A question about speed reading at 5th gear

[gorgeousgeorge]

Hi.

I am a new owner of a ZR-1(1991) and am enjoying it every weekend. I am very happy now:)

BTW, please let me ask you a beginner question. I would like to know whether the rear end of my ZR-1 is normal or not.

When I accelerate my ZR1 to 7000 rpm with 5th gear, the GPS speedometer reads 223.9 km/h(about 140 mph).

Is this normal?
Is the max speed of ZR-1 accomplished by 6th gear?

I am wondering if the rear end ratio of my ZR-1 is normal or not.

*the rear tires are Nitto NT555(315/35-17).
*the original speedometer reads about 219 km/h(136 mph) at 7000 rpm with 5th gear.

Thanks in advance.

Y. Honda
1991 ZR-1

[Aurora40]

7,000 in 5th gear would be about 200mph (320kph) with stock gearing. That would suggest you have around a 5:1 rear gear ratio. Some Jeeps use the D44, so there are probably some pretty extreme gears available out there.

It sounds like your speedometer has been corrected properly, though.

[bdw18_123]

It's possible his tach isn't reading correctly also.

[rhipsher]

Wow! That would be some serious rear gears. Mine with stock 3.45 hits 181mph at 7100 rpm,s.

[gorgeousgeorge]

Thank you for your interesting replies!:-D

I understand that 140 mph at 7000rpm in 5th gear is NOT normal for ZR-1. The problem would be that the rear end ratio is not normal or the tachometer is not correct. I think I have to inspect the rear end ratio at first.

Does the method below work well to inspect the ratio?
1. lift the car up.
2. mark a dot with chalk on circumference of the propeller shaft.
3. mark a dot with chalk on the rear tire wall.
4. rotate rear tire and drive the propeller shaft.
5. count the number of rotation of the tire required to have the propeller shaft make one complete rotation.

I am not a gearheaded guy:( and do not want to make any mistakes so I ask for the right procedure.

Thank you!

Yoshi Honda
1991 ZR-1

[tomtom72]

Hi! Yes that will work to get you a very close approximation of the gear ratio,

:cheers:
Tom

[cward]

Wow! That would be some serious rear gears. Mine with stock 3.45 hits 181mph at 7100 rpm,s.


I believe your tach is off too. You should be over 200 @ 7100 with 3.45 in 5th gear. At least mine was.

Curtis

[VetteMed]

My bet is inaccurate tach :)

[rhipsher]

I believe your tach is off too. You should be over 200 @ 7100 with 3.45 in 5th gear. At least mine was.

CurtisMy tach is off now. Corey even said it was off in the high direction 1000rpm's. Now it reads 8000 at 181mph which to me says 7100 is about right for that speed.

[HAWAIIZR-1]

Hi.

I am a new owner of a ZR-1(1991) and am enjoying it every weekend. I am very happy now:)

BTW, please let me ask you a beginner question. I would like to know whether the rear end of my ZR-1 is normal or not.

When I accelerate my ZR1 to 7000 rpm with 5th gear, the GPS speedometer reads 223.9 km/h(about 140 mph).

Is this normal?
Is the max speed of ZR-1 accomplished by 6th gear?

I am wondering if the rear end ratio of my ZR-1 is normal or not.

*the rear tires are Nitto NT555(315/35-17).
*the original speedometer reads about 219 km/h(136 mph) at 7000 rpm with 5th gear.

Thanks in advance.

Y. Honda
1991 ZR-1


Too bad you're so far way from Yokohama. I have a business trip in a few weeks so I'll be there for about 10 days. I sent you a PM on the other forum and will be meeting up with ZR-1 owners in Japan.

Craig

[gorgeousgeorge]

Hi.
I would like to thank everyone who responded to this thread. I inspected the ratio of my rear end today and found the propeller shaft rotate 3 and about a half turn while rear tire making complete 1 turn. I believe the final ratio is 3.45, i.e. stock.

http://www.zr1.net/forum/picture.php?albumid=104&pictureid=574

I did the inspection at my friend mechanic?s shop. My friend mechanic was under my ZR-1 and was counting the number of rotation.

I concluded that the tachometer is inaccurate.

DOES THIS OFTEN HAPPEN TO C4s?

How could I do to fix this problem?



http://www.zr1.net/forum/picture.php?albumid=104&pictureid=577

When the GPS meter read 223.9km/h(=140MPH), my inaccurate tachometer read 7000 rpm. The true rpm will be around 5200 to 5400 rpm, if the true speed of 7000 rpm in 5th gear is around 180MPH to 200MPH.

I am wondering what to do next:confused::confused:

http://www.zr1.net/forum/picture.php?albumid=104&pictureid=575
By the way, I talked with my friend mechanic about how we should improve our Corvettes till 10 oclock at night:-D

Yoshi Honda
1991 ZR-1

[Aurora40]

DOES THIS OFTEN HAPPEN TO C4s?
It's not uncommon, though yours is off by a huge amount. You can either buy a new instrument panel that hopefully is more accurate, or you will have to have yours fixed/repaired.

I don't know who repairs them, but people have had it done.

In the meantime, you can run the revs up a lot higher and really get to enjoy some of that acceleration the LT5 provides. The higher revs are where it's the most fun.

[cward]

My tach is off now. Corey even said it was off in the high direction 1000rpm's. Now it reads 8000 at 181mph which to me says 7100 is about right for that speed.


Using a 25.7" diameter tire, @ 7100 rpm with 3.45 gears and in 5th gear you should be at about 209 mph. It is a little easier to figure using 4th gear as it is 1:1 rather than 1:.75. Nevertheless, 180 should be about 6100 rpm in 5th with 3.45 gears and 25.7" diameter tire. Of course, that is assuming little or no strech on the tire or loss of traction. With 4.10 gears 180 mph is about 7400 rpm and that is about what my tach showed in the 2001 BBORR. I ran the race with 4.10 gears. It would run 180 in 5th and 181 in 6th. That was all it would do. I went across the finish line at about 7400 in 5th gear and the pedal to the metal (or fiberglass) trying to put my foot up to the radiator. Speedo said 180. At that time the speedo and tach were quite accurate according to testing on dyno. In the 2001 race I had about 372 rwhp. In the 2002 race, I had about 392 rwhp and 3.45 gears. Speedo was not as accurate after gear change even though we put in what we thought was the correct trans speedo gear for the 3.45. It read a little low. As well as I remember, the tach was bouncing some at around 7000 with the speedo at 201. Radar also got me at 201 but after 2 or 3 miles of 201 I let off before the end of the 7 mile straight. Don't know where the radar was located. I came out of the corner into the straight running about 160 or so and it took 2 or 3 miles to get to 201.

All the above is just for info.

Curtis

[gorgeousgeorge]

In the meantime, you can run the revs up a lot higher and really get to enjoy some of that acceleration the LT5 provides. The higher revs are where it's the most fun.

Bob,
Now I understand what you mean. I ran my LT5 up to over 8000 rpm (which is to be around 6300 rpm or a little higher if the tachometer is accurate) yesterday. LT5 really roared like a beast! I have been missing the last 1000 rpm which makes ZR-1 most fun due to the inaccurate tachometer. I have to look for an accurate tachometer..

Yoshi Honda
1991 ZR-1

[mgbrv8]

If this is still a problem this might help

http://www.corvetteforum.com/forums/c4-general-discussion/2662597-high-reading-tach-repair.html

David Hetrick