TRL12087

i just finished my 4.10 install now im wondering just how off is my speedo? i know its off quite a bit. also how about my odometer? does that change with the addition of the gears?

Mustangkiler

i doubt odometer does and speedo tends to be around 10 to 15 fro my experiences.

PRO50SC

The speedo and the odometer work of the same thing. Both will be off. As for how far, couldn't tell ya.

gspfunk

^^^Yup. Pro's right, if the speedo is off, the odometer is off as well by the same increment.

Grinster24

I think you need to change the gear to a 23 tooth for 4.10s thats what I have but the dealership said theres different color speedo gears??

lookinforastang

if the speedo bounces will that mess with the odometer.

i remember coming out of my friends house, after filling up my car earlier that night and realised my odometer read 66 miles. i go wtf? no way i drove 66 miles tonight but did i? i couldnt remember but my speedo was bouncing.

one of those mysteries

Hawk

Yes, by changing the gears in the rearend both your speedo and odometer are off. You need to change the speedometer gear in the transmission (not as bad as it sounds) to calibrate for your change. Please reference this tech article for the correct speedo gear you need to get for your application:

https://mustangforums.com/m_1108239/tm.htm

Please reference the following tech article for how to change the speedo gear (includes pictures):

https://mustangforums.com/m_1236295/tm.htm

Hope that helps.

nitrous_bob

if you had 2.73's it's off by 30%

i just did a 2.73 to 4.0 swap and i am using a 6t in the trans (drive gear) and the stock 18t driven gear from the 2.73's (yellow one)

it is still 10% error, but close enuff for now.

20t and a 6t would be perfect

90GTRagtop

As an example;
Calibrated Gear 2.73
Installed Gear 4.10

Speedo Reading Actual Speed
------------ 0 ------------ 0
----------- 10 ---------- 6.7
----------- 20 --------- 13.3
----------- 30 --------- 20.0
----------- 40 --------- 26.6
----------- 50 --------- 33.3
----------- 60 --------- 40.0
----------- 70 --------- 46.6
----------- 80 --------- 53.3
----------- 90 --------- 59.9
---------- 100 --------- 66.6
---------- 110 --------- 73.2
---------- 120 --------- 79.9
---------- 130 --------- 86.6
---------- 140 --------- 93.2


That is what it would be if your speedo is calibrated to a 2.73. Like others have said, your odometer will be off by the same amount (33.41%) so for every 100 miles showing up on the odometer, you would have actually only have traveled 66.6 miles.

g2_5point0

I have a 1990, and from what i see on the tech articles, i'd only be able to calibrate 3.55's to a correct reading. What if i installed 3.73's, will it always be off?