Speedometer correction
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Speedometer correction
Hi,
I finally got new wheels and tires on my truck, so I took some pictures. But with these new tires, my speedometer is quite a ways off. I was wondering what I can do to find the right gear to compensate for this difference.
I also had a problem with my speedometer before the tire change. I could go by a sheriff speed checker thing in my truck, and my speed in my truck would read about 26, when the speed checker reads 30. My odometer seems to be right on though. Then I could go by the same speed checker in another car, and it would be right on.
Anybody have that problem in their truck?
Thanks
85 F-150 4X4 351 4V H.O.
I finally got new wheels and tires on my truck, so I took some pictures. But with these new tires, my speedometer is quite a ways off. I was wondering what I can do to find the right gear to compensate for this difference.
I also had a problem with my speedometer before the tire change. I could go by a sheriff speed checker thing in my truck, and my speed in my truck would read about 26, when the speed checker reads 30. My odometer seems to be right on though. Then I could go by the same speed checker in another car, and it would be right on.
Anybody have that problem in their truck?
Thanks
85 F-150 4X4 351 4V H.O.
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I could do that, but I already used a speedometer error calculator, which takes my old tires size and new tire size and calculates my speed from speedometer reading, to actual speed. What I want is to have it be correct on my speedometer. I just went out and checked how many teeth are on the driven gear right now, and there are 20. How do i find out how many teeth are on the drive gear?
Post Fiend
Joined: Mar 2004
Posts: 24,418
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From: San Francisco, Ca.
jpjahn, I don't think you have to worry about the speedo drive gear.
I had the same issues with my 81 F150, only I had to figure out what speedo driven gear to get, after I changed both the tires (taller tires) and my rear end ratio.
What's the difference (circumference) between your stock tires and the new ones?
Just curious.
I had the same issues with my 81 F150, only I had to figure out what speedo driven gear to get, after I changed both the tires (taller tires) and my rear end ratio.
What's the difference (circumference) between your stock tires and the new ones?
Just curious.
Posting Guru
Joined: Sep 2003
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From: lino lakes, mn
one way to do that is go to a tune up shop with a chassis dyno that has its own speedo they will spin you vehicle to say 60 on their speedo see how far off it is then they'll correct it by changing gearsin your tranny or by using a varible ratio gear box or recalibrating your sppedo head but they do charge alot to do this
Fleet Owner
Joined: Sep 2005
Posts: 28,609
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From: Lake Tahoe, Nevada
Find yourself a good Ford parts counter man. Figure out the percentage of error by using the old and new tire diameters and the parts man will sell you a gear that will make it as close as possible.
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Post Fiend
Joined: Mar 2004
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From: San Francisco, Ca.
Originally Posted by Bdox
Find yourself a good Ford parts counter man. Figure out the percentage of error by using the old and new tire diameters and the parts man will sell you a gear that will make it as close as possible.
I did buy several speedo driven gears of different number of teeth, just in case I had to do any other changes in tires and/or rear end ratios.
My formula was a little bit harder, because of the rear end ratio change.
All in all, my speedo is very accurate.
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From: Winnipeg, Manitoba
Stock tires for an F150 are 235/75R15's That being said, stock diameter is 29 inches. Not all aftermarket tires are as advertised height. Some 31's are only 30 inches actual, some 32.5's are only 31 actual, some 35's are only 33 actual...you get the picture.
If you have access to a seamstress' measuring tape(made of cloth, and 1 yrd long, good ones measure in both metric and standard) jack a tire up, wrap the tape around it, and measure it's actual circumfrence, and use that in your equation, rather than it's advertised diameter.
If you have access to a seamstress' measuring tape(made of cloth, and 1 yrd long, good ones measure in both metric and standard) jack a tire up, wrap the tape around it, and measure it's actual circumfrence, and use that in your equation, rather than it's advertised diameter.
Post Fiend
Joined: Mar 2004
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From: San Francisco, Ca.
Actually original tire size vary from truck model to truck model.
My truck (81F150) came with P215x75R15 tires.
To find the tire size of your particular truck, locate the sticker that has that information. It could be located on the L.H. side door lock post ( on the cab).
It's usually a white label with tire size, tire inflation info, GVWR, and assorted body and drivetrain information.
Then, when you get the info, do the math.
It's easy.
My truck (81F150) came with P215x75R15 tires.
To find the tire size of your particular truck, locate the sticker that has that information. It could be located on the L.H. side door lock post ( on the cab).
It's usually a white label with tire size, tire inflation info, GVWR, and assorted body and drivetrain information.
Then, when you get the info, do the math.
It's easy.
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The new ones are 110 inches in circumference. The old tires that were on there were not stock though. They were 31X10.5X15, so I should use the stock circumference for the 29 inch tire. If my calculations are correct, a 29 inch tire would be 91.1 inches around. Does that sound right? So the difference is 18.9 inches. What do i do with that number now?
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Yeah, I thought my 31X10.5X15 were stock, because a friend of mine has the same size on his f-150. But then i looked on the door panel, and it says 235X75X15. That explains why my speedo was always alittle off I guess
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From: Missoula, MT.
my truck is going 69 when the speedo says 60 therefor i drive about 58 mph and that gives me roughly 65 mph simply...just get a gps and stick it on the seat, drive by that save yourself a huge headach and momorize the apporpriate speeds for the speed your actually looking for. thats what ive done. when im at 30 im technically going about 34-35 somewhre in there, so you see simply calculations in yur head and you got it made. Ive had my old 300 up to 95 on the speedo, i think, only goes to 85, lol but i think i was well over 100 actual speed, lol. isn't that fun