Tech Time Ė Gearing
By Kev Rooney
Note Ė This was originally published in the UKKís Gazette and is reproduced here with their permission.
Now this one may not make your car look cooler, but will almost certainly help your pocket. Thought that would get your attention!
Youíll need to check out the details for your particular engine, but on my old 2000 Pinto powered Capri the flat torque peak was about 3500rpm so I geared the car to cruise at 70mph at those revs (3.54:1 diff). This gave me almost 40mpg, but dropping to 60mph at 3000rpm also saw a drop in economy to 30mpg.Most Yank eights in stock form produce best torque at 2500rpm but remember tuning camshafts etc. alter these figures.
Right, now itís time to get those calculators out and follow the writing on the blackboard. The following specs are comparing the effects of dropping tyre sizes from 165x13 to 145x13.
145x13 = 22ins Tyre Diameter
165x13 = 23ins Tyre Diameter
1.00:1 Final Drive Ratio
letís work out the vehicle speed at a set rpm. We need to know the loaded
radius of the tyre and thatís the diameter divided by 2. Then we follow up by
working out the tyre revolutions per mile, which is 10,084 divided by loaded
radius of tyre (see last sentence). If youíre still with me the next formula
will tell us what we want to really know, cruising speed at a specific rpm.
Cruising speed = RPM x 60
Gear Ratio x tyre rev per mile
3500rpm x 60
3.9 x (10,084 / 11.5)
3500 x 60
3.9 x (10,084 / 11)
(RPM x 60) / tyre revs per mile
putting our known figures into this gives:
210,000 / 916.7272
equals 3.73 axle ratio
The nearest available in the Ford range is 3.77:1. I used a 3.54:1 to get my cruising speed up to 70 mph.
formula can be manipulated again to help work out engine revs at a given speed
if diff ratio and tyre size is known.
This formula (C) is:
RPM = Known Speed x diff ratio x tyre revs
RPM = 70 x 3.9 x (10,084 x 12.5)
RPM = 3670
Now use Formula B to work out diff ratio on 13inch wheels
(3670 x 60) / 876.8695
And the nearest available ratio is 3.54:1. Problem sorted.
the ratio right and youíll have better economy, more power and the ability to
hold a conversation without the engine note drowning you out.
Okay boys and girls, thatís all there is for this period. Class dismissed!
|Back to Top|
|All images and content Copyright © UK Hot Rods. Design by Alley Kat Internet. Content by Holmsey and UK Hot Rods members.|