I have a 2000 F150 XL 4X4 SC 4.6L Automatic. I discovered my speedo was reading about 4-5 miles faster than i was actually going. The tires on this truck were P235/70/16s. That is what came from the factory. I just replaced them with LT 265/75/16s. Can i assume that my speedo will now be reading correct or closer to correct since i went to a larger tire or is there something wrong with the calibration of my speedo?

 

How did you figure your speedo was off with stock sized tires?

 

I drove thru one of those radar detector signs that the sheriffs office puts on the side of the road and it flashes " your speed is "plus i travel alot with someone following towing travel trailers and using two way radios and compared what the drivers speedo behind me says compared to what my speedo says.

 

Does your speedo always read 4-5 mph off or is that just at some particular speed? If you went from the 235 to the 265 sizes like you show in your post, you would now be higer than before by 9.3% or so.

So if you were reading 60, and it was actually doing about 55, now you would actually be doing about 60 mph.

If you were reading 70, and actually going 65, now you would be doing 71 mph.

In any event, it should read closer than before, if you were indeed off.

Did the truck come stock with the 235's? The sticker on the door should say what the OEM size was. Maybe the size was cahnged somewhere along the way?

 

It read that 4-5 mph faster than what i was actually going throughout the speedo range. Ok lets say 40 to 70 mph that is. It did come stock with the 235 s and that is what the sticker on the door jamb says. I thought i had read somewhere in here also that in 2000 during the firestone tire debacle and the shortage of tires at that time that this truck was supposed to have 255s on it but because of a tire shortage they just threw 235s on it. I dont know how true that is. But could my computer really actually have been calibrated for the 255s?

 

Should i post my original question in the transmission forum???

 

Well first off you went with a larger diameter tire. THus you will actually be travelling at a higher speed then your speedo states. It is based on the ratio of the tire sizes. Take the diameter of the tires you have now and divide it by the diameter of the tires you use to have. Now take this number and multiply it by the speed you are traveling. Remember this will not be a consistant mph difference. It will be linear based on your traveling speed. Thus when the speedo says you are doing 35mph, you may be doing 36mph but at 60mph you may be doing 65mph.

The only way to fix your speedo on a 2000 is to take it to the dealer to get your ECU reprogrammed or buy a superchip. The superchip has a function to change your speedo for different tire sizes or gear ratios.

BTW, I have this on problem on my truck. I went from the stock 265s to 35" tires. Thus my speedo and odometer are way off. I am too cheep to pay to get the computer reprogrammed or a superchip so I just live with it. A standard rule of thumb for me is to just travel 5mph below the speed I want to be going at.

kev

 

Kev or anyone,
My speedo was already reading faster than what i was actually going with the smaller 235/70/16. These were the stock tires. I sized up when i replaced the 235s to LT265/75/16. Will my speedo read closer to correct MPH by going to the bigger tire or will my calibration of the speedo still be off???? I dont understand why my speedo was off with the stock tires unless, like i said in a previous post that the truck was actually supposed to have 255s on it from the factory.

 

Um I am not sure if I can answer that because I don't know what the calibration was on the original setup. There is a calculator for your speedo based on tire sizes and rear ratios somewhere on the web but I can't find the link anymore.

kev