Jayco RV Owners Forum
 


Reply
 
Thread Tools Display Modes
 
Old 07-10-2015, 08:47 AM   #1
Member
 
Join Date: Sep 2014
Location: near the crossroads of America
Posts: 84
How does +2 tire size affect towing?

This may be considered a dumb question by some. I was taught the only dumb question was the one that's not asked, so here it is.
I bought a 2006 Chevy 2500 4x4, then found out the tires are 285/75r16, instead of the oem size of 245/75/r16. I found out the speedometer reads about 5mph slow at 60pmh, and the odometer is off, too. It also lowers my fuel economy by about .7 %. What I'm trying to find out is, how does it affect my towing ability? Do I need to start thinking about going back to original size? They are Michelins with maybe 85% tread on them.
__________________

Poppy is offline   Reply With Quote
Old 07-10-2015, 09:15 AM   #2
Senior Member
 
Join Date: May 2013
Posts: 924
It depends how much weight you are towing, how many gears you have in the tranny, what your differential gear ratio is, and how much power you have from the engine.

As a general statement, it will reduce your available torque to some extent because of the greater rotational mass and diameter of the larger tire. Depending on the mentioned factors above, it may be so minimal that you don't notice it, or it could bog you down to the point that your tranny will hunt for gears or won't stay locked, overheating issues, terrible fuel mileage, etc.

Since your truck is a 2500, it probably has good gears and a beefy tranny so unless you are towing at the upper limits of the vehicle you should be just fine. I think 245/75/16 look dumb on those full size trucks anyways.

I'm towing with larger tires on mine with no problems at all. I run 285/55/20 which is 33.5in overall diameter and my 6500lb 28BHS it perfectly obedient behind it.
__________________

SkyBound is offline   Reply With Quote
Old 07-10-2015, 09:35 AM   #3
Senior Member
 
mike837go's Avatar
 
Join Date: May 2015
Location: Campbell Hall
Posts: 2,837
Quote:
Originally Posted by Poppy View Post
This may be considered a dumb question by some. I was taught the only dumb question was the one that's not asked, so here it is.
I bought a 2006 Chevy 2500 4x4, then found out the tires are 285/75r16, instead of the oem size of 245/75/r16. I found out the speedometer reads about 5mph slow at 60pmh, and the odometer is off, too. It also lowers my fuel economy by about .7 %. What I'm trying to find out is, how does it affect my towing ability? Do I need to start thinking about going back to original size? They are Michelins with maybe 85% tread on them.
Since SkyBound dealt quite well with the drivability issues, I won't comment further about that.

However, since you know the larger tires have thrown your speedometer and odometer off, how can you reliably calculate fuel mileage? Especially, down to 7/10 of a percent? MPG will vary by up to 10% just because the tank was filled differently (topped off vs. short-filled because it kicked off at $$.90).

The diameter change from just the normal amount of wear will throw the speedometer off by about 3 MPH @ 60.

75% of 2 CM X 2 (the difference in diameter of the tires) is 3 CM. It was mentioned that the tires are 33" tall. So the difference when new is 3.5% larger on the 285's. Not a heckofalot.

Actually, the gear ratio of the larger tire to the ground will give you slightly better highway fuel mileage that is impossible to measure since your odometer was calibrated for the smaller tires.
__________________
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck


Sitting in The Cheap Seats.
And proud of it!
mike837go is offline   Reply With Quote
Old 07-10-2015, 09:37 AM   #4
Member
 
Join Date: Sep 2014
Location: near the crossroads of America
Posts: 84
Thanks, SkyBound. My truck has the gas 6.0, 4 sp. automatic with a 410 gear. My last trip, I was running a speedo reading of 60 to 65, not realizing the +2" tire size. I ended up getting 7.5 mpg. I didn't think that was very good, so I will be slowing it down on the next trip.
Poppy is offline   Reply With Quote
Old 07-10-2015, 09:57 AM   #5
Senior Member
 
Seann45's Avatar
 
Join Date: Jun 2010
Location: Saskatoon Sask Canada
Posts: 9,701
With +2's your 4.10 rear end now becomes a 3.73. (approximate)
__________________
Seann
2004 Chev Silverado Duramax optioned past the max. 2009 Jayco Eagle 308 RLS 765 watts of solar, 6-6 volt batteries (696 amp hour), 2000 watt (4000 surge) whole house inverter.
175 days boondocking in 2017
215/2016, 211/2015, 196/14, 247/13, 193/12

Seann45 is offline   Reply With Quote
Old 07-10-2015, 11:36 AM   #6
Senior Member
 
Join Date: May 2013
Posts: 924
Quote:
Originally Posted by Seann45 View Post
With +2's your 4.10 rear end now becomes a 3.73. (approximate)
Which is just about perfect if you ask me. You are lucky to have those 4.10s! They are a perfect gear if you are looking to go with bigger rubber. How much does your trailer weigh? And yes, definitely keep the speed down a bit... You should be able to pull off 8-9 MPG without much else different.

I get 9.6-9.8 MPG with my set up doing about 55-58mph.
__________________
2013 F-150 EcoBoost MaxTow, Roush tuned (415hp 506tq), lifted on 33s, R.A.S.
2013 Jay Flight 28BHS Elite (Equalizer 10K hitch)
SkyBound is offline   Reply With Quote
Old 07-10-2015, 12:55 PM   #7
Member
 
Join Date: Sep 2014
Location: near the crossroads of America
Posts: 84
Mike, When I found out the difference in the size between the 2 tires, I knew I couldn't get an accurate mpg figure. I hadn't thought about the wear of the tires making a difference in the speedo readings, either. Somewhere on the WWW, I read that larger tires would lower the fuel economy. (They can't put it on the web if it isn't true, right? LOL!)

Sean, I wondered if the 3.73 gearing would be close to it.

SkyBound, I really wasn't looking to get larger tires. I was just concerned that these 285's were too much for the long hauls.
The MPG's is really what lead to me starting this thread. My first tank, I figured I got 8.4 mpg, but the second tank is when I ran the truck at 65 (speedo reading) and got 7.2 mpg. That lead me to some serious looking for a problem, until I seen the sticker on the door with the recommended tire size of 245. I know that is 2.36" difference.
That's when I found out I was actually doing 70 mph. I don't knowingly tow that fast.
BTW, my TT is a JayFlight 24FBS. I think GVW is 7200 lbs.
Poppy is offline   Reply With Quote
Old 07-10-2015, 01:12 PM   #8
Senior Member
 
Join Date: Mar 2010
Location: north central Iowa
Posts: 304
A decent GPS will give you fairly accurate speed and distance numbers. You could then reference those numbers to your speedometer and odometer to give you reasonable data without the GPS.
__________________
2014 Eagle Premier 361REQS
2013 Ford F350 Crew w/ 6.7 diesel
sailor is offline   Reply With Quote
Old 07-10-2015, 02:24 PM   #9
Senior Member
 
Edd505's Avatar
 
Join Date: Apr 2015
Location: Elephant Butte, NM
Posts: 1,190
Quote:
Originally Posted by sailor View Post
A decent GPS will give you fairly accurate speed and distance numbers. You could then reference those numbers to your speedometer and odometer to give you reasonable data without the GPS.
Works really well. Got a Mustang GT in a swap that came with 15's and had low profile 18's when I got it, speedo wasn't even close. I used a cheap GPS that went with car when sold.
__________________
2017 Durango G353KRT 2006 F350SD 6.0 LB Crew
2000 F250SD SRW 7.3 LB Extended Cab Air Bags
2014 Eagle 30.5BHLT (sold)
2007 Layton 245LF (sold)

Learn from others mistakes, you'll never live long enough to make them all yourself. Eleanor Roosevelt
Edd505 is offline   Reply With Quote
Old 07-10-2015, 02:50 PM   #10
Senior Member
 
mike837go's Avatar
 
Join Date: May 2015
Location: Campbell Hall
Posts: 2,837
Quote:
Originally Posted by Poppy View Post
Mike, When I found out the difference in the size between the 2 tires, I knew I couldn't get an accurate mpg figure. I hadn't thought about the wear of the tires making a difference in the speedo readings, either. Somewhere on the WWW, I read that larger tires would lower the fuel economy. (They can't put it on the web if it isn't true, right? LOL!)
From a straight, gear ratio standpoint, larger rear tires (bigger diameter) will turn fewer times over the same distance of road than smaller tires. Therefore, all other factors (too many to realistically consider) being equal, your actual MPG will be higher.

But, now you have to spend extra fuel in order to spin up the additional mass of the tires.

There was a huge hoopla in NYC back a bunch of years (1980's?) where taxicabs were deliberately installing much smaller than specified tires. The fare was charged based on subjective distance covered. The smaller tires amounted to 10-15% overcharges. Lots of fines levied!
__________________

__________________
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck


Sitting in The Cheap Seats.
And proud of it!
mike837go is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


» Virginia State Campgrounds

Reviews provided by

Powered by vBadvanced CMPS v3.2.3

All times are GMT -6. The time now is 03:20 PM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Copyright 2002-2016 Social Knowledge, LLC All Rights Reserved.