Different sized tire

SummitPond · 11-15-2016, 02:49 PM

My truck (F250 Super Duty Lariat 2WD) presently has LT275/65R18E tires on it. They need to be replaced. One of the local tire places (Pep Boys) suggests I use LT275/70R18E tires in lieu of the factory recommended size (they're roughly $40 less expensive per tire for the same brand). This larger tire will cause my speedometer to read about 3% low compared to actual speed. This doesn't concern me too much as it's a simple correction to make. However, my question is this: will the larger tire circumference put extra strain on the engine or transmission?

Thanks.

**JRTJH** · 11-15-2016, 03:17 PM

You don't indicate whether your truck is gas powered or diesel powered. I'd suspect that with the gas engine, you may have some issues with heavy towing if you've got a 3.55 gear ratio. If you've got the 3.73, probably not so much. With the diesel, both the 3.31 and the 3.55 rear end ratios are rated to tow the maximum weight, so if you've got the 3.55 rear end, the "worse case" would be a reduction to gearing around the 3.31 area which won't affect your towing.

So, I guess what I'm saying is that if you've got a gas engine and a high ratio rear end, you might notice some power loss with a heavy load, but if you've got a diesel, it probably won't make much difference.

**sourdough** · 11-15-2016, 04:29 PM

As John said, the drivetrain makes all the difference in the world when choosing larger tires. You have to know your axle ratio,

Why does the dealer think you need to go with a taller tire vs OEM? Trying to save a few dollars? I've upsized LOTS of tires on vehicles and never really gained anything unless I was trying to gain ground clearance. Then it was much more than 3%, requires new ring and pinions front and rear, new drive shafts, CV joints, wheels etc., etc. infinitum.

If it were me I think I would just stick with the OEM size unless there is some sort of problem with that. The tiny increase in height won't gain you anything...although the 70 series might be a little narrower. If you have an axle ratio higher (numerically lower) than a 3.55 I wouldn't even think about it.

SummitPond · 11-16-2016, 06:16 AM

Gentlemen

Thank you for your feedback.

The tire dealer was (I think) trying to save me some $, but in the long haul it doesn't should as if it's worth it. I'll stick with the OEM and keep researching price.

For the record the truck is gas, and while at one time I knew the gear ratio, I cannot seem to find it again in the owner's manual.

**JRTJH** · 11-16-2016, 07:36 AM

Quote:

Originally Posted by ShacterShack

Gentlemen

Thank you for your feedback.

The tire dealer was (I think) trying to save me some $, but in the long haul it doesn't should as if it's worth it. I'll stick with the OEM and keep researching price.

For the record the truck is gas, and while at one time I knew the gear ratio, I cannot seem to find it again in the owner's manual.

There are two easy ways to find your axle ratio (if your truck has the original rear axle).

First, locate the door jamb sticker and get the axle code from it, then go to this website and find the code. It will describe your axles: http://www.blueovaltrucks.com/tech/f...le_codes.shtml

Seecondly, you can go to this website and type in your truck VIN. It will produce the window sticker and in the bottom (options) section, your rear axle will be listed. http://researchmaniacs.com/VIN-Numbe...cker/Ford.html

Here is a picture of the doorjamb sticker. The axle code is along the bottom row of information. NOTE: I downloaded the picture from the internet, it is not from any vehicle that I own.

Desert185 · 11-16-2016, 08:08 AM

Quote:

Originally Posted by ShacterShack

Gentlemen

Thank you for your feedback.

The tire dealer was (I think) trying to save me some $, but in the long haul it doesn't should as if it's worth it. I'll stick with the OEM and keep researching price.

For the record the truck is gas, and while at one time I knew the gear ratio, I cannot seem to find it again in the owner's manual.

Yep. If gas, don't change the tire size if 3.55 or less. You'll be disappointed.

My diff ratio is on the glovebox door. Your's may not be.

CWtheMan · 11-16-2016, 01:38 PM

It’s quite unusual for a widely recognizable tire retailer to recommend a replacement tire that is not earmarked as a replacement for the vehicle.

The tire placard info and vehicle owner’s manual info will no longer be valid.

Plus sizing is part of their basic training. It’s not complicated and they should have had the information available to answer your questions. NHTSA authorizes the use of auxiliary tire placards for plus sizing. The installer should complete the form and affix it next to the original tire placard.

If they are just trying to make a quick sale I’d take my business elsewhere. If you asked for the less expansive plus sized replacements make sure the retailer provides you with all the pertinent information. The larger tire is 1.1” taller than the OE tire and has 626 revs per mile. The OE tires have 649 revs per mile.

Pull Toy · 11-16-2016, 03:29 PM

Had a Sears "Tire Guy" suggest a change in size that was "close". Turns out, that was all he had in stock, and they were getting "old". Commission for stock rotation of older sh*t?

IMHO if the vehicle manufacturer wanted a different size tire, they would have designed it that way, kind'a like coffee and beer... the first being cream and sugar... the second being ice. Wasn't meant to be! LOL

SummitPond · 11-16-2016, 05:59 PM

John (JRTJH)

Thanks for the links, but believe it or not, neither had information on my axle code (3E) or VIN (not released yet [!?]).

After searching several sites I found a one that had the axle code (http://www.ford-trucks.com/forums/10...xle-ratio.html) and it states that it is "3.73 Electronic Locking". I also downloaded an electronic version of my owner's manual from the Ford site and searched, without luck, for the code; all it had (on page 253) is a table based on GCWR, but no indication of how to determine what that was for my vehicle (there were two values listed in the table and I couldn't figure it out from the VIN). It turns out my gross combined weight rating) is 19,000 lbs. based on the axle ratio.

As stated earlier, I'm going with the OEM recommended tire. It is $120 more (for the four) than the oversized tire (and this surprises me ... bigger tire is less expensive?). I think the salesman was just trying to save me some money.

Again, thank you all.

**JRTJH** · 11-16-2016, 06:26 PM

I'm glad you found the information, sorry that my links weren't more help. Anyway, good luck with the new tires, I agree with you, staying with the OEM tire size is probably the best option, especially with heavy towing in your future.

Desert185 · 11-17-2016, 03:16 PM

Quote:

Originally Posted by Pull Toy

Had a Sears "Tire Guy" suggest a change in size that was "close". Turns out, that was all he had in stock, and they were getting "old". Commission for stock rotation of older sh*t?

IMHO if the vehicle manufacturer wanted a different size tire, they would have designed it that way, kind'a like coffee and beer... the first being cream and sugar... the second being ice. Wasn't meant to be! LOL

What is the effective difference between manufacturers changing tire size year to year and the consumer doing the same?

CWtheMan · 11-17-2016, 09:10 PM

Read this reference.... http://www.tiresafety.com/images/Tir...t%20Manual.pdf .... It has all the information oh how it's supposed to be done.

Desert185 · 11-18-2016, 06:09 AM

Quote:

Originally Posted by CWtheMan

Read this reference.... http://www.tiresafety.com/images/Tir...t%20Manual.pdf .... It has all the information oh how it's supposed to be done.

It tells how its supposed to be done (disregarding important wheel offset, BTW), and does not answer my question of the effective difference between the manufacturer changing tire size vs the consumer doing the same. The question is semi-rhetorical, FWIW, as I already know the answer. I was just wondering if the poster knew.

kfxgreenie · 11-18-2016, 08:09 AM

We live in the United States of Government, do not do anything other than written policy or you will either die or be TAXED! Furthermore, we as a society are dumbing down our citizens and our youth by teaching ONLY do as the book says. We wonder why our youth is stuck in front of electronic devices rather than in the garage tinkering with pa and or ma, and actually learning using their head and common sense. Oh wait, common sense was thrown out the window, today we use book sense, and then we wonder why the "millennials" (I'm one) are the most unproductive generation to ever grace society. Well let me tell you why its the way we are teaching youth.

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