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| | | | Joined: Nov 2015 Posts: 19 New Guy | | New Guy Joined: Nov 2015 Posts: 19 | I am running with 3.73 crown and pinion if I go taller will I get more top speed and how much and what else would I need to do.
Thanks
Mark | | | | | Joined: Sep 2011 Posts: 2,951 'Bolter | | 'Bolter Joined: Sep 2011 Posts: 2,951 | Mark,
In short, yes you will get more top speed at the same RPMs but you will notice that the truck will take a little longer getting up to speed.
I especially like gearing so maybe I can help. A 3.73 rear diff will give the truck plenty of power but the engine will really wind up at highway speeds. It really depends on what kind of driving you plan to do. If you want better highway RPMs (lower RPMs) and better fuel mileage, then you should look for an overdrive transmission with a 0.72 OD gear (0.68-0.78 would be the ideal range). With the OD gear you will not lose any performance in the lower gears at slower speeds and you will enjoy the drop in RPMs at higher speeds.
Many folks decide to swap the rear axle 3.73 gear for a 3.08. That also helps but the down side is a loss of performance in EVERY gear. This is not a problem for a V8 with lots of HP. However, it MIGHT be a problem for a tired inline 6.
If you are opposed to changing transmissions, then the only option is to swap for a taller rear diff gear.
Personally, I like my setup. I have an inline 6 and a 3.73 rear diff matched with a 5 speed transmission with the 0.72 OD gear. I cruise the highway with RPMs around 2100. My MPG is good too.
Last edited by Lugnutz; 11/18/2015 9:47 PM.
| | | | | Joined: Nov 2015 Posts: 19 New Guy | | New Guy Joined: Nov 2015 Posts: 19 | WOW and thanks, my only problem is that being based in the UK for me to purchase something like the OD transmission will cost me a fortune where as going to a 3.08 crown and pinion is only £120, ok i have already bought bearings and an overall kit at a cost of £100 so that hasnt been to much of a cost, as for the engine i have sorted that and it runs fine, good compression and all that but i am in the process of adding a 32/36 carb and having the manifold from stovebolt which will allow me to have the twin exhaust, so i hope i dont lose out on performance.
Many thanks
Mark | | | | | Joined: Sep 2011 Posts: 2,951 'Bolter | | 'Bolter Joined: Sep 2011 Posts: 2,951 | WOW and thanks, my only problem is that being based in the UK for me to purchase something like the OD transmission will cost me a fortune where as going to a 3.08 crown and pinion is only £120, ok i have already bought bearings and an overall kit at a cost of £100 so that hasnt been to much of a cost, as for the engine i have sorted that and it runs fine, good compression and all that but i am in the process of adding a 32/36 carb and having the manifold from stovebolt which will allow me to have the twin exhaust, so i hope i dont lose out on performance.
Many thanks
Mark
I think you will enjoy the change to the 3.08, especially if the truck is used as a daily driver. You will notice the highway RPMs will drop about 18%.
Last edited by Lugnutz; 11/19/2015 1:32 PM.
| | | | | Joined: Nov 2015 Posts: 19 New Guy | | New Guy Joined: Nov 2015 Posts: 19 | Just had a look throughout all you projects, you really are a clever fella, i can see you getting board with my questions that i ask as i am just a novice
Regards
Mark | | | | | Joined: May 2005 Posts: 8,983 Sir Searchalot | | Sir Searchalot Joined: May 2005 Posts: 8,983 | Didn't know you had the Autobahn in UK. Over here, in my experience, the 3:73 was always the perfect compromise ratio. We all like our quick starts now and then. Who knows? you may go to a V8 someday.
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
| | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | "Didn't know you had the Autobahn in UK."
Motorways in GB - 70 mp/h (112 km/hr) | | | | | Joined: May 2005 Posts: 8,983 Sir Searchalot | | Sir Searchalot Joined: May 2005 Posts: 8,983 | Understood. The comment was in jest indicating "what's the hurry"? (in a 6 cyl) (Autobahn has some areas of unrestricted/suggested speed limits only) I just don't go over 55 in an older truck. I get there soon enough. As indicated, I like a 3:73. That's all. I don't know what speed Ratcliffe is after.
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
| | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | No problem, Bartamos,
I only posted it because the Autobahn's has speed limits in only "congested" ("built-up") areas - enforced speed limit. There is no speed limit in many areas - but, you have to have your car under control.
It is incredible cruising Autobahn at 100+ MPH in a little car.
On Interstates here, I often cruise in my truck at 60-65 (with 3.55 R&P). | | | | | Joined: Nov 2015 Posts: 19 New Guy | | New Guy Joined: Nov 2015 Posts: 19 | I agree but I was looking for a crushing speed of around 65 to 70 on the motorways here and would like the lower RPM to give better fuel ecomany as I intend to go to the classic in le man's next year and that's close to 1000 miles
| | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | If your brakes and suspension and tires are good, cruising at 65-70 could be attained by using an over-drive transmission. That "solution" would most likely be your least expensive way to lower RPMs.
What is you engine and size truck? Someone will post helpful info. | | | | | Joined: May 2005 Posts: 8,983 Sir Searchalot | | Sir Searchalot Joined: May 2005 Posts: 8,983 | That does sound like a formula for 3:08 or 3:42. There are importers of American classic trucks and cars in UK. Maybe one of them could help you out with OD. Like stashing one in the lorry bed or car boot when shipping from U.S. 
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
| | | | | Joined: Nov 2015 Posts: 19 New Guy | | New Guy Joined: Nov 2015 Posts: 19 | I have the 230 in an 8 ft fleetside | | | | | Joined: May 2005 Posts: 8,983 Sir Searchalot | | Sir Searchalot Joined: May 2005 Posts: 8,983 | Chevy called the optional rear ratio 3.07 in 1963. That will probably be needed to wind up the 140HP to 70MPH without stressing motor or mileage if most of your usage is highway. The 3.73 was the std ratio. Sounds like you have a set of parts to do the swap. Good luck. Let us know what kind of effect it makes on both ends.
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
| | | | | Joined: Feb 2012 Posts: 506 'Bolter | | 'Bolter Joined: Feb 2012 Posts: 506 | My first truck was a 63 C10 with the original 235, granny low 1st gear trans and 3.90 rear end. I swapped in a 3.08 rear end out of early 70's C10 with an automatic trans and it worked great. It was very nice to drive. I lowered the highway RPMs to a reasonable level and didn't sacrifice much performance. I never raced it and was in no hurry to launch off the line so I was perfectly happy with the result. I even towed a small camper trailer with the truck without issue. That was the only time I ever started the truck in 1st gear.
Bottom line: I think you will be very happy with the 3.08 swap.
Thanks
Larry
| | | | | Joined: Nov 2015 Posts: 19 New Guy | | New Guy Joined: Nov 2015 Posts: 19 | Larry,
Many thanks for your input you have convinced me to go to a 3.08 as i was a little concerned and i was thinking about a 3.42 but you have settled my mind on the 3.08
Regards
Mark | | | | | Joined: Nov 2015 Posts: 19 New Guy | | New Guy Joined: Nov 2015 Posts: 19 | Hi I have tried to fit the 3.08 into my rear axle and when I put back the pinion gear seems to big as it causes the planet gear casing to stick out making it impossible to put back slide in my half shafts any ideas, also bought a 3.42 crown and pinion and thats the same can you get a thinner planner case or could I get another axle but I don't want to lose my 6 stud | | |
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