MR2 Owners Club Forum banner

1 - 20 of 30 Posts

·
Registered
Joined
·
1,197 Posts
Discussion Starter · #1 ·
I've noticed that sometimes I read 3500rpm while cruising at 60mph on a completely flat freeway, and other times it's 3000rpm. Why the difference? Is it because of cold conditions, or what? This is all done using cruise control to set speed and a boost gauge to determine when the engine is under load or not (thus increasing necessary rpms to maintain speed).

I might have screwed up the position of my tach needle when I did my indiglo gauges, because my idle is always around 1000rpm. How can I tell?
 

·
Registered
Joined
·
257 Posts
i dont think the rpm at 60 should ever change... not unless you changes to alot larger/smaller tires or your rear end gear. i always thought rpm at 60 was rpm at 60. its just gearing. I however I dont have a turbo so i dont know what the your rom should be at.
 

·
Registered
Joined
·
456 Posts
Tires and rims dont change any of this. You could run lawn mower rims and your speedo and tach would show the same thing monster truck tires would show. Gauges are not tire/wheel aware. Also, your RPM @ 60 should NEVER EVER change unless you changed gearing inside your transmission, which you are not doing. Therefore, your tach, and or speedo is messed up. Ive seen others complain about this after doing gauge faces, as you mentioned you have done.

A slipping clutch can affect the RPM to Speed relationship, but Im pretty sure thats not your problem.

Mark Sink
 

·
Registered
Joined
·
456 Posts
93smgturbo said:
Really? I am usually at about 3900rpm's at 78mph. I have 235/40/17's on the rear too.
...wheels and tires dont affect tach/speedo relationships. They affect actual speeds, but your tach and speedo behave the same regardless of tire size, unfortunately.
 

·
Registered
Joined
·
435 Posts
3000 rpms in 5th gear I am at a smidgen over 60. Sometiems if it is cold outside, my tach has been known to read rediculously low unless it has been being driven for a while. I.E. i will be driving at 3000 rpms in 5th gear and it reads something like 40 mph. It only does it if its somewhat cold, an after it warms up, it will fix itself automatically once i come to a complete stop. Dunno why.. just does.
 

·
Registered
Joined
·
1,289 Posts
Yeah, automatic transmission was never an option for the MR2 Turbo.

All 5 speed, baby!
 
G

·
err i think u will find they did infact make many auto mr2 turbos maybe u just dont have many in the states. here in the uk theres a few about but not as quick due to short gear changes. it is a sports box with 2 modes though!
 

·
Registered
Joined
·
5,392 Posts
In the states we never got any automatic turbos. There were some auto NA's and I have never heard of an auto turbo ANYWHERE. Are you sure they are turbo models?
 

·
Registered
Joined
·
2,993 Posts
Thats strange, ive never seen a single mr2 turbo with a auto and im in the uk, it didnt come as an option at all.

If it did ill stand very corrected, but i know alot of people in mr2s here, and someone whos breaks alot of them and have never came across a auto/3sgte combo

Graeme
 

·
Registered
Joined
·
1,983 Posts
My tach reads exactly 2000 rpms at 45 mph, 60 mph is ~2,600 rpms. I verify it all the time since my speedo is broken so my tach does double duty as a speedometer. Right now I'm on snow tires that are slightly larger than the stock tires, so with smaller tires the rpms would be slightly higher (this is measuring speed with a stopwatch and mile markers, not the speedo).
 

·
Registered
Joined
·
2,667 Posts
I dont think you can mess up the position of your tach. however i have heard that if you dont install the indiglos right (flush and flat) then the needles will stick. what year do you have? 91 and 92 have mechanical speedo cables so tires/wheel sizes can matter. broken speedos are common and something stripped or missing teeth will cause a lower speed reading. If you have a weird size combo then the circumference could be smaller or larger than the mechanical speedo was calibrated for. For 93+ doesnt matter cause they have electronic speedos. i have 16's and they required 45 series to come close to factory wheel+tire diameter so i would have the same circumference, i.e. the number of rotations would match the total distance traveled for the tire. hope that makes sense! use your geometry! something with twice the diameter will have a much larger circumference and thus when it has only turned half way the original tire will have made one full revolution. The speedo gear is on the driveaxle in the tranny, so i am pretty sure that it makes a difference in what tire/wheel size u have if it is not close to factory. however, you would have to have really big tires and wheels to make u go 60 at 3500rpm, usually the loss is negligible cause our cars are limited in how big our wheel/tire combos can be, unlike monster truck tires
 

·
Registered
Joined
·
1,983 Posts
91 and 92 have mechanical speedo cables so tires/wheel sizes can matter.
The speedometer is measured at the output shaft of the transmission, larger wheels and tires will affect your actual speed at a given rpm, but not your speedometer reading relative to your tachometer. That is, when the tachometer is reading 3000 rpms at the input shaft (or wherever it makes its reading), if the speedo is reading x mph at the output shaft, changing the size of your wheel/tire combo will not affect the relationship between the input shaft rotation speed and the output shaft rotation speed - that is affected solely by which gear you are in.
 
1 - 20 of 30 Posts
Top