I have a 98 f150 4x4 with the e40d transmission. I changed my tire size from a 33" tire to a 30.5". The calculators I can find online tell me I need a 16 tooth driven gear on the speed sensor to have the corrections speed on the speedometer. I got that on and its way off. At 25mph my speedometer reads 30mph and at 65mph my speedometer reads 80mph.

Any help fixing this would be appreciated.

 

Its highly unlikely your truck ever had any modification for 33" tires in first place.

What keep you from reversing what you did? What size was on door sticker? What were you trying to accomplish?

My speedo is off by 2 mph or so , I actually go by GPS not the speedometer when driving. Tire size can change by about 3/4 " between a new tire and a worn tire. Its never 100% accurate.

 

1. I plan on towing more with my truck. Larger tires reduce tow rating.
2. I adjusted the gear once because the original person that put the larger tires on the truck did not do that.
3. Original tire size for the truck is 235 70r16.
I currently have 265 60r18
4. I forgot to add that i have a 3.55 gear ratio

 

.no msg

 

I guess my point is you clearly did something wrong why don't you just reverse it as a starting point....

The difference between a 29-in tire and a 30.5 is 5% on speedometer....3 mph at 65 mph. It's really not worth fooling with.

 

Yourf 1998 does NOT have a speed sensor driven gear that you can change
Your speed sensor is in the rear axle
You change your speedo and odo using the PSOM
The tire size is the difference, and you tell the PSOM that info
You have to use a FORD NGS scan tool or ForScan to access the PSOM