4.10 Gears vs. 20 in rim speedometer adjustment

05saleen53 · 03-27-2008, 08:03 PM

I was reading a post a while back about people having about a 7-8% difference in their speedometer to actual speed, and this was being blamed on the rim size.However that should be calibrated from the factory. I bought my car used and when I test drove it, there were some issues so the dealer worked on it for a couple of days and then when I went to drive it again I asked to see the work order of what they didand they showed them to me and it said they shipped the ECU to saleen to get programed for a4.10 gear to adjust the speedometer. Does anyone know how I can tell if this car really has 4.10s or if the people at the dealer just assumed this and the program has been off the whole time Ive had the car? Everything seems fine but that old post makes me want to second guess the clowns at the forddealer because they really didnt seem to know what was going on half the time.

Jazzer The Cat · 03-27-2008, 08:11 PM

You can jack the rear wheels up and put trans in neutraland rotate the tires until the driveshaft spins exactly one revolution. The tires should spin 4.1revolutions aroundto equal 1 rotation of driveshaft.

Jazzer

05saleen53 · 03-27-2008, 08:19 PM

Thanks Ill try it when it gets a little warmer around here.

tim281 · 03-27-2008, 11:10 PM

Drive the car on the highway at 60 mph...if you have factory 3.55 gears, then your RPM will be around 1750.
if you have 4.10 gears, then your RPM will be around 2000.

desant89 · 03-28-2008, 07:52 AM

Easiest for me to tell if the speedo is accurate is to borrow a portable nav unit if you do not have one. It will tell you how fast you are goin. Compare that to the speedo.

As for the gears, I have done the driveshaft method that jazzer mentioned. Just mark the shaft, and tirewith chaulk(tape) or something so you can tell a full revolution. I actuallythink the shaft should spin 4.1 times per one wheel revolution though not the other way around. Icould bewrong since I have not done this for about 10 years, but I know whatever way it is, it works. I am sure someone will chime in, but I do think it is shaft spins per one full tire rotation. Anyone?

Jazzer The Cat · 03-28-2008, 08:01 AM

Quote:

ORIGINAL: desant89
"...I actuallythink the shaft should spin 4.1 times per one wheel revolution though not the other way around...."

Oops! your right, I had it backwards

Jazzer