My neighbor and I both have F-150s, SC, 4.6, auto tranny, 3.55 gears, towing package, etc. Mine's a '98 and his is a '99. Cruising the freeway at 70mph his '99 turns about 1850 rpm while my '98 turns about 2150 rpm. Does anybody know if the overdrive ratio in the automatic tranny was changed for 1999? If so, is there anyway to change/upgrade my '98? Of course, I'm looking for any mileage improvements I can get
Thanks,
Cannonball

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1998 F-150 XLT SC 4x2/Flareside/Shortbed/4.6L Triton/Auto/3.55 ls/towing package/Teal & Black/Rhino Liner

 

Cannonball -- Just curious, are you both running the same size tires?

 

no change is noted. must be the tires

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97 F-150 4.6 auto, 4x2, 3.55 LS, trailer tow pkg, ABM with K&N, Dynomax Super Turbo exhaust.
Oh yeah, it's Oxford White with crome wheels, Legacy shell & custom grill, mlaugh-Meridian, Idaho.

 

Geez I get 1950 rpm @ 70mph.
Maybe our tchs aren't 100% accurate.

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99 XLT 5.4L reg cab
4x2 120" Wheelbase
4 wheel disc brakes/ABS
5 star larait style wheels
Toreador Red/Silver
3.55 gears 255/70/16 OWL
Delta toolbox
Eclipse 5340 cd player
Memphis ARCS50 component speakers w/tweeters mounted flush in the door panels (excellent sound)
Sherwood 35x2 RMS amp for components.
Jensen 50x2 RMS for subs.
2 6.5" Bazooka tubes
Lowering shackles?
54regcab@duesouth.net in Columbia SC

 

I can see it now... hundreds of f150 owners out on the road doing 70 to see just exactly what rpm the tach is showing... I know at 120kph I run about 1800ish rpm's it's definatley under 2k.

cd

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Silver 99 F150 XLT Sport with Flare Sides...
Mods: K&N Air Filter, Tinted Side Windows, Reese Class 3, Pioneer Deck, Zapco Amp, JL Audio ('nuff said.)
Coming soon: Herculiner Bed Liner, Solid Tonneau Cover

 

Changing to bigger tires will most defintely change speedo accuracy.
However until you get your speedo recalibrated the RPM/MPH ratio will remain the same according to the dash gauges.
You will actually be going faster than the speedo shows until you recalibrate.
And yes Ford does calibrate each speedo differently depending on tires, gear ratio, etc.
My concern was how accurate are the stock tachs in our trucks.
Fords speedos must me within a certain amount to get DOT approval

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99 XLT 5.4L reg cab
4x2 120" Wheelbase
4 wheel disc brakes/ABS
5 star larait style wheels
Toreador Red/Silver
3.55 gears 255/70/16 OWL
Delta toolbox
Eclipse 5340 cd player
Memphis ARCS50 component speakers w/tweeters mounted flush in the door panels (excellent sound)
Sherwood 35x2 RMS amp for components.
Jensen 50x2 RMS for subs.
2 6.5" Bazooka tubes
Lowering shackles?
54regcab@duesouth.net in Columbia SC

 

54regcab,
Thanks for the info. Does the dealership have to recalibrate the speedo or can it be done by a home mechanic?

 

I know on the 99s it has to go to the dealer.
I think in 98 you can change the gear in the tranny not sure though.
Check both "wheels and tires" forums.
I sure someone has discussed it there somewhere.
Dont forget to change "show topics from past 30 days" to "Show all topics"

 

His tires are larger - 29.25" tall versus 28" tall for me. I didn't even think of the tires - DUH. Is 1.25" enough for 300 or so RPM difference? If so, I assume I could put on the larger tires and run 70 at lower RPM's. Does this mean his speedo is calibrated differently than mine? Would my speedo register incorrectly if I installed the larger tires?

Certainly Ford doesn't calibrate each truck's speedo at the factory... which means they must be calibrated for some constant (maybe the baseline tire) which would mean different size tires would cause the speedo to be incorrect... or am I complicating it too much?

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1998 F-150 XLT SC 4x2/Flareside/Shortbed/4.6L Triton/Auto/3.55 ls/towing package/Teal & Black/Rhino Liner

 

@ 74mph I run 2000rpm and 73.5 with 4wd

 

When I first bought the truck(2 weeks old) I has the the speedometer calibrated,Its free from AAA

Indicated Actual
speed speed
25 25
35 35
45 46
55 56
65 66
70 71 off 2%

odometer

100 107 off7 %

So I am realy going 75mph @ 2000 rpms,and getting 7% better gas milage.



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98,150,Excab,4x4,Lariat, Light prairie tan,4.6 auto,Super chip,Crome nurf bars,Orp,3.55ls,6-disk Changer,Electronic shift 4x4,Towing pkg, AC,Bed liner,Alarm Ecsondido, CA