the effect of gear ratio on fuel economy in spark ignition and combustion ignition en

[t walgamuth]

the question of what happens if you swap the gear ratios from this to that often comes up. i usually jump in and offer my thoughts on the subject, not being a shy individual. yesterday i was thinking and decided to list all the rear end ratio swaps that i have done just for fun.

1. 52 pontiac ambulance. originally came with 4.10 gears. this was a flathead straight eight engine with a two barrel carburator. it weighed something north of 4000 pounds and had a wheelbase that had been stretched about 20" giving it a wheelbase of about 135". one could observe the splices in the frame and the extension pieces. it was built by the national ambulance and hearse company and was based on a sedan delivery. it had a 3 speed with synchro on 2 and 3,. i tested and recorded speedo and odo error and corrected for tire size and drove the car through several tankfuls and several lengthly trips to establish base line figures. the fuel economy was in the low teens.

a. i swapped in a 3.66 rear end from a two door sedan. drove the car on several trips and calculated the improvment in fuel economy. i found that it increased directly proportional to the gear ration with very little deviation.

b. not satisfied, i searched out a 3.08 rear end from an automatic equipped car, bought it and installed it. although now taking off in first was a bit dicy if i got stopped on a steep hill with my trailer attached, once rolling the engine had more than adequate torque in every other situation both driving and pulling my 13' "love bug" travel trailer. i used the car in this configuration for a couple of years, driving and documenting the fuel economy. in this configuration it would get up to 18 mpg. again i found that the mileage was very directly related to the gear ratio.

i once also swapped a very small carburator (taken from my previous 61 mb 220b parts car) onto the engine using an adaptor and adapting the throttle linkage with small cable clamps. i found that the engine started and idled fine and drivabiliity was excellent but that the small carb limited high rpm power. i could open the throttle and the car would accelerate up to a speed commensurate with the amount of air the little carb would flow. it was as if i had the throttle about one third open on the original huge (by comparison) two barrell. the fuel economy on the one trip i took was exactly the same as with the bigger carb.

2. the second vehicle i experimented with was my 51 caddie hearse. i didnt change the gears on this one. by this time i had a larger 19' trailer and wanted to pull that with the hearse, so i actually fitted undersized tires for more mecahnical advantage. i did comprehensive calculations to find the comparative gear ratios generated by the smaller tire size.

3. the next vehicle was my 84 suburban with 6.2. these engines had the reputation for knocking out main bearing webs if equipped with the 410 rear end. so when the rear started showing signs of fatigue (its previous owner was the fleetwood travel trailer factory in crawfordsville indiana and it had over 150k of towing when i bought it), i bought a 3.73 rear with posi and had it installed. i did comprehensive documentation of mileage before and after. this truck with my 24' travel trailer attached would drive with no ill effect for hours on end at wot or very near it. and would top out at about 65 on the level. again i found the economy to pretty much direclty related to the gear ratios. this truck was equipped with a turbo 400 tranny. (a straight 3 speed aiutomatic)

4. the next vehicle was my 86 suburban. this too was equipped with a 6.2 diesel. again i changed the rear end from 410 to 373. again i found the mileage to relate very directly to the gear ratio. (turbo 400 tranny)

5. the next vehicle to receive the gear swap was my 81 280e. this car is a us spec vehicle that originally came with a 354 rear end. it had oodles of torque and an automatic tranny. (the tranny is from a 240d with its relatively low shift points, but ... hey it was paid for and when i put it in i didnt really know if the car was any good 'cause i bought it with a bad tranny). i swapped in a 288 gear from the 85 300dt. this vehicle is the only vehicle that disappointed me in its fuel economy after the swap. it did NOT correspond proportionately to the gear ratios in its economy improvment.

6. and finally when i did the swap of the euro 300d motor into my 83 240 i orignally had the 346 gear installed that came with the 300d. this was a very happy car. it performed very nicely. it had the same friendly driving 240dcharacteristics but felt about twice as lively due to the fact that the 240 has very little extra power to acclelerate after overcoming the power taken to transmit through the power train, run the water pump, power steering and charge the battery, not to mention the ac. i drove the car in this configuration for a month or two then ordered a 307 gear from the "benz store" and a speedo and had them installed. i found that the performance was not affected a lot, but taking off was not an automatic thing, taking a bit of skill to do smoothly. (it is a four speed stick). the engine noise on the highway was greatly reduced. the top speed was still about 100, the same as it was with the 346 gear. and driving at 75 it consistantly got 30 mpg on the highway, actually a bit better than the stock 240d. and with a lot less noise and a lot more usable performance.

i have kept files on all my cars and if i were to dig, i could come up with actual figures on most of these. my conclusion is that, from personal experience, in every case except the 280e, be it gas or diesel, if you change the gear ratio to slow the engine down say 15%, your fuel economy increases by very nearly exactly the same amount.

for years, i have listened to folks who are very knowledgable about cars, but have not done the conversions and done the math, expound eloquently about how a higher gear will result in a deeper throttle position and get into the enrichment circuits of the sysetm etc. etc. (cough cough...bull s###t) they havent done the swaps and done the documentation because they talked themselves out of it before actually doing anything.

but in actual practice, especially in a diesel, i have found that if fitted with a 20% od situation you will realize a 20% reduction in fuel use. my conclusion is that the engines are geared with so much reserve torque and power that they are virtually ALWAYS running at much less than their optimum fuel consumption mode to make them more drivable for the average situation for the average driver in the average weather conditions in the average country with the average..... you get the idea.

this is my experience, if any of you have read all this, i am impressed.

tom w