Stock speedo accuracy
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Laughing Gas
Joined: Dec 2014
Posts: 774
Likes: 11
From: Simsbury, Connecticut
Stock speedo accuracy
Folks - Just wondering about something regarding my stock speedo on my '56 F500. Considering the T98 and the 272 combo also comes in the F100 was there something done at the factory that adjusts the speedometer so it is accurate with my 6.12 D100 rear end and larger wheels? I'm running 8.25-20 tires now and I think the truck came with 7.50-20 but I'm not sure. Anyway, I ask because when I hit 40 it feels like I'm going a whole lot faster than that. And I'll be going even faster at "40" when I put on my new 9.00-20 tires. Thoughts?
Cargo Master
Joined: Aug 2014
Posts: 2,594
Likes: 72
From: North Central Arkansas
There is a gear on the tailshaft of your T98 just ahead of the driveshaft yoke that drives a small gear that the speedometer cable connects to. There are several different combinations of those gears that are matched to the axle ratio and tire size that the truck left the factory with. The speedometer will be off if you are running a different size tire than what belongs on it. It would also be off if a PO installed a transmission from a different truck but didn't change the speedometer gears.
Mark
Mark
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Have someone follow in a chase vehicle with a known accurate speedometer so the speed difference can be determined accurately. If you have some sort of GPS that will work too. Ford sells speedometer driven gears with varying number of teeth, you may have to try a couple different ones but that'll get you real close to spot on.
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Laughing Gas
Joined: Dec 2014
Posts: 774
Likes: 11
From: Simsbury, Connecticut
I learned something new today. Thank you for the clarifications. I was going to do the GPS thing but my truck requires, how do I say, concentration to stay on the road. I can only imagine how dangerous the roads would be if everyone did all of the things they do today while behind the wheel...if that wheel was in one of these classic rides!
Cargo Master
Joined: Mar 2001
Posts: 2,759
Likes: 70
From: Elk River, MN
Find a marked mile, like two mile markers on the highway. Hold any steady speed, say 40 mph indicated and time the mile in seconds. Divide 3600 by your time and you will get your actual speed. Like 3600 divided by say 80 seconds = 45 mph actual.
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