Tech Time – GearingBy Kev Rooney Note – This was originally published in the UKK’s 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!
You’ll 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 it’s 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 Next
let’s work out the vehicle speed at a set rpm. We need to know the loaded
radius of the tyre and that’s 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 you’re 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 60Gear Ratio x tyre rev per mile 165x13 example 3500rpm x 60 3.9 x (10,084 / 11.5) 210,000 3419.7910 = 61.40mph 145x13 example 3500 x 60 3.9 x (10,084 / 11) 210,000 3575.2360 = 58.74mph
(RPM x 60) / tyre revs per mile final speed putting our known figures into this gives: 210,000 / 916.7272 61.40 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. The
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 revs60 RPM = 70 x 3.9 x (10,084 x 12.5) 60 RPM = 3670 Now use Formula B to work out diff ratio on 13inch wheels (3670 x 60) / 876.8695 70 = 3.58 And the nearest available ratio is 3.54:1. Problem sorted. Get
the ratio right and you’ll have better economy, more power and the ability to
hold a conversation without the engine note drowning you out.
Okay boys and girls, that’s all there is for this period. Class dismissed! |
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