tires or re-gear rear end - Diesel Forum

country2 · 06-19-2007, 01:25 PM

Hi, I have been reading a topic on the 3.73 V/S 4.10 rear ends. I have a 99.5 F-350 PSD that I purchased about 3 months ago. It is a stock truck 4x4 with 3:73. I bought it to pull a 10K fith wheel. Well I pulled it last week end for the first time and do not think it is up to par. I know that I am going to do some mods, but my first question is how much are my tires hurting my performance? I am running 295/75/16 on a stock rim.

I have a F-250 5.4 with 4:10 and it seems like it towes better than the PSD. I know this should not be the case so I am in search of what could be wrong. Can tires make that big of a differance?

Thanks
Paul

GreyHorse · 06-19-2007, 01:35 PM

I would try going back to the stock tire size, that's the cheaper of the two options. My 2k 4X4 pulls large trailers just fine.

Also, how are your fuel and air filters?

Believer45 · 06-19-2007, 01:47 PM

Just to give you an idea where to start (and assuming the stock tire size at 235/75 x 16), a 235/75 x 16 tire is approximately 29.7" tall. A 295/75 x 16 should be approximately 33.3" tall.

The 235/75 x 16 travels @ 7'9" per revolution, the 295/75 x 16 travels 8'9" per revolution.

The 295/75 x 16 tires with 3.73 gears will give the same effective ratio as 235/75 x 16 with 3.32 gears. This scenario is what you have now.

295 tires with a gear ratio of 4.185 would give the same effective ratio as 235 with 3.73. This scenario is what you would have with the (I believe) stock tire size.

Dave / Believer45

country2 · 06-19-2007, 01:51 PM

Air and fuel filters OK. I am going to check tubo boots this week end. I just bought these tires though and if they are a big issue I am going to try and swap them out.

I was having to turn the OD off on the slightest grade and EGTs were up around 1000 at 60 - 65 mph. With out the trailer it seems OK.

Diesel_Aggie · 06-19-2007, 04:57 PM

You'll notice a substantial difference swapping out the tires. If you are mainly using the truck to tow that would be my recommendation. If you need the width to keep from sinking in the pasture that's a different story. I think you'll find the truck to be more stable, get better fuel mileage, stop better and tow better with stock sized tires.

country2 · 06-19-2007, 06:43 PM

my stock tires were 265/75/16. big difference between the 265 and 295.

would there still be noticable difference

Diesel_Aggie · 06-20-2007, 02:07 PM

My math shows your current tires are 10.2% taller than the stock size. Most tire makers will have a technical specs area on their web site showing revolutions per mile, tire hieght and width. All tires of the same size don't measure out the same for some reason!

265 x .75= 198.75
295 x .75= 221.25
198.75/221.25=0.898305 for 10.2% difference in height.

There are other ways to calculate this where you compare total tire height but you come up with the same percentage.

I feel it will make a big difference. I ran 305/70-16 Dunlops mud terrains on a '94 F-250 4x4 reg cab. Major difference in acceleration and much worse pulling hills with 7000 pound flatbed trailer. Mounting a wider tire on a stock wheel causes the sidewall to be a different profile and allows more sway side to side. Mounting on a wider wheel solves most of that. Most tire makers will have recommended rim size to mount on and I bet 16x7 is not the recommended one for your tires. I have 33/12.50-16.5 BFG Mud Terrains on 9.75 inch wheels on my '94 and it also took a big hit in accelleration and towing. Stability is great with the wide wheels. I'm willing to trade off the power and 1.5-2mpg hit because I use the truck in the pasture and on logging roads. I hope this helps you out some. If you can find someone with stock rims/tires see if you can swap them to your truck for a test drive.

country2 · 06-20-2007, 06:58 PM

Thanks for that info. I like the way these tires fill the wheel well and they do look good, but I bought the truck to tow and I can not sacrifice the towing performance for looks.

I plan on doing an exhaust, intake and a modest tuner or chip and a 6.0 trans. cooler. Will that off set the tires or would you still recommend stock.

I dont mean to ask redundent questions but I need to make a decision on swapping tires soon. I bought them from Discount tire and they guarantee your satisfaction.

DaveM · 06-20-2007, 11:13 PM

You might even consider 245/75/16. They are shorter and would gear you down more yet.

sandfun · 06-21-2007, 04:17 AM

my truck and trailer is 22,000 lbs gross combined.....towes good on the flat but hills kinda kill me....i tried 285/75/16....and it towed great....but couldnt live with the looks....oh vanity vanity.....anyway....the torque converter lock up mod kinda helps my situation.....if the hill is killing me ....i drop to second and locket....can to 45 in second ....on my 35s......but my advice is if you dont need the looks the tire size is absolutley you'r best option....

Diesel_Aggie · 06-21-2007, 05:03 PM

Sure that will offset the tires some and maybe be better than stock tire performance. No easy calculation for that. If you do the mods and have stock tires then it's even better. It comes down to the trade offs you are willing to accept. Do you want to pull the trailer down the road or yank it down the road? Only you can set priorities and decide what is best for your needs.

v_lombardi · 06-21-2007, 08:13 PM

i had 245/75-16E on my truck and it made a big difference for off the line power. i now have 285/75-16 and will never do that again. i took a 235/85-16 bfg a/t and a 265-75-16 bfg a/t and put them side by side, they were the same height, but my next set of tires will be the 235/85-16E's.

vince

Snowman269 · 06-21-2007, 10:12 PM

[ QUOTE ]
Thanks for that info. I like the way these tires fill the wheel well and they do look good, but I bought the truck to tow and I can not sacrifice the towing performance for looks.

[/ QUOTE ]

Yes, there is usualy a trade off between looks & function. It takes money to have both. Do you tow more often at 55mph on 2 lane blacktop, or on the highway at 70mph? How often do you tow & how far. How often do you drive truck unloaded?

[ QUOTE ]
I plan on doing an exhaust, intake and a modest tuner or chip and a 6.0 trans. cooler. Will that off set the tires or would you still recommend stock.

[/ QUOTE ] These mods will definately make a difference, & I recommend them regardless. Do the mods, then decide on tire size. Add Guages to your list, Exhaust Pyrometer, Trans Temp & Boost are good to start with.

[ QUOTE ]
I dont mean to ask redundent questions but I need to make a decision on swapping tires soon. I bought them from Discount tire and they guarantee your satisfaction.

[/ QUOTE ] How much time do you have before you can't take them back? I'd get modding right away, anything is a good reason to mod.

I have a US Gear, which makes my 3.73's closer to 4.56's. With the US Gear, you can run bigger tires & Tow. Then when unloaded, you can run in stock gearing for highway mileage. One thing to remember is, Bigger tires generally result in lower fuel economy.

Don't sweat the questions, yours are normal & there are many on here who enjoy helping others. Great Site.

country2 · 06-21-2007, 11:02 PM

Thanks for all the input. I installed guages 2 weeks ago and am ready to start mods.

I do not know but I think I am past the return date on the tires. I am thinking about going tomorrow and push their satisfaction guarantee and try to get them to swap them for some Goodyear Wrangler silent armors, 265/75/16E. I think they have the white letters and look pretty good.

I will be towing more highway at 70 mph and the truck will tow more than not.

country2 · 06-22-2007, 09:50 PM

Went today and was able to swap tires. I went with stock 265/75/16E. hey are smaller and do not look as sporty but even empty I can tell a difference. It does not take near as much to get the truck rolling and it accelerates so much faster.

Thanks for the input.