Speedometer reading slow

a1airboat · #1 (**permalink**) 05-05-2004, 04:05 AM

I have a problem with the speedometer reading about 10 to 12 mph slow on my Superformance. Car has T56 6 spd trans and Yokohama tires size P275/60R15 107T on the rear. Spdo gear in the trans has 19 tooth. Tried 18 and also 20 tooth gears and cant get it to read any better. Anyone have any idea what I can do to solve the problem ? Thanks Darrel

Ron61 · #2 (**permalink**) 05-05-2004, 06:08 AM

A1,

Less teeth should speed the speedometer up. The norm is about 4 MPH per tooth each way. Maybe try dropping down three teeth and see if you notice any difference. If not, then maybe the drive gear in the transmission itself will have to be changed but that is a job as you have to pull the transmission. I have also read on posts about adapters for speedometers but don't remember which ones they work on. Some of the others can give you better information on that.

Ron

Tom Howland · #3 (**permalink**) 05-05-2004, 09:33 PM

A1,
I just did mine yesterday. I was reading 10% too high. Went to 21 from a 19. I feels right on. Now I'm looking for one of those CHP radar signs they set up on the roads to check it.

Bruce Edwards · #4 (**permalink**) 05-06-2004, 04:13 AM

The others are correct you need to go smaller to speed up the speedo. You should check it at 60 mph. Every tooth is worth 2.5 mph if I remember. If you are to slow by 10 mph at 60 and you are running a 19 you would need to install a 15. Since one is not made that small the best bet short of changing the one on the output shaft to a larger size would be to get an adapter that fits on the output side of the speedo and changes the ratio from there. Check your local speedo repair shop they should have on to fit your car. They are not much money and they are easy to install. You may just have them do and calibrate the speedo at the same time.

a1airboat · #5 (**permalink**) 05-06-2004, 04:32 AM

I use a hand held GPS and it gives a fast and acurate reading on the MPH. I ordered a 16 tooth gear and i'll try it today and see if that makes any difference.

Ron61 · #6 (**permalink**) 05-06-2004, 06:22 AM

Bruce,

A question. when we were working on correcting a friends speedometer, Ford here told us that each tooth on the driven gear equates to approximately 4 miles per hour. If your 2.5 MPH is correct then I have been telling people the wrong thing. Could you please let me know which is correct?

Thanks,

Ron

a1airboat · #7 (**permalink**) 05-06-2004, 11:24 AM

Hi again, installed a 16 tooth gear in the trans and it corrected the slow reading speedo. It went from about 12 MPH slow to correct using the GPS for checking. I started out with a 19 tooth in the trans.

Bruce Edwards · #8 (**permalink**) 05-06-2004, 11:43 AM

Ron, from what I remember from working on Ford speedo's and working on them in the kits we build it is 2.5. He fixed it with a 16 which is very close to the 15 I thought he would need. The funny thing to me is I do not remember one as small as he found. Just glad he got it fixed.

Ron61 · #9 (**permalink**) 05-06-2004, 12:07 PM

Bruce,

Thanks for the correction. Glad he got it fixed too and it sounds as if your 2.5 per tooth is the correct one. Wonder why the Ford parts man here told us 4 miles per hour. I am going to have to contact that guy and see if he ever did correct his and how many teeth he changed.

Ron

Juggernaut · #10 (**permalink**) 01-06-2009, 12:54 PM

Quote:

Originally Posted by Ron61

Bruce,

Thanks for the correction. Glad he got it fixed too and it sounds as if your 2.5 per tooth is the correct one. Wonder why the Ford parts man here told us 4 miles per hour. I am going to have to contact that guy and see if he ever did correct his and how many teeth he changed.

Ron

I know this is an old post I have a quick question:

Is the 2.5 mph per tooth on the driven gear the same whether the tranny has a 7 or 8 tooth drive gear?

Thanks.

Tom Kirkham · #11 (**permalink**) 01-06-2009, 05:16 PM

No.

The rear end ratio and effective tire diameter are also variables that need to be accounted for...

Juggernaut · #12 (**permalink**) 01-06-2009, 06:24 PM

Quote:

Originally Posted by Tom Kirkham

No.

The rear end ratio and effective tire diameter are also variables that need to be accounted for...

Makes sense, thanks!

What'saCobra? · #13 (**permalink**) 01-06-2009, 09:05 PM

Push a little harder on the pedal on the right side to correct defect.

(just couldn't resist...)

STL Mark · #14 (**permalink**) 01-06-2009, 09:21 PM

My speedo seems about 10 to 15 mph slow too - I'm always passing people. Everybody drives so slow. But seriously - is this the speedo gear I need 16 tooth.

Here's the site where I found it:
http://www.speedometergears.com/

So, I have no clue - Where does it get installed?
Is this different than what came with the car/trans stock?

Vehicle info = SPF2693, TKO600, 15"wheels, dealer install 100%

Another dumb question - but would this problem effect the mileage count of the odometer?

strictlypersonl · #15 (**permalink**) 01-07-2009, 04:20 AM

If you, for instance, start out with a 19 tooth gear, you can calculate the change by dividing the proposed number of teeth by 19. i.e. 18/19=.95 > a 5% change (which translates to 3 mph at 60 mph).

Ron61 · #16 (**permalink**) 01-07-2009, 04:32 AM

Another dumb question - but would this problem effect the mileage count of the odometer?

That is not a dumb question and yes it would affect the mileage count. That friend of mine that I mentioned earlier who had one that was reading very fast kept telling us what great gas mileage he was getting with his built 429 and two fours. I used to race with him and one day was following him on the freeway and he never got over about 55 MPH. when we got home I asked him why he was driving so slow and he said he was doing 70 to 75. We took the car to a place where the distance was measured for several miles and his read almost 2/3rds more than what it was. I don't remember exactly but say it was 20 miles, his showed around 33 miles,hence the great gas mileage.

Ron

J Persons · #17 (**permalink**) 01-08-2009, 07:20 AM

To compute what speedometer gears you need -

Driven gear tooth count = Drive gear tooth count X Axle ratio X Tire revs per mile, divided by 1000

All T5 and Tremac trans have a replaceable drive gear. But it's a PITA to get to, the tailshaft housing requires removal to get to the gear. The good news is: Aftermarket T5 and Tremacs use a 7 tooth drive and don't normally require replacement. Most '90 through '95 OEM T5's use a 8 tooth gear, and it should be replaced with a 7 tooth drive gear.

There is a chart in most all Ford motorsport catalogs that list all the available drive and driven gears and what rear ratios they apply to.

Don't be tempted to use a 23 tooth driven gear to get the correct reading. These are not Ford spec gears, the pitch is wrong and sooner or later they will fail.

HI Cobra · #18 (**permalink**) 01-08-2009, 08:25 AM

When I got my CR with the T-5Z going it read high. I down sized twice before
I nailed it and in my case it was just a matter of disconnecting the speedo
cable on the side of the trans and swapping the gears, took longer to jack
the car up than to change it.

Juggernaut · #19 (**permalink**) 01-08-2009, 09:27 AM

Quote:

Originally Posted by J Persons

To compute what speedometer gears you need -

Driven gear tooth count = Drive gear tooth count X Axle ratio X Tire revs per mile, divided by 1000

All T5 and Tremac trans have a replaceable drive gear. But it's a PITA to get to, the tailshaft housing requires removal to get to the gear. The good news is: Aftermarket T5 and Tremacs use a 7 tooth drive and don't normally require replacement. Most '90 through '95 OEM T5's use a 8 tooth gear, and it should be replaced with a 7 tooth drive gear.

There is a chart in most all Ford motorsport catalogs that list all the available drive and driven gears and what rear ratios they apply to.

Don't be tempted to use a 23 tooth driven gear to get the correct reading. These are not Ford spec gears, the pitch is wrong and sooner or later they will fail.

Thanks guys, I ran the numbers, looks I need a 17 tooth gear, right now there's a 16 tooth installed.

Just so there's no confusion, I'm not the original poster for this thread, my speedo is reading too fast, not too slow.