Toyota Celica Supra Forum banner

1 - 20 of 23 Posts

·
Registered
Joined
·
73 Posts
Discussion Starter #1 (Edited)
At 3,000 rpms, my spedometer reads 65 mph, when it's more like 75 mph. My question is, if I go on a trip and it says I only went 35 miles when I actually went 40 miles, doesn't that mean I'm actually getting better gas mileage than I calculate? If it records less miles, it'll say I'm getting less mpg.

PS: Are all MKII spedometers off at high speeds? If so, how much roughly?
________
New mexico marijuana dispensaries
 

·
Super Moderator
Joined
·
10,577 Posts
you ever change your rear end gears or do you run a different sized tire then stock (other then 225/60-14)? If not your speedo shouldn't be out unless something is out of wack.

And you have it backwards, if your actual speed is higher then what your speedo says, then your traveling fewer miles then what your trip meter reports and your actually getting worse milage then it appears. The worst thing about this scenario is that the odometer will report the car aging faster then it actually is.
 

·
Registered
Joined
·
1,713 Posts
i have the same problem. nothing has been done to my car except a 6mge swap and i see on the spedo 10% less than what i'm actualy going. if the spedo is registering lower than actual speed you are getting better than calculated fuel milage. i calculate about 18 mpg my actual is closer to 20.
 

·
Registered
Joined
·
634 Posts
No he would be getting better mileage.

If he is traveling faster than his speedo says.

Then in a given hour he would go say 75 miles compared to the speedo reporting only 65 miles traveled.

Secondly you might want to make sure you are changing your oil that much sooner also....
 

·
Registered
Joined
·
2,800 Posts
easiest way to tell is go thru speed trap at 90 mph with your
eye on speedo so officer can give u an exact measurement.
take 20 mile trip (or more) on hiway and cal your odometer
to markers. Run car on cruise over a fairly flat hiway and
time with stopwatch (convenient to use your recently calibrated
odometer here) and calc speedo correction and note tach rpm.
now u can drive by tach if neccessary which's a lot better than
my first suggestion. also, speedo tends to age and read higher
while tach is very stable except it's very sensitive (reads lower
as i remember) when reglatir starts to go.

StanS
 

·
Registered
Joined
·
4,871 Posts
Check your odometer for accuracy from point A to point B using highway mileage markers like Stan recommended and see if your odometer is accurate. The speedo itself may just be out of calibration due in part to a weak hairspring from years of use while the odometer may actually still be accurate.
 

·
Registered
Joined
·
73 Posts
Discussion Starter #8 (Edited)
Dave A. said:
Check your odometer for accuracy from point A to point B using highway mileage markers like Stan recommended and see if your odometer is accurate. The speedo itself may just be out of calibration due in part to a weak hairspring from years of use while the odometer may actually still be accurate.
I go to LA tomorrow morning, I'll do that and let you guys know what's up.

Also, my needle wobbles at low speeds, that could be from the weak spring.
________
Built Ford Tough Series
 

·
Super Moderator
Joined
·
10,577 Posts
ok, I have it backwards. I was thinking it was the same as my situation as I have smaller tires then stock. But my car travels slower then the speedo says. Its great for avoiding tickets, kinda like setting your watch ahead to keep you on time :D
 

·
Registered
Joined
·
347 Posts
HMM? at 3,000 RPM I'm clocking at around 80 or 85-which I typically do on highways. Does that sound about right?
It's an 85 ptype with 6mge if that matters. Minor mods for engine.
 

·
Registered
Joined
·
634 Posts
I have all my clocks set ahead and I am still late???

Maybe its because I dont drive my supra?

Anyway I reread my post in response to SupraFiend and feel that I may need to make an apology for it being short and not so firendly. Being short on time, and at work I really dont reread my posts for a few days if ever....

Anyway back to work.


And mods to the engine shouldnt have anything to do with the speedo being accurate or not I dont believe. They just effect how quick you get to 60 mph.
 

·
Registered
Joined
·
73 Posts
Discussion Starter #12 (Edited)
Ok, so I clocked my mileage but I can't get mapquest to give me an exact number for the amount of miles I went. The trip was right about 50 miles and my car clocked 34 miles. It's all freeway driving which is why it's so off, I think my speedo is only off at high speeds. At 3,000 rpms It says I'm right under 65 mph but I know that can't be correct because 65 mph is slow as hell and I always pass traffic at that speed. Anyone know what stock MKIIs do at 3,000 rpms?
________
Honda Super Cub
 

·
Registered
Joined
·
9,445 Posts
teng,

80 - 85mph @ 3K rpms???? Something is not right.

Same questions for you on your 85. 5-speed or Automatic??? 4.30 rear or a 4.10 rear??? Or has it been swapped out for something different?? Any change in your tire size, or are you still on 225/60/14's???

On my 85/5-speed/4.30 rear, at 3K rpms, I'm at 65 mph. This is on the stock 225/60/14 size. Since I went to great pains to insure that I still have the same speedo readings when I did a wheel/tire upgrade, my 16" Compomotive's w/ 245/45/16 Yokohama's ALSO says 65mph @ 3K rpms.

Let us know what you have.
 

·
Registered
Joined
·
73 Posts
Discussion Starter #14 (Edited)
I have an 85 5 Spd. I highly doubt that any mods were done to the car so assume completely stock as it was used as a daily driver its whole life. I'm on 225/60/14s but I don't think there's any way that my speedo could be right.

My trip to LA is 42 miles on the freeway according to mapquest, but it doesn't add the 5 miles that it takes to get off the freeway to my destination so it's right about 47 miles and I clocked it at 34. Now, my speedo isn't always this much off but at highway speeds I pass people when my speedo says 65 mph. There's no way that I could pass people on LA freeways in the fast lane at 65 mph. I've lived here my whole life and I drive many other cars (99 sentra, 94 legend, 92 legend, 96 tahoe and 00 grand cherokee) and I can tell that my speed is higher then it says, but it only seems to be off at highway speeds.

Is there any way to calculate exactly how fast I should be going at a given rpms? I know that at 3,000 rpms, my speedo says about 63 mph on flat ground. If all 85s had 4.3 rears then I'm pretty sure mine is 4.3 but I don't know how to tell.
________
XXX
 

·
Registered
Joined
·
9,445 Posts
Abomination,

Going by what you say, in that you still have the original 4.30 rear & are still on the 225/60/14 tires, use the calculations that I use. Starting at 55mph in 5th gear, you should be doing 2600 rpms. 60, you should be doing 2800, 65: 3K, 70, 3200, etc etc.... Every 5 mph, you should go up 200 rpms.

This is how mine works out. Still on the 5-speed/4.30 rear/225/60/14's or the 245/45/16's. Same reading every time.
 

·
Registered
Joined
·
4,871 Posts
Cruising at 65MPH at 3000RPM puts you right near the torque peak of a stock 5MGE. Passing other cars shouldn't be too difficult. Bring it on up to 70MPH and passing will be a breeze. 8)
 

·
Registered
Joined
·
73 Posts
Discussion Starter #17 (Edited)
Cool, thanks CJ.

Something's definitely wrong with my speedo then. But this isn't the only electrical problem I have with my car. I seem to have a bunch. The biggest problem is that I can turn my car on without the key in it :?: my dome light doesn't come on when I open my car door, my door locks have a mind of their own and my clock never turns off, which could be normal I guess, but seems a little weird.
________
James j. couzens
 

·
Registered
Joined
·
73 Posts
Discussion Starter #18 (Edited)
Dave A. said:
Cruising at 65MPH at 3000RPM puts you right near the torque peak of a stock 5MGE. Passing other cars shouldn't be too difficult. Bring it on up to 70MPH and passing will be a breeze. 8)
Passing people is no problem, my car is %$#ing fast but there's no way that my speedo is right. OMG! :!: I just remembered that my differential was replaced before I bought it.

Okay, I searched through my records, but it doesn't say if it was a differential for an 85 or for some other year. It must be from some other year. What would make my car go about 75 when at 3K rpms?
________
Lsv
 

·
Super Moderator
Joined
·
10,577 Posts
its probably a 3.72. The only way to tell for sure is to jack the rear end up and figure out the ratio by making some marks and rotating the drive shaft and counting how many times the axels rotate. Find out for sure what gear it is (could be a 3.9, 3.72, or 4.1) then get the plastic speedo gear to match from a dealer and install it in your tailshaft and everything should be back to normal.
 

·
Registered
Joined
·
73 Posts
Discussion Starter #20 (Edited)
SupraFiend said:
its probably a 3.72. The only way to tell for sure is to jack the rear end up and figure out the ratio by making some marks and rotating the drive shaft and counting how many times the axels rotate. Find out for sure what gear it is (could be a 3.9, 3.72, or 4.1) then get the plastic speedo gear to match from a dealer and install it in your tailshaft and everything should be back to normal.
Cool, thanks for the help everyone, I hope it's just this and nothing really complicated electrically.
________
Honda lock s.c. specifications
 
1 - 20 of 23 Posts
Top