speed changes
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Laughing Gas
Joined: Aug 2000
Posts: 1,005
Likes: 63
From: Eastern, Idaho
speed changes
If I put bigger tires on my truck will it change the speedo reading? The new ones will be 35 from stock 32. What do I need to do to adjust so it will read right?
Cargo Master
Joined: Jul 2000
Posts: 2,281
Likes: 18
From: Northern California
speed changes
Most have a gear that connects to the tranny or the TC if it's 4x4. The gears can be changed and the ford dealer has many of them. Some companys make them and re-adjust the speedo for you. If you have an electronic auto, that may be a different story. Some of your local 4x4 shops might have some good info for you.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
speed changes
Chris, Yes it will change your reading a little. The best thing to do is find the shop in your area that does the speedo calibrating. All cop cars in NC (before electronic) had to have their speedometers calibrated once a month, in order to be declaired certified. That shop can test yours and either adjust it or at least let you know what your actual speeds are. A lot of people have had their speeding tickets reduced after having their speedos adjusted.
My trouble has always been I was caught speeding enough over that the adjusting wouldn't help me. LOL
John
jowilker
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66F100s Rule
In the cool still quite of night you can hear chevies rusting away.
My trouble has always been I was caught speeding enough over that the adjusting wouldn't help me. LOL
John
jowilker
[link:www.ford-trucks.net/users/jowilker|Club FTE since 01 01] My FTE Page
[link:www.ford-trucks.net/users/jowilker/NCFTE.html|NC Truck Owners] NC Ford Truck owners group
66F100s Rule
In the cool still quite of night you can hear chevies rusting away.
Post Fiend
Joined: Feb 2000
Posts: 5,130
Likes: 8
From: Brea CA
speed changes
Actually, putting on bigger tires will not change your speedo reading or your tach reading but you will be going down the road faster for the same readings. Think of it this way: say you put your truck on a lift and it tachs at 3000 rpm @70 mph (don’t really do this), now put bigger tires on it. The reading will be exactly the same because the bigger tires have nothing to do with it. Now put the truck back on the ground, the reading are still the same but the larger diameter tires are covering more distance than the smaller tires for the same wheel rpm. That’s where the difference comes in.
Barry
Barry
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
speed changes
So Barry, If a big ol road cop pulls you over for speeding, what will you get the ticket for, going beyond the posted speed limit, or faulty equpment(bad speedo reading)?? 
John
jowilker
[link:www.ford-trucks.net/users/jowilker|Club FTE since 01 01] My FTE Page
[link:www.ford-trucks.net/users/jowilker/NCFTE.html|NC Truck Owners] NC Ford Truck owners group
66F100s Rule
In the cool still quite of night you can hear chevies rusting away.
John
jowilker
[link:www.ford-trucks.net/users/jowilker|Club FTE since 01 01] My FTE Page
[link:www.ford-trucks.net/users/jowilker/NCFTE.html|NC Truck Owners] NC Ford Truck owners group
66F100s Rule
In the cool still quite of night you can hear chevies rusting away.
Senior User
Joined: Mar 2000
Posts: 113
Likes: 0
speed changes
I am in the process of changing my Speedo gear (the driven gear) for my 35 x 12.5 x 15's. I ran a 16 speedo gear on the 33's and it was right on. My 25's run 5 miles an hour slow on my speedometer. I need either a 15 or a 17 tooth speedo to compensate for this difference. An older Ford mechanic said that you need to change your speedo in tooth count 1 tooth for every 4 to 5 miles an hour difference. So I have ordered both the 15 and 17 to see wich one is needed. Hope this helps. Also for running P235 x 75r15's the gear is the 21 tooth gear.
BJ 1977 F150 4x4 Flairside "Bubba Grape"
400 C/6 B LT Mega Shifter
6" suspension lift 35x12.5x15 BFG,s
"Have you driven under a Ford Lately?"
BJ 1977 F150 4x4 Flairside "Bubba Grape"
400 C/6 B LT Mega Shifter
6" suspension lift 35x12.5x15 BFG,s
"Have you driven under a Ford Lately?"
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Junior User
Joined: Dec 2000
Posts: 72
Likes: 0
speed changes
Thats odd I always that that a bigger circle went around slower that a small one, so if you had say 32" tires and went to a 36" tire, when your speedo says you going 65 your acually going 70. I saw this on Mr Wizards world once.
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