We're shopping for a '98 or '99 F150 with a 5.4 V-8 and would sometimes tow a travel trailer. Reading the Ford specs shows quite a difference in towing capacity between the 3.08 and 3.55. Since we're looking a a used truck, is there an easy/reliable way to determine which gear ratio is installed? I've read a couple of messages about swapping out rear ends - would that void factory warranty if done at other than a Ford dealer?

Thanks!

 

The rear end cover has a metal tag on one of the bolts. It has the gear ratio, if it is limited slip, and the size. Example = 3L55-975-9M19 . 3 55 = 3.55 ratio, L = limited slip (try for this), 9.75 = size (all 5.4's have this size). Gear changes done at Ford will not void warranty.

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
4-wheel disc/ABS,Chestnut/Parchment,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.

 

jtbvirginia,

Open the driver's side door and look on the door pillar just below the strike. On the sticker there will be an axle code. Do a search on this site. The codes have been posted here several times.

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98 F-150 XLT 4X4 ORP Super Cab Short Bed, 4.6, Auto. Superchip, Gibson sing. cat-back, Airaid, 80/100w German head lamps. Warn HS9500i winch with semi hidden mount. Wrangler dual battery kit with red & yellow top Optima batteries. Torsion bars cranked up 2 turns. Needs one more door and a lot more power. I think I need to talk to Mr. Whipple.

 

Thanks for your help! I'm new to this website, so next time I'll try a "search" first.

 

jtbvirginia, make sure not only to check the axle code (rear end ratio) but the OEM tire size as well. When buying used vehicle you can run into "up sized" tires that will cut your effective rear end ratio substantialy. Look for at least a truck that has 3.55's or 3.73's rear end with the 255/70 R16 tires lsited as OEM on its door sticker. Do not go by what is presently mounted on the wheels!. They probably have been changed if they don't match the door sticker. And if increased you are no longer at the same *effective* rear end ratio as when it was stock. Same door sticker for this info as for axle codes. If you find one with the 235/70 R16's on the sticker beware that you will have to live with that size if you don't want to lose towing power.

Play around with this site to see how much "up sizing" tires can reduce your *effective* gear ratio. http://www.4lo.com/

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2000 F-250 Super Duty, SuperCab XLT, Oxford white with medium grey cloth captains chairs, 5.4L Triton V8 (260 HP - 345 FP Torque version!), 4R100 4 Speed Automatic Transmission, 4 wheel ABS, Deluxe Aluminum Wheels, Trailer Towing Package, Class IV Hitch, Reece load distribution system, Escort Sensor II Electronic Brake controller, Sure Stop towing skirt, "Westin" black powder coated step bars, "White-White" head lamps, Herculiner bed liner, ISSPRO Tranny Temp Guage, Excursion center console, Ford moulded mud gaurds, K&N Air and Oil filter, Amsoil XL7500 5W-30,Air filter minder.

 

If all else fails you can use this formula to calculate diffrential gear ratio based on the rpm, speed and tire diameter of the truck.

RPM x Tire Diameter/MPH x Transmision Ratio x 336.14 = Diffrential Gear Ratio

For example: 1789.95 rpm x 28" tire diameter/60 mph x .7 tran ratio x 336.14 = 3.55 diffrential gear.



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97 4.6 F150, 1/8 = 10.885 @ 64.10 mph, 88 5.0 Mustang 1/8 = 7.191 @ 94.76 mph

 

I dont see how the towing of a 3.55 equiped truck with 255/70/16 is going to be any different then one with a 235/70/16. If the truck came with 235/70/16 and you put on 255/70/16, the truck should have no different tow rating then a truck that came with 255/70/16 and still has 255/70/16

 

Look at the axle code inside the driver's door frame. What you're looking for is the code, H9. That's the 3.55LS (limited slip).

 

DarnEFNET, It does make a difference. A truck with 3.55 rear end that came with 235's when upped to 255's would then have an effective ratio of about 3.3. Now taht same truck with 3.73's would end up near 3.55 with the same change. Either way it will lose at least 500 pounds towing capacity and a bit of low end get up and go. The towing guide demonstrated this in fact with the move form 255's to 17 inch tires (265/70's)that are about 1.5 inches taller. It demonstrates this loss as 500 pounds. But it only covers OEM options. However the same calculations apply when you move up in any tier size that changes the overal diameter. If it didn't there would be no change in the towing guide for different rear end ratios now would there. And there most certainly is. *Effective* ratio is very important. And tire changes affect that.

 

I have 235/70/16 tires now. I am considering going with 235/60/16 tires soon. This would up my gearing 6.4% due to a 1.85 difference in tire diameter. (28.9528 - 27.1024 = 1.8504/28.9528 = .0639) Tire diameter is certainly a way of changing gearing.

[This message has been edited by LarryS (edited 06-13-2000).]

 

jtb, hurry up and get that truck before our Norfolk rally on 28 July.

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1999 F-150 Lariat 4x2
Reg cab Short box Styleside
5.4L 3.55LS
Dark Toreador Red/Gold
VentShades
Tow Pkg
Line-X

 

cphilip and others - thanks for the heads-up on tire size effect on effective ratio. As a non-mechanic type, I need a little help on one point. Is the OEM tire size shown on the door sticker a limiting factor in future tire size that can be installed? In other words, if the OEM size was 235/70 R 16, would that prohibit changing out tires to something like 255/70 R16, which I take it is what I should be looking for in order to get better towing capacity? Guess I'm really asking how much latitude I'd have in changing tire size, based on the OEM number. Unless the trucks are built differently to correspond to the OEM tire size, I guess I assumed there was a good bit of latitude on changing tire sizes, within obvious broad limits (but those assumptions are what gets ya sometimes!) Thanks!

 

I agree with the posts by CPhilip and LarryS. A friend of mine runs some really small slicks at weekend trips at the local strip (looks funnier than running on one of those compact spares) -- and it changes his effective ratio from 3.?? way up one full size to 4.?? -- It is a really cheap way to have a dual-purpose truck.

<A HREF="http://www.net-link.net/~n8jg/y2k/mvc-008s.jpg" TARGET=_blank> [img]//www.net-link.net/~n8jg/y2k/mvc-008s_small.jpg[/img] </A>


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Y2K™

Toreador Red, Keyless XLT SC SB 5.4L E4x4 4wDisc/ABS, 3.73LS, Skid, HD 7700# Towing, LT-245's on Chrome, Tube-Steps, Captain's, 6CD, Tonneau, named: "Nick"

 

If you can find one that had OEM tires of 255/70 R16's I would do it. And even better yet had 3.73 rear end and 255's tires OEM. That will give you the most latitude for tire sizes. Nothing precludes you from putting any size tire you can put on there. It's just for towing you will hurt your overall ability if you do. I just wanted you to be aware of it when buying.

Example: You spot a good used one and look at the tires and they are in great shape but they are 265/75 R16's and you like the looks of these bigger tires but then you look at the door sticker and discover that its OEM set up was for 235's and it has a 3.55 rear end. Ooops! you now have closer to a 3.0 something rear end. Not good for towing. And you then might be faced with buying new smaller tires right away to correct this. And you may not like the truck as well with the smaller tires either. But at least you could then bargain for a tire change if you really like the truck,. But only if you could point this problem out to the dealer and then save yourself some money. Or you might chose to pass it up instead not thinking the smaller tires would look good.

But I strongly recommend the 3.73 rear end (3.55 minimum) and at least 255/70 R16's for what you want to do and to have good sized tires with the ability to go up a little if you wanted too later (with the 3.73 rear end anyway).

My 2 cents.

 

Changing from 235's to 255's gives you 20mm extra tread width, but also changes the tire height, speedo, and gear ratio slightly (providing both have a 70 aspect ratio). Each type/brand of tire has it's unique weight ratios, speed ratings, etc. The aforementioned situation creates very slight changes. If you change to say 285's, most tire companies recommend wider wheels than the stock 7"ers. Example 285/60/16 = same overall height but recommended for 8-9" rims. Curiosity can be endulged looking at major tire manufacturer's fitment guides (ex. tire basics) ex. BFGoodrich.

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
4-wheel disc/ABS,Chestnut/Parchment,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.