveries on a smooth HT profile tyre vs a tyre with some moderate profile (AT tyres). 

Going for larger tyres results in higher weight on the axles, BUT seems to help with the contact patch area which is a good thing. So there is a balance that we are trying to strike. As a result of that you will see most offroaders in the dunes using similar tyres from the stock 31" to about 33" but not often 35" or more because of the negative effects that go along with that. (see re-gearing further down this topic).

 

Category 2: Offroading in the mountains

What are the key parameters that determine the performance for driving in the mountains ?

* Surface area of each tyre on the rocks when deflated.

* Tyre profile: AT Tyre is needed (HT tyre is not suitable for regular offroading in the mountains).

* Ground clearance.

As you can see, the key parameters for driving in the mountains are somewhat different because during rock-crawling you will be mostly in Low gearing and won't be needing the full top-end horsepower of your 4x4. The focus is more on using good torque and getting good clearance to overcome those boulders and other obstacles.

As a result of that, bigger tyres would be performing better, simply due to the fact that you will win ground clearance without having to lift your 4x4, and the extra surface area of those tyres will give you better grip as well.

And this is the main reason why we see so many Jeeps and Bronco's in the US on 35"-37" tyres crawling over boulders

 

Re-Gearing: What's that all about ? 

source: https://www.4-wheeling-in-western-australia.com/differential-gear-ratio.html

The differential gear ratio on a stock standard 4x4, are made to suit the tire diameter which the vehicle is originally came with. 

Using bigger diameter wheels/tires with stock ratios will cause more wear and tear on clutch, driveline and engine.

Low range gear will also suffer as 1st gear low will now roll too fast as the tires are bigger, this becomes a problem when descending steep hills and climbing rocks and logs.

Example: a stock 2005+ Toyota Hilux has 29 inch wheels/tires and fitting 33 inch rubber is quite bigger (4 inches in diameter bigger) this will cause the engine and entire driveline more stress and more power is then required due to the larger rolling resistance.

The final gear ratio is suited for 29 inch, this ratio in this particular case is 3:5, referring to a chart it will state the same (29' recommended ratio is 3:5) and looking at 33 inch diameter the chart will recommend 4:1.

Going by the chart any higher will make the 4x4 rev too much at highway speeds causing poor fuel economy.

Changing the crown wheel and pinion is how the ratio is modified. Basically installing a new or reconditioned crown wheel and pinion with the recommended ratio will bring the 4wd driveline back to stock specification rpm and fuel usage. Once differential gear ratio mod is done the vehicle will regain its 1st gear low range and take off in high range will be improved or even back to stock.

This chart is an approximate tyre size to differential gear ratio; it’s based on an RPM at 105 km/h (65 mph) and 4th gear on a manual transmission or 3rd gear on an automatic gear box (1:1 gear ratio).

This is only a guide to get a general idea of which ratios would suit the vehicle in question. To get an accurate RPM the following steps and formula will give an accurate answer:

Take a drive and record the rpm at 105 km/h (65 mph), the next step is to work out what the current ratios are in the vehicle. This can be revealed by calling the manufacturer or even calling a differential repairer giving him/her the Vehicle Identification Number, also known as VIN.

Once all this information is known use the following formula to get the accurate differential gear ratio for the larger tires:

Example:

The rpm is 2806 at 105 km/h (65 mph), the current tires are 29 inch, the factory ratio is 3.73 and the new size upgraded tires are 33 inch.

RPM: 2806

Stock tires: 29 inch

Stock ratio: 3.727

New Tyre size: 33 inch

2806 = 65 x 3.727 x 336 / 29 (rpm = mph (not km/h) x diff ratio x 336 / tire size)

Now the correct rpm is known for the vehicle (2806), the next step is working out what ratios are available for the vehicle, some makes and models won’t have as many choices as others.

Say the following 3 ratios are available 3.909, 4.100 & 4.88.

The new tire size wanting to be fitted is 33 inch and the rpm needs to be a close to the 2806 as possible in order to keep it to stock specs. Just a quick scan at the differential gear ratio chart it’s obvious that a ratio of 3.909 is too low at 2587 rpm, ratio of 4.88 is too high at 3230 rpm. But looking at 4.10 with the rpm at 2713 is the closest to 2809 stock rpm.

New Specs:

RPM: 2713

New tires: 33 inch

New ratio: 4.10

 

Please share your experiences / feedback !

[Benjamin]

Hi @Frederic very interesting topic just thought I would drop my personal point of view on tire size; So I run stock Sahara wheels and tires. I believe the tire size is 275/55R20 which is 31inch tyre (please correct if wrong) they are also AT tires. 

Since starting at Carnity I've only run these tires and I have had barely any issues in the sand, I do notice that bigger tyres have much more surface area when deflated but apart from that I don't see much of a difference atleast performance wise (once these tires wear out I am looking at potentially moving up a size) 

continuing this I have also taken these tires up steep mountains in Ras Al-Khaimah for camping trips, still to my surprise these stock tires (with the added lift) perform exceptionally well. 

I'm very interested to hear from others that have experienced both type of tires in the sand (as I'm looking into maybe going bigger) 

[Frederic]

53 minutes ago, Benjamin said:

Hi @Frederic very interesting topic just thought I would drop my personal point of view on tire size; So I run stock Sahara wheels and tires. I believe the tire size is 275/55R20 which is 31inch tyre (please correct if wrong) they are also AT tires. 

Since starting at Carnity I've only run these tires and I have had barely any issues in the sand, I do notice that bigger tyres have much more surface area when deflated but apart from that I don't see much of a difference atleast performance wise (once these tires wear out I am looking at potentially moving up a size) 

continuing this I have also taken these tires up steep mountains in Ras Al-Khaimah for camping trips, still to my surprise these stock tires (with the added lift) perform exceptionally well. 

I'm very interested to hear from others that have experienced both type of tires in the sand (as I'm looking into maybe going bigger) 

Hi @Benjamin

https://tiresize.com/comparison/

You are right, these are 32" tyres as you can see from above screenshot.

That size works great but has a couple of downsides:

- 20 inch offroad tyres are a bit less common in the market.

- the 55 sidewall is quite low and would be a bit more prone to popouts vs a 70 or 75 size sidewall (but that would make the overall wheel bigger again.

The Jeep OEM tyres will do fine and are suited for a nice blend of on-road and different styles of offroad, and that is why you've been happy with them.

In comparison with the Rubicon, that has smaller rims (17") they are able to go for a bigger sidewall as they have 285/70/17 if i remember correctly. That would be a 33" inch overall diameter approx.

If you want to go bigger on that same 20" rim i would advise to go for a 275/60/20 or 275/65/20, and not much more than that. Otherwise once you reach 35" overall size you will be entering re-gearing terrain  

 

[Benjamin]

10 minutes ago, Frederic said:

Hi @Benjamin

https://tiresize.com/comparison/

You are right, these are 32" tyres as you can see from above screenshot.

That size works great but has a couple of downsides:

- 20 inch offroad tyres are a bit less common in the market.

- the 55 sidewall is quite low and would be a bit more prone to popouts vs a 70 or 75 size sidewall (but that would make the overall wheel bigger again.

The Jeep OEM tyres will do fine and are suited for a nice blend of on-road and different styles of offroad, and that is why you've been happy with them.

In comparison with the Rubicon, that has smaller rims (17") they are able to go for a bigger sidewall as they have 285/70/17 if i remember correctly. That would be a 33" inch overall diameter approx.

If you want to go bigger on that same 20" rim i would advise to go for a 275/60/20 or 275/65/20, and not much more than that. Otherwise once you reach 35" overall size you will be entering re-gearing terrain  

 

Thanks @Frederic for the advice! I really don't want to regear my jeep, but extra sidewall might be something to consider maybe I should go down a Rim size but for now my tires are perfectly fine 😃

[Gary F]

@Benjamin I believe they are 255/70 r18  not r20  the Sahara wheel is 18 inch.

[Zed]

I changed my front diff & rear diff 4.30 gear ratio to 4.88 and I like it: both the torque and the speedometer correction since my tires are 285/70/17 (33") .  Since I like it so much for sand use, for my next project I'm ordering UnderDrive Gear for my Transfer Case.  Because my LC100 is Full Time 4WD, after transmission/gearbox it goes to Transfer Case before going to front & rear diffs. By modifying the Transfer Case gear, I'm re-gearing without changing both front & rear diffs (actually should have done this but parts was not available at the time). When I change the Transfer Case Output Gear, the output of front & rear diffs will be increased from 4.88 to 5.29, which will give me more torque.

The only downside to regearing is high RPMs and worse fuel mileage, but hey I can tow trucks and mileage for V8 sucks anyways 😂

Link for underdrive gear example: https://cruiserteq.com/sumo-gear-land-cruiser-transfer-case-10-underdrive-gear-set-fits-8x-100-series 

 

[Frederic]

17 hours ago, Zed said:

I changed my front diff & rear diff 4.30 gear ratio to 4.88 and I like it: both the torque and the speedometer correction since my tires are 285/70/17 (33") .  Since I like it so much for sand use, for my next project I'm ordering UnderDrive Gear for my Transfer Case.  Because my LC100 is Full Time 4WD, after transmission/gearbox it goes to Transfer Case before going to front & rear diffs. By modifying the Transfer Case gear, I'm re-gearing without changing both front & rear diffs (actually should have done this but parts was not available at the time). When I change the Transfer Case Output Gear, the output of front & rear diffs will be increased from 4.88 to 5.29, which will give me more torque.

The only downside to regearing is high RPMs and worse fuel mileage, but hey I can tow trucks and mileage for V8 sucks anyways 😂

Link for underdrive gear example: https://cruiserteq.com/sumo-gear-land-cruiser-transfer-case-10-underdrive-gear-set-fits-8x-100-series 

 

Would that basically turn your 4H into a semi-4Lo ?

[Zed]

2 hours ago, Frederic said:

Would that basically turn your 4H into a semi-4Lo ?

So let's calculate that:

A. 4Lo Crawl Ratio on 1st Gear 4Lo

My 4Lo ratio is 3.12 : 1, so if i hit 1st Gear on 4Lo (1st gear ratio is 2.80), my Crawl Ratio will be 👇🏻

[ Transmission Ratio of Current Gearstick x Transfer Case Ratio x Axle Ratio ] 

= 2.80 x 3.12 x 4.88 = 42.63 on 1st Gear 4Lo

B. 4Hi Crawl Ratio on 1st Gear 4H

Current 4H ratio is 1:1, so if I hit 1st Gear on 4Hi my Crawl Ratio will be 👇🏻

= 2.80 x 1 x 4.88 = 13.66 on 1st Gear 4Hi

C. Future project, gear reduction of 4H by 10%

This would mean my axle ratio is increased by 10%, so 4.88 + (10% of 4.88) becomes 5.37

= 2.80 1st Gear x 1 4Hi x 5.37 increased = 15.04 on 1st Gear 4Hi

So Mr @Frederic from point A, B, C, no my 4Hi won't be any close to 4Lo, it will actually just be 10% more torque than current 4H. Because of my 3.12 : 1 4Lo ratio, my 4L and 4H are like sand & sky apart (42 vs 13 crawl ratio) 😄

Edited June 1, 2023 by Zed

[Zed]

Sorry @Frederic I wrongly compared 1st Gear 4Lo with 1st Gear 4Hi, should be 4th Gear 4Lo (cos my transmission is 4-speed) and 1st Gear 4 Hi., So the new calculation

D. 4Lo Crawl Ratio on 4th Gear 4Lo

= 0.75 4th Gear x 3.12 4Lo x 4.88 front & rear diff = 11.42 on 4th Gear 4Low

E. 4Lo Crawl Ratio on 3rd Gear 4Lo

= 1.00 3rd Gear x 3.12 4Lo x 4.88 front & rear diff = 15.23 on 4th Gear 4Low

Yes you're right, my 3rd Gear 4Lo (point E) will now be similar to my 1st Gear 4Hi (point C) once I upgrade to 10% underdrive gear on the transfer case 😁

[Srikumar]

On 6/1/2023 at 1:31 PM, Zed said:

Yes you're right, my 3rd Gear 4Lo (point E) will now be similar to my 1st Gear 4Hi (point C) once I upgrade to 10% underdrive gear on the transfer case 😁

That's always the case. In my XJ the 3rd gear in 4L is almost the same as 1st gear in 4H. 

So there have been times in some nasty areas I have toggled between 2nd and 3rd gear in 4L which gives enough speed without compromising significantly on the low range torque to get out