hi i dont know if anyone can help ive got a formula i use to work out speed from gear ratio, diff ratio and wheel revs.

i work it out like this 8000 rpm x 60 = 480000

wheel revolutions a minute 897, diff ratio 3.64, 5th gear .80.

so 897 x 3.64 x .80 = 2612.064

480000 / 2612.064 = 183.7 at 8k in fifth standard wheels.

but i have it on good authority that with 17s and 40mm tyres actual speed is 195 ish.

so i tried to work out wheel revolution a mile by calculating circumference of larger wheels and it works out at 165 mph

now the only thing i didnt check for myself in the above equation is the wheel revs which i was told is 897 per mile on standard wheels.

does anyone know what im doing wrong

 

most folks use this calculator...

http://www.gearboxman.co.uk/inforatios.html

 

also apparently ford state 149 mph at 6500k revs so can i do this.

149 mph divided by 6500rpm then x 8000rpm = 183.3 mph ay 8k

 

its rolling radius of the wheel that make all the differance mate.

 

i work out to 200 mph in 5th at 8k using a 0.8 final 5th and 3.64 final drive on 235 40 17 tyres.

Not 100% sure that is correct for a 2wd final drive tho

 

ford workshop manual states 3.64 diff. when you have 215 tyres front and 235 tyres rear which size do you use for calculation ?

 

rear tyres as its rear drive

 

 

thanks euan

 

don't forget that this is a purely basic ratio calculation and it doesn't take ito account tyre swelling the faster you go which can account for an extra bit of speed

the maclaren F1 was geared to hit the limiter at 225 mph but their first run out saw it doing 231, so there was some serious head screatching going on there, because you aren't supposed to be making that sort of mistake when you build a car that expensive

 

it is strange my car was mapped to hit the limiter at 7800 rpm which according to the calculator is 156. but my speedo shows just over 140 hitting limiter in 4th although i know the speedo is under reading a bit but no more than 5 mph so ? i thought my speed was around 145 unless my limiter is a bit lower for some reason cant tell as rev clock only goes to 7k

 

What size tyres are you running?

 

Measure your tyre again in mm's.

Then divide... 1 609 344 millimeters x the number of mm's your tyre measures. (my guess is it'll be around 820) thats your revolutions per mile.

Then do your equation again like you did at the top of this page

ps.. its revolutions per MILE not per MINUTE

 

Also... if you limiter is set too 7800 i would guess you have a soft cut limiter around 7600 which you will prob only feel in 5th.

 

not sure on that mate stu said he was doing 8k limiter and soft limiter at 7.8k but on the mapping paperwork just says limiter set to 7800 rpm

 

i meant mile

im confused by the equation above ? 17 in rims and 235 40 tyres. can you show me the equation for working that into revolutions per mile please mate ?

nice 3 door to its

 

17 inch rims, 235 40 tyres.

 

you will never be totally accurate unless you accurately measure the actual diameter or better still circumference of your tyre as it deforms and goes along. using standard tyre measurements will not be accurate as there can be a big difference between new and worn tyres, tyre pressure etc

 

As a rough guestimate (going by a fixed* tyre circumference of 1947mm for a 235/40x17):
7800rpm = 194.45mph on 0.80 fifth gear and 3.64 final drive.
7700rpm = 191.96
7600rpm = 189.46
7500rpm = 186.97
7400rpm = 184.48
7300rpm = 181.98
7200rpm = 179.49
7100rpm = 177.00
7000rpm = 174.51

*due to centrifugal force, the diameter actually grows slightly, so you may achieve fractionally higher speeds than quoted, or as Nick points out, the above circumference is based on a new tyre and an almost fully worn item could be up to 1.5% less speed.

 

thanks mike

 

spoke to kenny at msd today 7800 is soft limiter and 8000 is limiter

maybe i am doing 150 on the limiter in 4th

cant check it with sat nav as id have to hold it on the limiter for a few seconds and i dont wana do that else il be