Speed question.

[henry]

My car makes 90mph (stock speedometer) in 3rd. gear at 6000rpm. is it possible to calculate the top speed in 5th, gear with this numbers?

[Daniel]

Yes !


Put it in 5th Gear and floor the gas pedal you should be able to caculate from there hehe




Daniel

[mbouse]

Daniel, I BELIEVE that henry has the federated 85 mph speedo and therefore cannot calculate his speed above the federally mandated and obtrusively BIG-BROTHERLY arbitrary and silly limit. our government attempted fuel conservation by limiting the recorded maximum speeds we were seeing on our dashboards back in the late 70's-mid 80's (as if that helped).

henry, i believe that your speedo reads a max of 85, not 90. that was the law here in the states when your car was shipped in. if it helps, when i am in fifth gear i am pulling roughly 4-4100 rpms at 80 mph. What have you seen as a maximum rpm while in fifth gear?

and, you realize, i hope, that you can swap that speedo for a ridiculously and overly optimistic 140 mph speedo..

[henry]

maximum rpm while in fifth gear 4-5000rpm.

55 MPH Speed limit on Highway!! NO GAS!! on weekwends..... Remember these. 1975, I exchange an Akai reel to reel tape recorder for my first Spider.....

[mbouse]

ack !! yes i remember those... how about alternate days of week for fuel purchases.. odd verses even?

sorry, i cannot recall the last time i did 5,000 rpm. i can check this weekend, but maybe someone else already knows.

i'd still consider replacing that speedometer if i were you. then, you'd be sure what you're doing.

[So Cal Mark]

it's quite easy to calculate once you have the tire diameter, rpm and rear gear ratio

[kmead]

There is the calculated speed your car could go and then there is the maximum speed your car can go with the existing mechanicals.

As I recall the Spider in US trim will max out well below 110mph (108 as I recall from old magazine tests). They don't have a lot of HP and the aerodynamics of the car are not exactly smooth, particularly with the top down.

[mbouse]

Karl, if you have been following Henry's posts.... he's been on a year long quest to increase the output of his Spider. those stock answers from the factory brochures aren't gonna fit his application.

but, i get what you are saying about the ceiling... i guess i won't be taxing the upper end of my 140mph speedo any time soon, 'eh?

[So Cal Mark]

with the top down, use loads of "slickum" in your hair to improve aerodynamics. That will help top speed

[narfire]

Lucky you... having hair to put goop into.

[kmead]

So the way to know how fast you ARE going (within the limits of accuracy of the factory tachometer) is of course to find a nice open, flat road without interfering OPP or RCMP friends and floor it. After about 5 minutes you should be at terminal velocity in 5th gear (given that it is slightly overdriven, you may get a higher speed in 4th depending on your engine and where it reaches peak).

Record your max RPM you reached in whichever gear and then use a calculator such as:
http://precisionsite.com/auto-x/tools/
or
http://www.kabamus.com/garage/gears.html

Let us know how it went and if the RCMP took your car or not for exceeding 150kph...

[henry]

Mark, How can I get the rear gear ratio? Anyway I will change the speedo although is gonna be hard to find one with the top speed of 240MPH.

[henry]

Without any calculation, I can see the speedo shows 85 - 90MPH on 3rd. at 6000RPM. I''ll make the calculations later and let you know.

[Daniel]

will max out well below 110mph (108 as I recall from old magazine tests).

you mean to tell me i only had 8 mph to go that time i took my 79 up to 100 mph my speedo says 140

[mbouse]

will max out well below 110mph (108 as I recall from old magazine tests).

you mean to tell me i only had 8 mph to go that time i took my 79 up to 100 mph my speedo says 140

unless you have something special under the hood, that is exactly what we mean.