Formula for working out the "potential" top speed of your car

[Glenn Butcher]

          

Calculating your top speed in MPH / and RPM
To  work out the MPH you will be doing at XXX RPM use this table:
Formula #1: mph= (rpm * circumference) / (gear * final * 88)
RPM	MPH	Tyre Circumference (in feet)	Differential Ratio (Final drive)	Gearbox Ratio (Top Gear I.e. 5th)	Var. Conv. Factors (leave at 88)
163.0636335	6500	6.507874632	4.1	0.719	88
To  work out the RPM you will be doing at XXX MPH use this table:
Formula #2: rpm= (mph * gear * final * 88) / circumference
RPM	MPH	Tyre Circumference (in feet)	Differential Ratio (Final drive)	Gearbox Ratio (Top Gear I.e. 5th)	Var. Conv. Factors (leave at 88)
7972.3478	200	6.507874632	4.1	0.719	88
RX-7  Differential Ratios:
Auto	3.9090
Man	4.1000
RX-7  Gearbox Ratios:
Auto(4th)	0.6940
5th	0.7190
To  work out the circumference of your tyres:
1mm=	0.0394	Inches
	Tyre width	Profile%	Tyre height (incl. Profile)	Wheel diam. / wheel rim height	1/2 rim height+tyre height
Metric  (mm\'s)	225	50.00%	112.50	406.4008
Imperial  (Inches)	8.8583		4.4291	16.0000	12.4291
PI	PI * 2
3.1416	6.2832
Total  tyre and rim circumference in inches:	78.0945		Calculated Tyre and Rim Diameter:	24.85825
Total  tyre and rim circumference in feet:	6.5079
Example Tyre Circumference calculation: You can estimate the rolling circumference from the tyre-width, profile and rim height. For example, a 195/60R-15 tyre will have a section height of 60 percent of 195mm, (195 x .60) = 117mm, which converts to 4.6063 inches. Add 4.6063 to one-half of the 15-in. wheel diameter (4.6063 x 7.5), and you get 12.11inches. Multiply by 2pi (12.11 x (3.141593 x 2) = 76.0661 gets the circumference using inch units; then dividing by 12 gets you to the circumference in feet (76.0661 / 12) = 6.3388.

[grinder]

not being thick here - but can you actually work this thing on line

cause if i copy and paste into excel i will loose all forumula\'s !

Just a thought !

[Glenn Butcher]

Grinder!

Yes - here it is.  I didn\'t attach it because not many people have Excel, and I don\'t have the Javascript skills to build it (hint hint anyone).

Main point is that this formula applies:

mph= (rpm * circumference) / (gear * final * 88)

[grinder]

thanks !

so if i can red line my 1988 N/A in 5th i can get 174 mph

i dont think so !

i assume that the chart takes into consideration that you have enought BHP etc... to do the set RPM in the first place !

still pretty coool though !