Quick question?? I have a 1998 F-150 4x4. I've purchased and installed a few after market goodies. They are super-chip, exhaust system, K&N intake system and a few other goodies. My question is this. Do bigger tires make you speedometer read fast or slow? I'm not sure what the size of the stock tire was. I do know I installed the biggest BFG's that I could without lifting it. I know I've lost some towing power. I had saved up to have the gears changed, but spent the money on a new stereo for my boat. I wanted 4.10 gears, but I don't have $1300.00 laying around. Anyway back to my main question. Is my speedometer reading slow or fast. I drove next to anothe vehicle today and here is what I found. The other vehicle said 70 and my truck was reading 76. I was told this could not be possible. What do you think?

 

richgonfishn,

It is possible, simply because you saw it,
but it should read slow. For the same number of revolutions your larger tire goes further so your speedo should read slow. What tires did you start with and what did you change to?

Regards

Jean Marc Chartier

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00 F-150 XLT SC Flairside 4x4 4.6 w/5spd 3.55LSD
Warn XD9000i, skid plates, Draw tite class III,
Rancho RSX Reflex shocks,
BFG 285/75 R16 MTs,
Borla Cat Back, Headman Headers,
Modified Air Cleaner Outlet Tube
and air box, Ported Throttle body,
Shortened MAF sensor tower,
K&N air filter,
Superchips

 

With bigger tires, your speedo will read slower. This other car that you were reading their speedo from, were you talking with them at the time or just trying to glance over at their speedo??? Try getting a friend (w/a phone) to drive next to you and test it that way.

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2k F250 CC SWB 6 speed 4x2 Diesel XLT Super Duty, Woodland Green
Power Mirrors, 3.73 LS, Captains Chairs, ARE tonneau top, rear privacy glass
TT Pkg, Class IV hitch, sliding rear window, running boards, mud flaps
265/75/16 AS OWL tires, am/fm/cassette/cd, rechargable MagLite
Clarion APA 5240 amp, Memphis speakers:Components front, 5x7 rear, 10"sub
SuperChip, Stepshields, Ventshields on windows, rubber bed mat
rugged liner bedliner on tailgate, quick disconnect jumper cables
unaFORDable (decal on tailgate) Western Diesel Turbo Brake (whoa boy)
Finally gauges!!: Pyrometer & Boost on the A-Pillar mount

 

I went from stock 255/70/16 to BFG 285/75/16 tires.

 

By my calcs, you should be reading approx 9.9% (might as well say 10%) slower than what you are actually going. I. E. if you speedo reads 70 you should be going approx 76-77 MPH (exactly the opposite of what you said happened)
question: Is the other guys speedo correct?

Solution: try this, using the miles markers on the highway, time how many seconds it takes for you to go 1 mile at a set speed. Then use this formula 3600/(time you recorded). That should give you your actual speed (pretty close anyway)

 

richgonfishn, I changed my tires to the same one's as you. But I went from 265/70/17(31.7") to 285/75/16's (32.8"). I did lose some get up and go but not that bad. I have 3.73 gears so mabe that helped. I haven't tried to match speedo's with another drive yet but I did calculate that my rear end is at 3.6 on a website calculator.


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99 Expedition XLT,5.4L,4X4,3.73LS,285-75-16 KO'S9(old 17"AT/S),Airaid Intake,Superchip,Edlebrock shocks,Dynomax Super Turbo 3",Helwig Sway Arm,Polk speakers