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How will this affect my gear ratio or mpgs?

How will this affect my gear ratio or mpgs?

Postby jdelrizzle [OP] » Apr 21 2012, 4:15am

Well I'm about to get rid of my general tires, 245/70/17 and replace them with all terrian 265/70/17. My questions are how much will this change my gear ratio and will this hurt my rear end, tranny, or engine. Will it throw off my computer/speedo? Is the 245 to 265 a big difference?
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Re: How will this affect my gear ratio or mpgs?

Postby jdelrizzle [OP] » Apr 21 2012, 4:16am

0h by the way I have 3.23 gear ratio
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Re: How will this affect my gear ratio or mpgs?

Postby 1500VHO » Apr 21 2012, 4:51am

jdelrizzle wrote:Well I'm about to get rid of my general tires, 245/70/17 and replace them with all terrian 265/70/17. My questions are how much will this change my gear ratio and will this hurt my rear end, tranny, or engine. Will it throw off my computer/speedo? Is the 245 to 265 a big difference?


Hi,

Will this hurt your rear end = No
Will it hurt the tranny = No
Will it hurt your engine = No

Will it throw off your computer/speedo = Yes
Your rear gear of 3.23 will be more like 2.98 (approx)

The difference will be 20mm in overall height. 245 = 30.5in 265 = 31.6in
Also 245/70-17 tires are 9.65 in wide
265/65-17 tires are 10.43 in wide

At any given indicated speed the rpm will be lower OR at any given rpm you will be going faster. You can always use the +1 rule and put 265/60-18 on your truck with no difference. If you want or need to keep the 17 in rims the closest size would be 265/65-17 = 30.56

The computer can be re-programmed for the tire height, with a hand held or perhaps by the dealer (for a charge).

BTW - Here's a link to a tire height calculator: Link: Wallace Racing

I hope this helped, :-)
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Re: How will this affect my gear ratio or mpgs?

Postby jdelrizzle [OP] » Apr 21 2012, 2:47pm

So if at any given rpm the speed is faster, does that not mean that the truck isn't working as hard to turn those tires? Or is it the other way?
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Re: How will this affect my gear ratio or mpgs?

Postby 1500VHO » Apr 21 2012, 3:54pm

jdelrizzle wrote:So if at any given rpm the speed is faster, does that not mean that the truck isn't working as hard to turn those tires? Or is it the other way?


Hi,

Yes at any given rpm, say 2,000. You will be traveling further and "true" speed will be higher because your tires are gaining 20mm per per revolution... Your indicated speed via the speedometer will read what it use to be. It does not "realize" you have larger tires, unless you tell it so via a handheld programmer.

In the old days you use to have to change out a little gear in the tranny. Now it's all digital via the computer.

I'll post back with some examples....

:D
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Re: How will this affect my gear ratio or mpgs?

Postby 1500VHO » Apr 21 2012, 8:01pm

jdelrizzle wrote:So if at any given rpm the speed is faster, does that not mean that the truck isn't working as hard to turn those tires? Or is it the other way?


Hi,

I hope my math is correct. Please feel free to check it...

Stated Facts:
Engine, Trans and Differential = same/no change
Rim width, diameter = same/no change

There are 63,360 inches in a mile, and you’ll travel 3,801,600 inches per hour at 60mph.
3,801,600/63,360 inches per minute = 60.00mph

What is changing are the tires.

245/70-17 = 30.5 inches in height = 661 revolutions per mile

265/70-17 = 31.6 inches in height = 638 revolutions mile. So in order for the tire to cover the same distance, it turns less. Which equates to the engine turning less rpm to cover the same distance in the same time… 60 miles in one hour.

Tire dimensions will vary a little from tire manufacturer to manufacturer, but generally they are equivalent. The larger tires will cover more ground per revolution.

The formula looks like this:

245/70-17 tires = 30.5 x 3.14 = 95.77inches per one revolution. Almost 8 feet.
63,360 in per mile/95.77 = 661 revolution per mile

265/70-17 tires = 31.6 x 3.14 = 99.22inches per one revolution. More than 8 feet.
63,360 in per mile/99.22 = 638 revolutions per mile

Now at a given or set engine rpm for 60 mph in overdrive with 245/70-17 tires
.
We need to cover 63,360 inches per minute to reach 60mph.
63360/95.77 = 661
661 x 3.23 = 2,136rpm in 3rd gear
2,136 x .7 (Trans OD) = 1495rpm

You should be running about 1,500rpm at 60mph in Over-drive with 245/70-17 tires

Now at a given or set engine rpm for 60 mph in overdrive with 265/70-17 tires.

63,360/99.22 = 638
638 x 3.23 = 2,060rpm in 3rd
2,060 x .7 (Trans OD) = 1,440rpm

So you should be running at 1,440rpm at 60mph in overdrive with 265/70-17 tires.

If you run at 1,500 rpm in Overdrive with 265/70-17 tires, you’ll actually be doing 63 mph.. Just remember this difference gets larger as you increase the rpm… At an indicated 75mph you'll actually be doing 79mph... That would be enough to get you a ticket in some states..
And the Odometer will also be off by the same factor. You’ll be going further, but the odometer will read less… As to your other question, the truck will be fine, either way.

BTW - I used this calculator for tire size/dimensions: Tire Calc

I hope this helped,, :D
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Re: How will this affect my gear ratio or mpgs?

Postby jdelrizzle [OP] » Apr 21 2012, 9:53pm

Ook, so if at lower rpms I'm going a bit faster cover more area wouldn't that equal more mpgs? How much do you think my odometer is off by? Hey thanks really appreciate it.
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Re: How will this affect my gear ratio or mpgs?

Postby 1500VHO » Apr 21 2012, 10:11pm

Hi,

For every 1,000 miles your odometer reads, you'll have "actually" traveled 1,040 to 1,050 more. Or there abouts.

As for the gas mileage. It will probably go down.. Again the engine and truck are "actually" going farther than the odometer reads. It still takes gas to cover those extra miles.

I apologize if I hit you with too much info on my previous post. I just wanted to make sure you understood my logic...

:D
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Re: How will this affect my gear ratio or mpgs?

Postby EKinMN » Apr 21 2012, 10:24pm

All that math looks good. But simply stated, the new tires are going to weigh more, and it will take more torque to turn them. It will probably take more gas, but since the speedo is off a bit you'll never really know, so who cares!? Enjoy the new tires!
The difference is so small, this is not something I'd spend too much time being worried about. I intend to go from 265/65/18 to 285/65/18, and I am not going to do a single thing differently after the switch.

Enjoy!
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Re: How will this affect my gear ratio or mpgs?

Postby jdelrizzle [OP] » Apr 21 2012, 10:26pm

Yea I understand I looked up all that stuff myself and couldn't understand it that well, but you broke it down pretty good for me thanks, I like the Yokohama geolander at-s p265/70/17
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