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I just bought a new speedometer cable to replace my old one because the needle was jumping around and I didn't want to pull the cable, lube it and reinstall it. I have been driving many years making adjustment to my speed because I'm running 235 70 R15 tires now; the original tires for this 1980 F100 were 205 70 R15. I believe my rear end is a 9" with 3.25 gears. Is there a math formula to determine my actual speed vs what the speedometer tells me? Is a different gear for the cable at the trans connection available?
Ok first you "CANT BELIEVE" you need to know what the numbers are you are dealing with.
Factory tires were 215 / 75R / 15 for both of my 81 F100's.
Option would be 235 / 75R / 15 and what I have on my flare side: 2wd, 300 six, granny 4sp, and a 2.75 rear gear.
So you need to know the tire size on your truck and what rear gear it has as the numbers will be off if you use the wrong ones.
See if this one will give you what you are looking for. https://www.garysgaragemahal.com/calculators.html
I forgot yes there are different gears for the speedo cable to dial it in.
What makes you think it may be off?
A few ways to check.
Find one of them radar speed signs that tells you how fast you are going in that speed zone and check the speedo.
Get a app for your phone that can measure your speed by GPS.
I used my trucker GPS that displays speed when checking the speedo in my truck and can tell you it is so close that I dont use anything but the trucks speedo.
Dave ----
My tire numbers are correct (door sticker states 205s came with the truck). The axel code is 15 and according to General Information Section 39 1980-89 Light Truck that translates to a 3.25 rear end. I guess a 2.75 would be possilble as someone once rattled that off to me in a matter of fact way in a discussion. With no tag on the rear end how do I know?
I guess 205 could be factory but you sure on the 70 and not 75? Both what came on the truck and with the 235's you have on now?
On the rear gear could jack it up and count the number of turns the drive shaft turns for 1 turn of a tire.
2.75 turns or 3.25 would tell what you have.
Do you have a tach in the truck as that would help too.
But I think a GPS speed app on your phone would be the best for checking speed.
Dave. ----
You can't believe those electronic speed signs along the road, some are correct, some are not. You did not change your tire size that much, so it's probably not off very far. The easiest way is to get someone to ride with you with a stopwatch, and get on the interstate where the miles are marked off with markers along the side of the road. Go 60 mph (try not to get run over going that slow) and it should take you 60 seconds to go 1 mile when the speedo says 60 mph. If after 60 seconds you are not at the next marker yet, your speedo is reading faster than you are actually going. Speed up just a little bit and try it again. If you are already past the marker when 60 seconds come up, your speedo is reading slower than you are actually going.
Adjust your speed till you get 60 seconds between the markers. Once you get that you can tell how far it's off.
The one I go by at times is pretty close to me speedo
I was also thinking the same on the tires, not that big of a difference to worry about.
We dont know what his issue is with the speed reading?
It could on the money and he just wants to check to make sure.
App on phone I think is still the best.
Dave ----
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