New 3.73 gears, new tune, speedometer is incorrect

[dwright463]

I have a 2006 4.0L V6; I'm the original owner. Had a garage install new Auburn limited slip differential and new 3.73 gears. They said they could not flash the PCM. Speedo registered 50 mph and a radar sign indicated I was only going 42-43 mph. Took it to a Ford dealer and they set the PCM to 3.73 gears and the OEM wheels (215/50R16). Had no effect on the speedometer. Ordered a tuner from American Muscle, installed the tune. No luck.

The only other modifications are a K&N CAI, sequential tail lights and daylight running lights.

Any ideas on how to correct this problem? I'm kind of stumped.

 

[Postman]

What tuner did you buy from AM? When you down loaded the tune the option screen should have come up and given you the option to change the gear ratio.

 

[mfergel]

Are you sure they actually put 3:73's in? If the tuner/Ford have all taken into account that you have 3:73 gears and stock wheels, then one of two things is happening. Either you don't have 3:73's or that radar sign is fubar.

 

[Wes06]

Jack the car up. Chalk mark the tire and driveshaft, turn it and count revolutions

 

[dwright463]

I bought the bama x4-xf4. The tune had the correct 3.73 on the tune. Thanks for the suggestions. We re due for snow this Sunday so I probably won't get it up on stands until next week. Have to do my daughter's car brakes this weekend too.

 

[oldVOR]

The OEM tire size is 215/65-16 and the speedo difference is due to the ratio difference in the 50 series to 65 series

 

[dwright463]

Correct. My tires are the 215/65R16s. I have not put on the 235/50R18s yet. The Ford dealer gave me the print out showing the correct tire size and gear ratio. What are the chances that American Muscle sent me 4.10 gears by mistake and the garage didn't catch it. And I didn't bother to look at the box - just put it the car and drove it to the garage. Now I'm wishing I should have looked more carefully.

 

[TexasBlownV8]

look at the invoice from american muscle. it'll have the part number of the gears you bought.

and +1 on raising the car up in the back (safely of course) and turning the wheel one complete rotation and counting the number of turns made by the driveshaft to get the actual gear ratio that is in there.

 

[Macman45]

Do the math out from the engine RPM out to the speedo and see if it matches. So find your 1-1 gear (4th in a 3650) and set the engine at say 2,000RPM exactly. then do some unit conversions to get MPH. Cross reference that with the radar sign and see if it matches 3.73 or 4.10s

 

[JimC]

Are you sure that the tire RPM is set correctly?

 

[dwright463]

I took it back to the shop that did the original work. They knew a car computer guy. I'm not completely sure what he did but the car seemed to be set on 3.90 gears. He flashed it with his tuner and it seemed to take. One of the mechanics knew the local police and had an officer come out with his radar gun to verify it at 35 and 60 mph. Now every's fine and it is a blast to drive! If I had known it was this good, I would have done it the day after the warranty expired. I strongly recommend a new differential and 3.73 gears for the V6 cars.

 

[07TR]

Glad to hear you got it fixed