just a few questions that have been bothering me...

[sr975j]

alright well here we go,
first off, i plan on changing my plugs, wires, and fuel filter tomorrow. is this necessary already at 68k miles? if so, in your opinions, which are the best bang for the buck?
second, sometimes when i am cruising, doesnt matter at what speed, ill push the clutch in to put it in neutral and just roll to a stop sign, then let it back out but the rpm's will stay up for 2 seconds or so then drop back down to idle. any ideas what could be causing this?
also, i went from the factory size tires to a 31 inch tire. does anyone know about how far off my speedometer is?

sorry for the hefty post, any help is appreciated

Sam

 

[Big Jim M]

I don't mess with the firing system till it misses.. Usually that is beyond 150,000 miles. At 68K I wouldn't even think about replacing the plugs.

Putting 31" tires on there makes the engine think it is climbing a hill ALL the time. Your milage has worsened and your power has also.
Changing the plugs won't help that any at all.
Big JIm

 

[sr975j]

i dont mean to be a dick but thats the way this is gonna come across ...sorry. you basically just told me what i already know. of course bigger tires are going to make the engine work harder, but what is it going to do to my speedometer? i just want to change the plugs wires and fuel filter as simple maintenance. im not trying to offset the loss of power from the tires.

Sam

 

[Chance]

It's a three litre... You don't ever need to change anything on it
But like said, you don't need to change ignition unless something is tweaked or problematic...

as for speedo off... well on 32's my truck is dead on according to my gps... on 30's which was stock-ish it was under by 3-5 mph...

So... how far off is your truck when you pass a radar box? That'll give ya an answer... I dunno if I trust my gps speed but it lines up with radar.

 

[sr975j]

hmmm...ive never thought to run it by one of those radar things. i always seem to be accelerating past them because im young and stupid.
does anyone have any input on the whole rpm issue? to add on, when the rpm's finally do go down, they stop at around 1000-1100, dont the 3.0s idle around 750? could this be an issue with my TPS?

Sam

 

[Chance]

Honestly... I don't know... mine does the same thing... I just ignore it... I figure I redline the thing enough that 1100 aint bad for it...

I'm assuming someone else has some insight but I don't think it's an unusual thing.

 

[oachs83]

First off preventive maintenence is never a bad idea. Changing your plugs/wires is a good idea. Why wait for something to happen when you can prevent it from happening? You need a good quality wire and plug set, look in the tech forum for recomendations. Your idle issue could be TPS related, it is normal for a engine to "step down" on RPM's but the idle does seem a little high. As far as your tires google it. There are a few calculators to factor your answer. You will need to know original tire size and gear ratio.

 

[triumphrider-1]

The idle could be your IAC.

 

[TomB1269]

Sticks use the IAC to control idle when clutch is in............

 

[macarchie]

Here's my two cents worth...

speedo: measure the perimeter of the tires and compare to the perimeter of the original size. The speedo measures the rotation rate of the tires and multiplies that by the distance covered with each tire rotation to get the distance. If the perimeter (for example) is twice that of the original, then the speed indicated would be half because the tire would only turn half as many times to cover the same ground. If the perimeter is 10% more then the speedo will be lower by about 10%. I have 225R14 on my '91 (which originally came with 215R14. The perimeter difference is 9% and the speedo reads about 9% lower than actual speed. When it indicates 55 mph I am actually traveling about 60.

rpms: an easy and cheap possible solution may be to disconnect the battery for a couple of hours and then reconnect and run it for 10-15 minutes to allow the computer to reset the environmental parameters.

plugs and wires: I can always tell when mine need to be changed because it starts to cutout under a load (like accelerating up a hill). If it is running smooth then I would change them out. However, if you are planning a long trip and they have been in there a long time - it might save you some headaches and money to replace before you leave instead of on the road.
hope this helps...

 

[sr975j]

not familiar with the term IAC? im gonna feel stupid when someone tells me what this is but i cant think of what that means

 

[oachs83]

IAC is your "idle air control". I am sure macarchie's answer to speedo change is good and accurate but if you want to save time and any type of work go here http://www.miata.net/garage/tirecalcold.html . Type in your sizes and your done.

 

[oldfordman]

Here's my experience with larger tires on my 99 ranger. 3 liter, auto trans, 4.1 gears 4WD. But that doesn't really matter, it's a matter of ratios between the two tires. I went from the stock 215-15 tires to 235-15 as soon as I got the truck, so I don't have a baseline. However, even with going up 2 sizes, my speedo only reads 1-2 mph low at 60, and the odometer is right on. Checked this with the GPS, measured mile, mile markers on long stretches on the interstate. 30+ vehicles experience tell me that the manufacturers set the speedos and odometers to read about 5% high. Not a big deal, keeps some people from getting a ticket, but I believe the main reason is that it's a free way for the manufacturers to get you out of warranty a bit faster. 5% on a 60K warranty is 3 thousand miles, so the warranty runs out when you actually have only 57. Or, figured another way, for every 20 vehicles they sell, it's the same as them not covering one vehicle. After a few million cars, it adds up to serious money. Also makes you think you are getting better gas mileage than you really are. On all my new vehicles, I clock the speedo and odometer, then document it with the dealer. I don't expect them to fix it, just want it documented in case something happens when I'm just over the "magic" mileage. Most service managers give me a dirty look about it, so I figure either I'm correct, or it's just me being anal (a real possibility).