axle ratios
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axle ratios
I just burned out a 355 rear and thought it was a good time to get something a little more for the highway; swapped out rear end for a 308. Anyone know of a formula or some way to figure out how much my speedo would be off; I think it now reads lower than real speed. Also called one Ford place and found that I might not be able to get an exact match for replacement speedo gear; using a C6; parts guy said he had a 325 for a C6 and said something about a 300, but I think it was for a different tranny. Any help is appreciated.
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axle ratios
The best way I know is to compare the difference between your odometer and the actual distance you travel using Hwy mile markers, its best if you can do it over an extended distance, (50 miles or more). You can then count the number of teeth on your odometer gear and make a change reflecting the difference : such as 100 miles traveled / 110 miles on your odometer 10% increase in your odometer. If you count 20 teeth on your gear you would decrease by 10% (or 2 teeth) to an 18 tooth gear. If you want to be even more exact,there are several adjustable boxes made to work in line between your tranny and odometer(on older vehicles that dont use a speed sensor). You should be able to find a speedo calibrating shop in your yellow pages(usually under calibrating) and they will also be able to make suggestions or help.
Jeff
Jeff
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Joined: Dec 1999
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From: Copley, OH
axle ratios
The 3.25 and 3.00 ratios are probably for the 9" rear. If you had a 3.55 and your speedo was correct before then with a 3.08 your speedo is off by:
3.55/3.08 = 1.1526
At an indicated 56 you'll be doing 65.
At an indicated 48 you'll be doing 55.
Any half way competent speedo shop or Ford dealership can supply the speedo drive gear you need. It's a 10 minute job to install.
3.55/3.08 = 1.1526
At an indicated 56 you'll be doing 65.
At an indicated 48 you'll be doing 55.
Any half way competent speedo shop or Ford dealership can supply the speedo drive gear you need. It's a 10 minute job to install.
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axle ratios
Thanks for the replies; I did some quick math after posting the question and got the same results as StrangeRanger: 56 reading is an actual 65; there's about a 13 or 14% speed difference, which is of course directly related to the change in final drive. I'm going to try and find a correct gear; it's definitely worth it.
Fleet Mechanic
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From: Lower 48
axle ratios
Get it out on the interstate and time yourself between mile markers. 60 secs = 60 mph, 65 secs = 55 mph, 55 secs = 65 mph. Take the time it takes in secs and divide it into 3600 and it will give you mph. 3600 divided by 60 secs = 60 mph.
Trail Rider
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axle ratios
I just burned out a 355 rear and thought it was a good time to get something a little more for the highway; swapped out rear end for a 308. Anyone know of a formula or some way to figure out how much my speedo would be off; I think it now reads lower than real speed. Also called one Ford place
and found that I might not be able to get an exact match for replacement speedo gear; using a C6; parts guy said he had a 325 for a C6 and said something about a 300, but I think it was for a different tranny.
In responce to your question I also had a 355 rear end in my 1990 F Series truck with a 355 and burnt it out I had it replaced with a 308 in 1997 and when I got pulled over for going (according to my speedo) 135 Klms in a 100klms zone the cop wrote me up for doing 136 klms. So my guess is that the speedo is not off very much at all.
and found that I might not be able to get an exact match for replacement speedo gear; using a C6; parts guy said he had a 325 for a C6 and said something about a 300, but I think it was for a different tranny.
In responce to your question I also had a 355 rear end in my 1990 F Series truck with a 355 and burnt it out I had it replaced with a 308 in 1997 and when I got pulled over for going (according to my speedo) 135 Klms in a 100klms zone the cop wrote me up for doing 136 klms. So my guess is that the speedo is not off very much at all.
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