4:10 to 3:55

[dlefebvre]

My 91 E350 dually currently has a LS differential with a 4:10 ratio. Purchased a set of 3:55 for it. The heaviest thing I pull with it is a 6000# fifth wheel(for now anyway) The lower rpm's at cruising speed is what I'm after, not to mention the better fuel economy that comes with it. I was wondering what changes, if any, have to be done to the speedo. Speedo gears in the transmission, or maybe gear on the speed sensor? Any info would be great. Thanks guys.

 

[Hutchew]

Stealer or a speedo shop.........

Since you went with an OEM ratio, you can get the correct adaptor (speed sensor) at the dealer for @ $65. 00. It will be correct as long as you are running stock tire size. If you are not, or prefer to not mess with the dealer, you can go to a speedo shop and they will build you a new gearbox. Last time I did that was on a '90 Jeep Cherokee with 33" tires and 4. 56 gears about 7 years ago. Cost @ $125. 00, but it was dead on accurate.

 

[paccool]

If the 91 is set up the same as my 93, you don't have to change the speed sensor, just the gear inside it. Cost me about $12 at the local speedometer shop. I did the exact thing you are going to do. The truck pulls my 6K travel trailer better with the 3. 55 setup better than the 4. 1 ratios due to a better RPM range. You will be happy with it.