Correct speedo vs. performance?
 

Ah,

Here's a good question, probably even for Ken.

If I put larger tires on my truck, much larger, and my speedo says I'm going alot slower than I really am, and the computer is thinking I'm going alot slower than I really am, will this have an effect on my auto trans shift points and timing? Or is the speedo not linked to anything through the computer at all.

Tom

Yes it will. It will make it shift late and will change the fuel curve. It might not make it drive bad, but it will shift late.

No, it will shift at the same RPMs it always had, but not at the same sped you are ACTUALLY travleling. get my drift? it will however on the speedo look like it is shifting at the same RPMs

Funny thing i've noticed is that now, it doesn't ALWAYS downshift when I floor it. SOMETIMES if I'm my tranny is in 2nd gear at about 2500 rpm, and I floor it, it will just bog down and wont down shift to first, even though it has plenty of room to downshift to first. This only happens occasionally, but always when I need that downshift the most. This is why I was thinking about the speedo being off and the shift points.

Id like to get Kens input on this one.

Ken, are you there??

Tom

That is exactly what I am talking about. The speedo will read slow, changing the actual shift points as compared to actual speed which can cause it to lack power.

Could that also affect gas mileage??

The larger tires is what will actually affect gas mileage. You have alot more weight to move around. This will also keep it from wanting to downshift. I myself dont understand WHY exactly I would think that it would make it more willing to downshift but from everyhting I have read seen or done it makes it want to downshift less.

What makes it want to downshift less, simply having larger tires or the fact that the larger tires threw the speedo off. I know that larger tires adds more friction and reduces aerodynamics, which will give you poorer gas mileage, but i'm strictly referring to the speedo being off. Does the fact that the speedo is off confuse the computer enough to alter it's shifting qualities and gas mileage performance. After I get my 35's, I'm probably going to have Ken re-program my chip anyway.

Tom

So, how do you recalibrate the speedo on these? Is it as easy as doing the old PSOM units?

N Tom. the speedo does not REALIZE it is off your truck does ot realize that it has new tires on it it shold theoretically shift at the exact same RPMs as before the reason it is not is because the much larger tires also weigh ALOT more. this added load on your drive train tryin to turn 4 tires that weigh alot more. Remember teh tires themselves may only weigh say 10 punds more per tire, or 40 lbs for the set. but when turning they equal alot more probably a few hundred if not a thousand or so in centrifugal weight.


Bigric
Speedos get recalled in two ways depending on the vehicle old cars/trucks there was a mechanical gearbox that changed with the correct gears for tire size and the gearing of the rear end.

Some newer vehicles it is just simply a few numbers in the cars computer that need to be changed.

The gearbox setup there is only 2 gears. 1 gear is always the same teh other gets larger or smaller and is calculated to equal both changing dimensions into one. teh driviline is driven off the rear of the tranny I believe at the final output. as it cannot do it off the crank, but the driveshaft speed which is a constant in proportion to your speed. I.E. if you have 30 inch tires (stock) for every single rotation of your driveshaft (with the rear gears beign stock 3.55:1) you vehicle will travel just over 50 inches. regardless of your engine speed or what gear you are in with your tranny. This corolation does not exist between final speed and engine RPMs solely as the tranny gearing is also a variable in the final output gearing hope I didnt lose you sorry.

Jimmy Dean

Yeah, I know how speedos are calibrated from the gear-driven types through the PSOM units. I should have been more specific--how are the speedos calibrated on the '97-03 models? Is it as simple as grounding out a wire and dialing in a constant on the speedometer like on the PSOM units?

On most tire sites they have computer recalibration kits that connect to your tranny harness. You can adjust your speedo to any speed you want depending on how many times you change your tire size.

Jimmy, What country were you born in??? My Swampers weigh 85 pounds EACH!!!!!!

Still, I wouldn't know why this would cause my truck NOT to downshift sometimes in second gear at 2500 rpm, it has plenty plenty plenty of room to downshift to first.

Ah, maybe I'll just disconnect the battery and reset the computer.

Tom

Tom, couldn't Ken just program your chip for it?

Yes, but not yet. I'm going up to a larger tire size and then possibly 4.56 gears, so I want to wait until either i decide if i'm going to do the gears or not or at least wait until I get the 35" tires. But I wont know if I need the 4.56 gears until I drive my truck with the 35" tires on it. I don't want to keep sending my chip to ken every time I make a change. The recalibration kit is electronic and can be manually adjusted every time you make a change. And costs the same as a reprogram from Ken. So I might just take this route. I'll have to look into it further.

Skinny Nate was talking about taking out his 4.56's to install 4.10's....shoot him an email Tom.!!

Garrett