I have the stock 255/70R16 but I want to put on larger diameter Mud tires with different wheels for the Fall (back road hunting/Colorado ski trips) and Winter. I know I will deviate the speedo but what else could I hurt by the change, rear end, suspension? What does Ford do differently for the ORP setup to accommodate the 265/70R17’s, a 1.5" diameter difference?

2001 F150 SuperCrew 4x4
5.4 EFI
3.55 LSA
CLASS III Towing


[This message has been edited by larrygio (edited 04-05-2001).]

[This message has been edited by larrygio (edited 04-06-2001).]

 

I went from the stock 255/70/16 to a LT265/75/16. Big difference in looks and it changed the speedo. When it says 60, I'm actually going 63 or 64. Seems to be getting worse mileage too. About 1 mpg less (14 to 13). I don't think you while "harm" any of the other suspension parts. If you went to an even larger tire, you should change the gears so the engine/transmission won't "bog" down too much.

You might want to consider getting the same size spare tire. If you run the smaller spare with the larger tire on the rear with the LS rear end, it'll ruin it pretty fast. I'm not too sure on the front with 4x4 disengaged, but definatly don't engage it!!

Here's a cool site that has calculators for tire height, gear ratios and mph change.

http://www.bncoffroad.com/tire_gear_cal.html

Since the taller tire makes a 3.55 act like a 3.37, you're pulling power is less. The only way to bring it back is to install a higher ratio, like a 3.73.

I'm going to just live with it for now. If it really becomes a problem with towing/hauling, then I may consider swapping the gears. I need to check about the speedo. I think it is just a matter of changing out the speedo gear in the tranny. Like one with one less tooth or something?

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1997 F150 4x4 Off Road, SC, Short Box, 5.4, Royal Blue/Silver, K&N, Gibson SS, LT265/75/16 Wild Country APT's

 

Thanks for info...

 

Mitchf150
you said you had about a 1 mpg loss after switching tire sizes but did you take into account that not only your speedometer is now off but so is your odometer and trip meter. If not take that into account also and then check your mileage.

 

Thanks big pa s,

Didn't even consider that!! Duh!! It makes sense now. Speedo is slower, so the odometer is going to show LESS miles then I have actually gone. Guess I need to figure out the percent that it is off and then re-compute. Come to think about it, what's the difference anyway....... It's a 5500 pound truck. Of course it's not going to get any kind of gas mileage. If I wanted gas mileage, I would have gotten a (sic) Honda!

I really like the bigger tires anyway. Will have to go for the speedo gear change I guess. Thanks again for pointing out that oversite on my part.

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1997 F150 4x4 Off Road, SC, Short Box, 5.4, Royal Blue/Silver, K&N, Gibson SS, LT265/75/16 Wild Country APT's

 

I find that's pretty common (the forgetting about the actual mileage increase due to larger diameter) On my 93 f-150 I ran 265's & it came with 235's. All I did is got out on a decent stretch of highway with mile markers, hit my tripometer at a marker & counted off ten miles worth of markers then looked to see what I was short on my tripometer. If I remember right I ended up adding in about 6% to my miles traveled on my odometer before calculating mileage (pretty much added about 6 miles to every hundred registered on the meter)There is probably a better way to do it but it worked for me. I plan on running 285's on my 97 which came with 255's so I plan on having to do the same thing unless I can get the speedo set for the bigger tires. I think in some cases you may actually see mileage increases with a little larger tire as long as it's not too wide. And of course you would probably only see gains if any at highway speeds but possibly a drop in city driving.

 

I did that very thing coming into work this morning. Turns out I lose one tenth every two miles, so I need to add one mile for every twenty (5%). When I compute that in, I am back to the 14 mpg.

I just hope I can find a speedo gear that will adjust for only 5%. I seem to remember reading somewhere that the minimum was 9%. I'm going to be doing some looking around, but if someone already knows something, that would be great. (this is a great site!)


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1997 F150 4x4 Off Road, SC, Short Box, 5.4, Royal Blue/Silver, K&N, Gibson SS, LT265/75/16 Wild Country APT's

 

Quote: What does Ford do differently for the ORP setup to accommodate the 265/70R17’s, a 1.5" diameter difference?

Answer: Nothing! They adjust the speedo and lower the towing rating in the towing guide! It's true...

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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, 3.73, 4 wheel ABS, Deluxe Aluminum Wheels, Trailer Towing Package, Telescoping Trailer Towing Mirrors, Class IV Hitch, Reece load distribution system, Escort Sensor II Electronic Brake controller, Sure Stop towing skirt, "Westin" black powder coated step bars, 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, Perma Cool 8" (800 CFM) Tranny Cooler Fan (manualy switched), Deflect-a-Sheild Aluminum Dimond plate Tool Box.

 

Glad that I could help. I'm going through the same thing trying to figure out what my mileage is. I just recently put 35's on my 97 and I have seen a drop in mileage. But the tires are 13.5" wide too. Oh well I didn't buy them for their mileage helping abilities.

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big pa s
1997 F150 ORP 3" Performance accessories Body lift
35x13.5x16 Mickey Thompson Baja Claws
16x10 AR Outlaw II's
Grill Guard
Side Steps
100 watt off road lights
55 watt fog lights
mud in every crevice of the truck