speedometer off by 10 mph

have a 1966 with a 170 6cyl / 3spd. with 14inch wheels. i used my gps and found out im going 10 mph slower than my speedometer is reading how can it be so far off ? far as i know everything is orginal. how many teeth sould i have on my speedometer gear

Is it 10 MPH slower at all speeds? Or is it 10 MPH slow while driving 60, and 5 MPH slow while driving 30 MPH?

only tested it at 50 & 60. 50 it ran 40 at 60 it was showing 50

i'm waiting for an answer on this as well. my van does the progressive misreading, the faster i go, the more it's off.

If you have the wrong gears somewhere, it should be off by a percentage. My not-so-old Econoline reads one MPH faster for every 30 miles. It reads 31 when I'm going 30, and it reads 62 when it's really doing 60. If I was to go 120, the speedometer should read 124, but if I was going 120, I wouldn't be looking at the speedometer!

If it is always off by 10, then you might have a problem with the speedometer. Is it at 0 when you are stopped?

gh0st wrote:i'm waiting for an answer on this as well. my van does the progressive misreading, the faster i go, the more it's off.

There is a small gear where the speedometer cable goes into the transmission that can be changed. I remember reading about the different gears and how to identify them on one of these forums. Try searching for speedometer gear.

On GM tranies, ! tooth less wil increase speed 5 mph at 50mph. don't know on Ford set up.

There are sevral different gear available for the end of the spedometer cable. If yours is off by a percentage then you need to change the gear. IO have a formula somewhere that calculates it for you.
Fred

i was told this van was orginaly from a air force base. most military veh. are governed at 55-60 mph any way i will take it for a slower ride and see if its stil off by 10 mph

Possibly spedo gear change is warranted?? Also consider wheel size=?/ and Tire sizes also =ive seen everything from 13-15 inch wheels on the older econos= you can also consider a spedo compensator -goes inline on cable & spedo gear-where cable goes into-Happy Vannini]

delraydoug wrote:i was told this van was orginaly from a air force base. most military veh. are governed at 55-60 mph any way i will take it for a slower ride and see if its stil off by 10 mph

If I had to guess and since you said it was an Air Force van it might have had a 4.56 rear end ratio and it was probably changed out by a previous owner. Like Fred said, you probably need a different gear for the speedometer (at the transmission end). Pretty simple fix once you find out which gear you need.

yes i do think it orginally had 4.56 and was changed out to a 3.50 and they never changed speedo gear in tranny. thats why its off so much. what color gear or how many tooth should i have running 3.50's ?

so can anyone comment on what tooth speedo gear was stock for the 3.03 manual econolines?

The info is out there if you look.

http://www.transmissioncenter.net/speedometer_calibration_______va.htm

There isn't an exact speedo gear for a rear axle ratio, the tire diameter also affects the final drive ratio.

RodStRace wrote:The info is out there if you look.

true, but the info is also in here if i ask.

anyone with a 61-67 econoline van or truck, with a 3 spd 3.03 manual transmission, with driver's side speedo cable drive gear hole, with a 3:50 rear end, and your speedometer is accurate:

what is the tooth count of your speedo cable drive gear?

pulled out my speedo drive gear today it is a - white 17 tooth . while i was under there with the belly pan off i took the rattle out of the shifting linkage with plastic wire ties thru the holes on the radiator suport underneath the rods [ worked great have to post a picture of it]

thanks for the info Doug, that's great!

checked mine last night, ended up being a 19 tooth type 3A gear.



since my speedo is reading about 16mph high, i will be replacing it with a 20 tooth gear to slow it down. it better be close though, because 20 is the highest 3A gear i can find. or maybe i could just put some 22" wheels on the back, haha.

I just did the GPS test in my '64 G-10 with 15" wheels with 215R70 series rear tires and found that my speedo reads 4-5 MPH faster than GPS speed, consistent through all ranges...at 35, I'm really doing 31, at 55, 51 and at 65, about 60.  The same GPS unit in a modern car matches the speedo perfectly.  Looks like larger than stock wheel and tire diameter inflates the speedo reading....good to know, at least I'm not speeding all the time and not that far off to worry about a gear change if you can even get one for a '64 Saginaw!